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USGS2011

Hydrogeology, chemical characteristics, and water sources and pathways in the zone of contribution of a public-supply well in San Antonio, Texas

In 2001, the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey initiated a series of studies on the transport of anthropogenic and natural contaminants (TANC) to public-supply wells (PSWs). The main goal of the TANC project was to better understand the source, transport, and receptor factors that control contaminant movement to PSWs in representative aquifers of the United States. Regional- and local-scale study areas were selected from within existing NAWQA study units, including the south-central Texas Edwards aquifer. The local-scale TANC study area, nested within the regional-scale NAWQA study area, is representative of the regional Edwards aquifer. The PSW selected for study is within a well field of six production wells. Although a single PSW was initially selected, because of constraints of well-field operation, samples were collected from different wells within the well field for different components of the study. Data collected from all of the well-field wells were considered comparable because of similar well construction, hydrogeology, and geochemistry. An additional 38 PSWs (mostly completed in the confined part of the aquifer) were sampled throughout the regional aquifer to characterize water quality. Two monitoring well clusters, with wells completed at different depths, were installed to the east and west of the well field (the Zarzamora and Timberhill monitoring well clusters, respectively). One of the monitoring wells was completed in the overburden to evaluate potential hydrologic connectivity with the Edwards aquifer. Geophysical and flowmeter logs were collected from one of the well-field PSWs to determine zones of contribution to the wellbore. These contributing zones, associated with different hydrogeologic units, were used to select monitoring well completion depths and groundwater sample collection depths for depth-dependent sampling. Depth-dependent samples were collected from the PSW from three different depths and under three different pumping conditions. Additionally, selected monitoring wells and one of the well-field PSWs were sampled several times in response to a rainfall and recharge event to assess short-term (event-scale) temporal variations in water quality. For comparison purposes, groundwater samples were categorized as being from regional aquifer PSWs, from the well field (wellhead samples), from the monitoring wells (excluding the overburden well), from the overburden well, from the PSW depth-dependent sampling, and from temporal sampling. Groundwater samples were analyzed for inorganic, organic, isotopic, and age-dating tracers to characterize geochemical conditions in the aquifer and provide understanding of the mechanisms of mobilization and movement of selected constituents from source areas to a PSW. Sources, tracers, and conditions used to assess water quality and processes affecting the PSW and the aquifer system included (1) carbonate host rock composition; (2) physicochemical constituents; (3) major and trace element concentrations; (4) saturation indices with respect to minerals in aquifer rocks; (5) elemental ratios, such as magnesium to calcium ratios, that are indicative of water-rock interaction processes; (6) oxidation-reduction conditions; (7) nutrient concentrations, in particular nitrate concentrations; (8) the isotopic composition of nitrate, which can point to specific nitrate sources; (9) strontium isotopes; (10) stable isotopes of hydrogen and oxygen; (11) organic contaminant concentrations, including pesticides and volatile organic compounds; (12) age tracers, apparent-age distribution, and dissolved gas data used in age interpretations; (13) depth-dependent water chemistry collected from the PSW under different pumping conditions to assess zones of contribution; and (14) temporal variability in groundwater composition from the PSW and selected monitoring wells in response to an aquifer recharge event. Geochemical results indicate that the well-field and monitoring well samples were largely representative of groundwater in the regional confined aquifer. Constituents of concern in the Edwards aquifer for the long-term sustainability of the groundwater resource include the nutrient nitrate and anthropogenic organic contaminants. Nitrate concentrations (as nitrogen) for regional aquifer PSWs had a median value of 1.9 milligrams per liter, which is similar to previously reported values for the regional aquifer. Nitrate-isotope compositions for groundwater samples collected from the well-field PSWs and monitoring wells had a narrow range, with values indicative of natural soil organic values. A comparison with historical nitrate-isotope values, however, suggests that a component of nitrate in groundwater from biogenic sources might have increased over the last 30 years. Several organic contaminants (the pesticide atrazine, its degradate deethylatrazine, trichloromethane (chloroform; a drinking-water disinfection byproduct), and the solvent tetrachloroethene (PCE)) were widely distributed throughout the regional aquifer and in the local-scale TANC study area at low concentrations (less than 1 microgram per liter). Higher concentrations of PCE were detected in samples from the well-field PSWs and Zarzamora monitoring wells relative to the regional aquifer PSWs. The urban environment is a likely source of contaminants to the aquifer, and these results indicate that one or more local urban sources might be supplying PCE to the Zarzamora monitoring wells and the well-field wells. Samples from the well field also had high concentrations of chloroform relative to the monitoring wells and regional aquifer PSWs. For samples from the regional aquifer PSWs, the most frequently detected organic contaminants generally decreased in concentration with increasing well depth. Deeper wells might intercept longer regional flow paths with higher fractions of older water or water recharged in rural recharge areas in the western part of the aquifer that have been less affected by anthropogenic contaminants. A scenario of hypothetical contaminant loading was evaluated by using results from groundwater-flow-model particle tracking to assess the response of the aquifer to potential contamination. Results indicate that the aquifer responds quickly (less than 1 year to several years) to contaminant loading; however, it takes a relatively long time (decades) for concentrations to reach peak values. The aquifer also responds quickly (less than 1 year to several years) to the removal of contaminant loading; however, it also takes a relatively long time (decades) to reach near background concentrations. Interpretation of geochemical age tracers in this well-mixed karst system was complicated by contamination of a majority of measured tracers and complexities of extensive mixing. Age-tracer results generally indicated that groundwater samples were composed of young, recently recharged water with piston-flow model ages ranging from less than 1 to 41 years, with a median of 17 years. Although a piston-flow model is typically not valid for karst aquifers, the model ages provide a basis for comparing relative ages of different samples and a reference point for more complex hydrogeologic models for apparent-age interpretations. Young groundwater ages are consistent with particle-tracking results from hydrogeologic modeling for the local-scale TANC study area. Age-tracer results compared poorly with other geochemical indicators of groundwater residence time and anthropogenic effects on water quality, indicating that hydrogeologic conceptual models used in groundwater age interpretations might not adequately account for mixing in this karst system. Groundwater samples collected from the well field under a variety of pumping conditions were relatively homogeneous and well mixed for numerous geochemical constituents (with the notable exception of age tracers). Groundwater contributions to the PSW were dominated by well-mixed, relatively homogeneous groundwater, typical of the regional confined aquifer. Zones of preferential flow were determined for the PSW, but groundwater samples from different stratigraphic units were not geochemically distinct. Variations in chemical constituents in response to a rainfall and aquifer recharge event occurred but were relatively minor in the PSW and monitoring wells. This observation is consistent with the hypothesis that the response to individual recharge events in the confined aquifer, unless intersecting conduit flow paths, might be attenuated by mixing processes along regional flow paths. Results of this study are consistent with the existing conceptual understanding of aquifer processes in this karst system and are useful for water-resource development and management practices.

Texas↗

USGS2025

Fluvial sediment dynamics in the Shoshone River and tributaries around Willwood Dam, Park County, Wyoming

Sedimentation affects many of the aging reservoirs in the United States. Dams and water diversions from rivers have been central elements of infrastructure supporting agricultural irrigation in the arid and semiarid regions of the Western United States for more than a century. The Willwood Irrigation District diversion dam (hereafter referred to as “Willwood Dam”) in Park County, Wyoming, is approximately 12 miles northeast of Cody, Wyo.; has a structural height of 70 feet; and impounds the Shoshone River for diversion into the Willwood Canal. Willwood Dam is part of a larger irrigation scheme supported by water storage in the much larger Buffalo Bill Dam, which is approximately 20 miles upstream. In October 2016, renovation construction activities at Willwood Dam and the Willwood Canal caused an unplanned evacuation of nearly 96,000 cubic yards of fine sediment. The fine sediment release in 2016 raised concerns that ongoing sediment management at Willwood Dam could impose limits on the long-term health of the aquatic ecosystem and fish populations. The U.S. Geological Survey, in cooperation with Wyoming Department of Environmental Quality and Willwood Work Groups 2 and 3, initiated an investigation of the dynamics of sediment transport in the Shoshone River and selected tributaries between Buffalo Bill Dam and Willwood Dam. The goal of the study was to quantify sediment transport into and out of Willwood Dam on an annual, seasonal, and event basis to better understand the relative quantities of sediment coming from natural sources and human activities on the landscape. The study ran from March 2019 through October 2021 and used observations of streamflow, turbidity, and acoustic backscatter collected at streamgages upstream and downstream from Willwood Dam to quantify suspended-sediment loads into and out of the dam during irrigation and fallow seasons, precipitation-runoff events, and deliberate sediment releases. Each tributary’s relative contribution to the sediment load upstream from Willwood Dam was examined using discrete measurements of suspended-sediment concentration and bedload during irrigation and fallow seasons, precipitation events, and stable conditions. Analysis of daily precipitation and temperature data indicated that conditions in the study area during the 2019 agricultural year were wetter and colder than period of record normal, and drier and near normal temperatures for the 2020 and 2021 agricultural years. Not all sediment load records between 2019 and 2021 are complete because of rejected observations (outliers), instrument failures or fouling, and instrument removal for calibrations. Statistical modeling of suspended-sediment concentration using paired values of turbidity and acoustic backscatter produced four models that, after refinement, had coefficients of determination indicating that more than 84 percent of the variance was explained by either turbidity or acoustic backscatter. A system of rules was developed to select the model predictions based on the seasonal operations of Willwood Dam, assumptions about the grain sizes mobilized during these operations, and assumed accuracy of the models at the downstream streamgage (Shoshone River below Willwood Dam, near Ralston, Wyo. [streamgage 06284010]) under different operational conditions. The sediment budget between upstream and downstream estimates of loads was interpreted using the mean predicted values bound by their respective model prediction intervals. When mean predicted loads of one streamgage were contained in the prediction intervals of the other streamgage, and vice-versa, difference in the sediment budget were interpreted as “indeterminate.” Modeled sediment load balances demonstrated the depositional and erosional behaviors expected from the conceptual model of dam operations whereby sediment tends to accumulate during irrigation seasons when the dam is spilling over the top, and sediment tends to evacuate during the fallow seasons when it is flowing through the sluice gates at the base of the dam. The sediment load calculations using the rules-based model criteria indicated that between 14,200 and 380,000 tons of suspended sediment moved through the Shoshone River around Willwood Dam during the irrigation seasons of 2019, 2020, and 2021; 380,000 tons of suspended sediment were transported during the cool, wet year of 2019, and 14,200 tons of suspended sediment were transported in 2020, which was relatively dry. During fallow seasons 2019, 2020, and 2021, which had fewer complete records, between 1,140 and 106,000 tons of suspended sediment was estimated to have moved through the river. For all seasons except fallow season 2022, the models estimated that more sediment was released from the dam than entered the dam, but the modeled mean loads at each streamgage were nearly always within the prediction intervals of each other, making the sediment balance indeterminant. Examination of suspended-sediment loads during irrigation seasons indicated that between 65 and 85 percent of fine sediment was transported during annual high flows and storm events, with the remainder transported during steady, lower streamflows. Examination of suspended loads during fallow seasons indicated that deliberate sediment releases through Willwood Dam accounted for between 39 and 67 percent of the total sediment moved during the fallow seasons. Deliberate sediment releases from Willwood Dam had estimated net exports of between 1,360 and 22,400 tons. Between August 2017 and July 2023, suspended-sediment concentration and bedload sediment samples were collected from 9 tributaries to the Shoshone River during 137 sampling events, including stable and precipitation-runoff conditions. During irrigation season precipitation events, the mean total sediment yields ranged from 0.33 to 9.51 tons per day per square mile; during fallow season precipitation events, the mean total yields ranged from 0.04 to 0.95 ton per day per square mile. The mean total sediment yield per unit area across all samples at each tributary site ranged from 0.26 to 3.08 tons per day per square mile. Bedload was a minor fraction of the total load, constituting a mean of 4 percent across all samples; 3 and 6 percent for events and nonevents, respectively, during irrigation season; and 3 and 1 percent for events and nonevents, respectively, during the fallow season. With the exception of one tributary, Dry Creek, these mean yield values were within the range of watershed-scale background sediment yield values estimated from reservoir surveys and previous suspended-sediment studies. Imagery from irrigation seasons 2012, 2015, 2017, 2019, and 2022 was used to determine the planimetric backwater extent of the pool area in the Shoshone River behind Willwood Dam to identify any changes in sediment storage. Active river channel widths in the Shoshone River upstream from Willwood Dam were all similar between years except 2015, which was determined to be statistically different from all other years. Bathymetric data taken in the pool behind Willwood Dam during three different surveys between November 2017 and April 2022 indicated no statistically significant differences in bed elevations between the years. Results from the planimetric and bathymetric survey data provide multiple lines of evidence indicating that sediment did not accumulate behind the dam within the error of the methods used. Examination of how precipitation affects sediment transport in the Shoshone River upstream from Willwood Dam indicated that accumulated rainfall from the natural runoff events captured during the study period varied from a trace to as much as 4.26 inches, with associated predicted suspended-sediment loads varying from 112 to 232,000 tons of suspended sediment. The behavior of the sediment loads relative to accumulated precipitation did not appear to change depending on irrigation or fallow season. A model of suspended-sediment concentrations relative to the 2-day accumulated precipitation indicated that suspended-sediment concentrations in the Shoshone River upstream from Willwood Dam increased exponentially for accumulations of 0.3 inch or more; such storms accounted for 10 percent or less of precipitation events observed during the 1981 to 2018 period of record. The gaps in records, precision of the instrumentation, and large variation in grain sizes in suspended-sediment mixtures downstream from the dam made closing the sediment budgets for most seasons unattainable. The biggest recent change in sediment storage measured using the planimetric area of deposits behind Willwood Dam took place between 2015 and 2017. The main event between these two measurements was the installation of new Willwood Canal gates in October 2016, which resulted in the large unplanned sediment release. Because the sediment budgets were nearly always indeterminate and the planimetric and bathymetric data indicated little change in the bed and bank material, it is likely that the change in sediment storage behind the dam during the study period was small relative to the precision of the statistical models and other uncertainties. This body of evidence suggests that, averaged during the 3-year study period, no major changes in storage took place, and that the current operations may be keeping storage at near-equilibrium. This condition could have been initiated because the middle sluice gate has now been operational since 2014, and the sediment release in October 2016 evacuated a large amount of legacy sediment from storage. Although the uncertainties are large, sluicing events allow for controlled releases of sediment that contributed to the near equilibrium conditions observed over an annual basis during this study.

Wyoming↗

USGS2002

Water-quality and physical characteristics of streams in the Treyburn development area of Falls Lake watershed, North Carolina, 1994–98

Treyburn is a 5,400-acre planned, mixed-use development in the upper Neuse River Basin of North Carolina. The development, which began in 1986, is located in the Falls Lake watershed near three water-supply reservoirs-Lake Michie to the north, Falls Lake to the southeast, and Little River Reservoir to the west. A study began in 1988 to determine the water-quality characteristics of surface waters in and around the Treyburn development area. Data to characterize water quality at five different sites were collected from July 1994 through September 1998. Data from a previous study are available for some sites for the period 1988–93. The sites were selected to characterize water quality and quantity in and near the Treyburn development and included an undeveloped basin, a relatively small basin containing single-family residences and a golf course, a basin downstream from the western part of the development with some industrial land use, and two basins unaffected by the development where agricultural land is being converted to urban and forested land use. Suspended-sediment concentrations ranged from less than 1 to 581 milligrams per liter and were fairly uniform among the five sites. Median suspended-sediment concentrations ranged from 12 to 21 milligrams per liter. Few concentrations of metals and trace elements, except aluminum, iron, and manganese, exceeded the laboratory reporting levels or water-quality criteria. At one site, concentrations of silver exceeded both the action level and the reporting level; copper was detected at each site and exceeded the action level of 7 micrograms per liter at one site. The lowest range and median concentrations of total organic nitrogen, nitrate, ammonia, total phosphorus, and orthophosphorus occurred in the relatively undisturbed, forested site. The maximum concentration of organic nitrogen (1.97 milligrams per liter) occurred at one of the sites unaffected by the Treyburn development where agricultural land is being converted to urban land use. At all sites, ammonia concentrations ranged from less than 0.02 to 0.36 milligram per liter, and median concentrations were near the reporting level. Nitrate concentrations ranged from less than 0.05 to 0.80 milligram per liter. Phosphorus concentrations at all of the Treyburn study sites were low compared to phosphorus concentrations that typically exceed 0.1 milligram per liter at sites sampled nationally for the U.S. Geological Survey National Water-Quality Assessment Program, including the Albemarle-Pamlico study area in North Carolina. Total phosphorus concentrations ranged from less than 0.01 to 0.87 milligram per liter, and orthophosphorus concentrations ranged from less than 0.01 to 0.76 milligram per liter as phosphorus. The maximum concentrations of total phosphorus and orthophosphorus occurred at the Treyburn residential and golf-course site, likely as a result of the fertilizer applications associated with these two types of land use. Of the 119 different pesticides tested, 11 were detected in concentrations that exceeded the laboratory reporting levels, though in very low concentrations. Water samples from the residential and golf-course site contained the greatest number of pesticides (10). Five of six samples collected at this site had detectable concentrations of simazine, atrazine, and pendimethalin-all herbicides used to control weeds in crops or turf. Channel geometry was assessed at eight sites in the study area in February 1997. These sites were separated into three groups based on mean bank angle and mean channel width-to-depth ratios. Channel gradient ranged from 0.04 to 1.63 percent, and mean cross sectional area ranged from 31 to 1,227 square feet. Three macroinvertebrate samples were collected from each of 10 sites. These three samples were from areas designated as richest targeted habitats, depositional targeted habitats, and qualitative multitargeted habitats. Over 230 taxa were identified fromthese 10 sites. The North Carolina Biotic Indices ranged from 4.98 (excellent) to 6.82 (fair). River sites tended to have higher total taxa richness (91-108) than did the small, intermittent streams (49–84) or the midsize Mountain Creek (85). Intermittent streams represent fairly hostile environments for most aquatic organisms. Samples from richest targeted habitats typically were more than twice as rich as samples from depositional targeted habitats and represented from 50 to 75 percent of the taxa found at each site (mean of 62 percent). The industrial site lacked many of the mayfly taxa that were present at the undeveloped site. Mayflies are very sensitive to metals contamination, and their absence may indicate a possible problem. The supporting chemical information is not available for the industrial site, and additional study would be necessary to substantiate this possibility. The two sites with residential and golf-course land use tended to support more different types of sensitive invertebrates (that is, mayflies, stoneflies, and caddis flies) than did the forested/residential site, though the abundances of these taxa were very similar. Land-use effects were not evident based on a comparison among these sites. Indirect gradient analysis was used to determine patterns in the distribution of invertebrates and to examine the relations between these patterns and physical and chemical site characteristics determined in this study. This analysis supports the contention that the dominant factors accounting for the distribution of benthic invertebrates are associated with natural factors, such as basin size, rather than land use. Constituent loads at five study sites were calculated for nutrients, suspended sediment, and total organic carbon. The median annual total nitrogen yield ranged from 0.635 to 1.63 tons per square mile. The median annual phosphorus yield ranged from 0.046 to 0.619 ton per square mile, and the median annual orthophosphate yield ranged from 0.022 to 0.379 ton per square mile. Orthophosphate accounted for more than half of the phosphorus yield at the residential and golf-course site. The maximum suspended-sediment yield was 422 tons per square mile, and the minimum yield was 32 tons per square mile. The suspended-sediment yield at one of the sites unaffected by the Treyburn development where agricultural was being converted to urban land use was high compared to other forested basins in the Piedmont of North Carolina. Total organic carbon data sufficient for estimating loads were available at three of the five sites. Of these three sites, the undeveloped site had substantially more organic carbon yield than the other two sites. The only significant water-quality trend (alpha=0.05) was a downward trend for total nitrogen and organic nitrogen at the undeveloped site. The trend slope was small, only 0.019 milligram per liter as nitrogen or less than 9 percent of the median organic nitrogen concentration. No trend was observed for nitrite plus nitrate or for ammonia, indicating that the downward trend in total nitrogen was due only to organic nitrogen.

North Carolina↗

USGS2017

Groundwater quality for 75 domestic wells in Lycoming County, Pennsylvania, 2014

Groundwater is a major source of drinking water in Lycoming County and adjacent counties in north-central and northeastern Pennsylvania, which are largely forested and rural and are currently undergoing development for hydrocarbon gases. Water-quality data are needed for assessing the natural characteristics of the groundwater resource and the potential effects from energy and mineral extraction, timber harvesting, agriculture, sewage and septic systems, and other human influences. This report, prepared in cooperation with Lycoming County, presents analytical data for groundwater samples from 75 domestic wells sampled throughout Lycoming County in June, July, and August 2014. The samples were collected using existing pumps and plumbing prior to any treatment and analyzed for physical and chemical characteristics, including nutrients, major ions, metals and trace elements, volatile organic compounds, gross-alpha particle and gross beta-particle activity, uranium, and dissolved gases, including methane and radon-222. Results indicate groundwater quality generally met most drinking-water standards, but that some samples exceeded primary or secondary maximum contaminant levels (MCLs) for arsenic, iron, manganese, total dissolved solids (TDS), chloride, pH, bacteria, or radon-222. Arsenic concentrations were higher than the MCL of 10 micrograms per liter (µg/L) in 9 of the 75 (12 percent) well-water samples, with concentrations as high as 23.6 μg/L; arsenic concentrations were higher than the health advisory level (HAL) of 2 μg/L in 23 samples (31 percent). Total iron concentrations exceeded the secondary maximum contaminant level (SMCL) of 300 μg/L in 20 of the 75 samples. Total manganese concentrations exceeded the SMCL of 50 μg/L in 20 samples and the HAL of 300 μg/L in 2 of those samples. Three samples had chloride concentrations that exceeded the SMCL of 250 milligrams per liter (mg/L); two of those samples exceeded the SMCL of 500 mg/L for TDS. The pH ranged from 5.3 to 9.15 and did not meet the SMCL range of 6.5 to 8.5 in 22 samples, with 17 samples having a pH less than 6.5 and 8 samples having pH greater than 8.5. Generally, the samples that had elevated TDS, chloride, or arsenic concentrations had high pH. Total coliform bacteria were detected in 39 of 75 samples (52 percent), with Escherichia coli detected in 10 of those 39 samples. Radon-222 activities ranged from non-detect to 7,420 picocuries per liter (pCi/L), with a median of 863 pCi/L, and exceeded the proposed drinking-water standard of 300 pCi/L in 50 (67 percent) of the 75 samples; radon-222 activities were higher than the alternative proposed standard of 4,000 pCi/L in 3 samples. Water from 15 of 75 (20 percent) wells had concentrations of methane greater than the reporting level of 0.01 mg/L; detectable methane concentrations ranged from 0.04 to 16.8 mg/L. Two samples had methane concentrations (13.1 and 16.8 mg/L) exceeding the action level of 7 mg/L. Low levels of ethane (up to 0.12 mg/L) were present in the five samples with the highest methane concentrations (near or above 1 mg/L) that were analyzed for hydrocarbon compounds and isotopic composition. The isotopic composition of methane in four of these groundwater samples, from the Catskill and Lock Haven Formations and the Hamilton Group, have sample carbon isotopic ratio delta values (carbon-13/carbon-12) ranging from –42.36 to –36.08 parts per thousand (‰) and hydrogen isotopic ratio delta values (deuterium/protium) ranging from –212.0 to –188.4 ‰, which are consistent with the isotopic compositions reported for mud-gas logging samples from these geologic units and a thermogenic source of the methane. However, the isotopic composition and ratios of methane to ethane in a fifth sample indicate the methane in that sample may be of microbial origin that subsequently underwent oxidation. The fifth sample had the highest concentration of methane, 16.8 mg/L, with an carbon isotopic ratio delta values of -50.59 ‰ and a hydrogen isotopic ratio delta values of -209.7 ‰. The six well-water samples with the highest methane concentrations also had among the highest pH values (8.25 to 9.15) and elevated concentrations of sodium, lithium, boron, fluoride, arsenic, and bromide. Relatively elevated concentrations of some other constituents, such as barium, strontium, and chloride, commonly were present in, but not limited to, those well-water samples with elevated methane. Three of the six groundwater samples with the highest methane concentrations had chloride/bromide ratios that indicate mixing with a small amount of brine (0.02 percent or less) similar in composition to those reported at undetermined depth below the freshwater aquifer and for gas and oil well brines in Pennsylvania. The sample with the highest methane concentration and most other samples with low methane concentrations (less than about 1 mg/L) have chloride/bromide ratios that indicate predominantly anthropogenic sources of chloride, such as road-deicing salt, septic systems, and (or) animal waste. Brines that are naturally present may originate from deeper parts of the aquifer system, while anthropogenic sources are more likely to affect shallow groundwater because they occur on or near the land-surface. The spatial distribution of groundwater compositions generally indicate that (1) uplands along the western border of Lycoming County usually have dilute, slightly acidic oxygenated, calcium-bicarbonate type waters; (2) intermediate altitudes or areas of carbonate bedrock usually have water of near neutral pH, with highest amounts of hardness (calcium and magnesium); (3) stream valleys, low elevations where groundwater may be discharging, and deep wells in uplands usually have water with pH values greater than 8 and highest arsenic, sodium, lithium, bromide concentrations. Geochemical modeling indicated that for samples with elevated pH, sodium, lithium, bromide, and alkalinity, the water chemistry could have resulted by dissolution of calcite (calcium carbonate) combined with cation-exchange and mixing with a small amount of brine. Through cation-exchange reactions between water and bedrock, which are equivalent to processes in a water softener, calcium ions released by calcite dissolution are exchanged for sodium ions on clay minerals. Thus, the assessment of groundwater quality in Lycoming County indicates groundwater is generally of good quality, but various parts of Lycoming County can have groundwater with low to moderate concentrations of methane and other constituents that appear in naturally present brine and produced waters from gas and oil wells at high concentrations."

Pennsylvania↗

USGS2017

Baseline assessment of groundwater quality in Pike County, Pennsylvania, 2015

The Devonian-age Marcellus Shale and the Ordovician-age Utica Shale, which have the potential for natural gas development, underlie Pike County and neighboring counties in northeastern Pennsylvania. In 2015, the U.S. Geological Survey, in cooperation with the Pike County Conservation District, conducted a study that expanded on a previous more limited 2012 study to assess baseline shallow groundwater quality in bedrock aquifers in Pike County prior to possible extensive shale-gas development. Seventy-nine water wells ranging in depths from 80 to 610 feet were sampled during June through September 2015 to provide data on the presence of methane and other aspects of existing groundwater quality in the various bedrock geologic units throughout the county, including concentrations of inorganic constituents commonly present at low values in shallow, fresh groundwater but elevated in brines associated with fluids extracted from geologic formations during shale-gas development. All groundwater samples collected in 2015 were analyzed for bacteria, dissolved and total major ions, nutrients, selected dissolved and total inorganic trace constituents (including metals and other elements), radon-222, gross alpha- and gross beta-particle activity, dissolved gases (methane, ethane, and propane), and, if sufficient methane was present, the isotopic composition of methane. Additionally, samples from 20 wells distributed throughout the county were analyzed for selected man-made volatile organic compounds, and samples from 13 wells where waters had detectable gross alpha activity were analyzed for radium-226 on the basis of relatively elevated gross alpha-particle activity. Results of the 2015 study show that groundwater quality generally met most drinking-water standards for constituents and properties included in analyses, but groundwater samples from some wells had one or more constituents or properties, including arsenic, iron, manganese, pH, bacteria, sodium, chloride, sulfate, total dissolved solids, and radon-222, that did not meet (commonly termed failed or exceeded) primary or secondary maximum contaminant levels (MCLs) or Health Advisories (HA) for drinking water. Except for iron, dissolved and total concentrations of major ions and most trace constituents generally were similar. Only 1 of 79 well-water samples had any constituent that exceeded a MCL, with an arsenic concentration of about 30 micrograms per liter (µg/L) that was higher than the MCL of 10 µg/L. However, total arsenic concentrations were higher than the HA of 2 µg/L in samples from another 12 of 79 wells (about 15 percent). Secondary maximum contaminant levels (SMCLs) were exceeded most frequently by pH and concentrations of iron and manganese. The pH was outside of the SMCL range of 6.5–8.5 in samples from 24 of 79 wells (30 percent), ranging from 5.5 to 9.2; more samples had pH values less than 6.5 than had pH values greater than 8.5. Total iron concentrations typically were much greater than dissolved iron concentrations, indicating substantial presence of iron in particulate phase, and exceeded the SMCL of 300 µg/L more often [35 of 79 samples (44 percent)] than dissolved iron concentrations [samples from 8 of 79 wells (10 percent)]. Total manganese concentrations exceeded the SMCL of 50 µg/L in samples from 31 of 79 wells (39 percent) and the HA of 300 µg/L in samples from 13 of 79 wells (about 16 percent). A few (1–2) samples had concentrations of sodium, chloride, sulfate, or TDS higher than the SMCLs of 60, 250, 250, and 500 mg/L, respectively. However, dissolved sodium concentrations were higher than the HA of 20 mg/L in samples from 15 of 79 wells (nearly 20 percent). Total coliform bacteria were detected in samples from 25 of 79 wells (32 percent) but Escherichia coli were not detected in any sample. Radon-222 activities ranged from 11 to 5,100 picocuries per liter (pCi/L), with a median of 1,440 pCi/L, and exceeded the proposed and the alternate proposed drinking-water standards of 300 and 4,000 pCi/L, respectively, in samples from 60 of 79 wells (75 percent) and in samples from 2 of 79 wells (3 percent), respectively. Groundwater samples from all wells were analyzed for dissolved methane by one contract laboratory that determined water from 19 of the 79 wells (24 percent) had concentrations of methane greater than the reporting level of 0.010 milligrams per liter (mg/L) with a maximum methane concentration of 2.5 mg/L. Methane concentrations in 18 replicate samples submitted to a second laboratory for dissolved gas and isotopic analysis generally were higher by as much as a factor of 2.7 from those determined by the first laboratory, indicating potential bias related to combined sampling and analytical methods, and therefore, caution needs to be used when comparing methane results determined by different methods. The isotopic composition of methane in 9 of 10 samples with sufficient dissolved methane (about 0.3 mg/L) for isotopic analysis is consistent with values reported for methane of microbial origin produced through carbon dioxide reduction; an isotopic shift in 1 or 2 samples may indicate subsequent methane oxidation. The low concentrations of ethane relative to methane in these samples further indicate that the methane may be of microbial origin. Groundwater samples with relatively elevated methane concentrations (near or greater than 0.3 mg/L) also had chemical compositions that differed in some respects from groundwater with relatively low methane concentrations (less than 0.3 mg/L) by having higher pH (greater than 8) and higher concentrations of sodium, lithium, boron, fluoride, arsenic, and bromide and chloride/bromide ratios indicative of mixing with a small amount of brine of probable natural occurrence. The spatial distribution of groundwater compositions differs by topographic setting and lithology and generally shows that (1) relatively dilute, slightly acidic, oxygenated, calcium-carbonate type waters tend to occur in the uplands underlain by the undivided Poplar Gap and Packerton members of the Catskill Formation in southwestern Pike County; (2) waters of near neutral pH with the highest amounts of hardness (calcium and magnesium) generally occur in areas of intermediate altitudes underlain by other members of the Catskill Formation; and (3) waters with pH values greater than 8, low oxygen concentrations, and the highest arsenic, sodium, lithium, bromide, and methane concentrations can be present in deep wells in uplands but most frequently occur in stream valleys, especially at low altitudes (less than about 1,200 feet above North American Vertical Datum of 1988) where groundwater may be discharging regionally, such as to the Delaware River in northern and eastern Pike County. Thus, the baseline assessment of groundwater quality in Pike County prior to gas-well development shows that shallow (less than about 1,000 feet deep) groundwater generally meets primary drinking-water standards for inorganic constituents but varies spatially, with methane and some constituents present in high concentrations in brine (and connate waters from gas and oil reservoirs) present at low to moderate concentrations in some parts of Pike County.

Pennsylvania↗

USGS2016

Baseline assessment of groundwater quality in Wayne County, Pennsylvania, 2014

The Devonian-age Marcellus Shale and the Ordovician-age Utica Shale, geologic formations which have potential for natural gas development, underlie Wayne County and neighboring counties in northeastern Pennsylvania. In 2014, the U.S. Geological Survey, in cooperation with the Wayne Conservation District, conducted a study to assess baseline shallow groundwater quality in bedrock aquifers in Wayne County prior to potential extensive shale-gas development. The 2014 study expanded on previous, more limited studies that included sampling of groundwater from 2 wells in 2011 and 32 wells in 2013 in Wayne County. Eighty-nine water wells were sampled in summer 2014 to provide data on the presence of methane and other aspects of existing groundwater quality throughout the county, including concentrations of inorganic constituents commonly present at low levels in shallow, fresh groundwater but elevated in brines associated with fluids extracted from geologic formations during shale-gas development. Depths of sampled wells ranged from 85 to 1,300 feet (ft) with a median of 291 ft. All of the groundwater samples collected in 2014 were analyzed for bacteria, major ions, nutrients, selected inorganic trace constituents (including metals and other elements), radon-222, gross alpha- and gross beta-particle activity, selected man-made organic compounds (including volatile organic compounds and glycols), dissolved gases (methane, ethane, and propane), and, if sufficient methane was present, the isotopic composition of methane. Results of the 2014 study show that groundwater quality generally met most drinking-water standards, but some well-water samples had one or more constituents or properties, including arsenic, iron, pH, bacteria, and radon-222, that exceeded primary or secondary maximum contaminant levels (MCLs). Arsenic concentrations were higher than the MCL of 10 micrograms per liter (µg/L) in 4 of 89 samples (4.5 percent) with concentrations as high as 20 µg/L; arsenic concentrations were higher than the Health Advisory level of 2 µg/L in 27 of 89 samples (30 percent). Total iron concentrations exceeded the secondary maximum contaminant level (SMCL) of 300 µg/L in 9 of 89 samples (10 percent). The pH ranged from 5.4 to 9.3 and did not meet the SMCL range of greater than 6.5 to less than 8.5 in 27 samples (30 percent); 22 samples had pH values less than 6.5, and 5 samples had pH values greater than 8.5. Total coliform bacteria were detected in 22 of 89 samples (25 percent); Escherichia coli were detected in only 2 of those 22 samples. Radon-222 activities ranged from 25 to 7,400 picocuries per liter (pCi/L), with a median of 2,120 pCi/L, and exceeded the proposed drinking-water standard of 300 pCi/L in 86 of 89 samples (97 percent); radon-222 activities were higher than the alternative proposed standard of 4,000 pCi/L in 12 of 89 samples (13.5 percent). Water from 8 of the 89 wells (9 percent) had concentrations of methane greater than the reporting level of 0.24 milligrams per liter (mg/L) with the detectable methane concentrations ranging from 0.74 to 9.6 mg/L. Of 16 replicate samples submitted to another laboratory with a lower reporting level of 0.0002 mg/L, 15 samples had detectable methane concentrations that ranged from 0.0011 to 9.7 mg/L. Of these 15 samples, low levels of ethane (0.00032 to 0.0017 mg/L) were detected in 6 of 7 samples with methane concentrations greater than 0.75 mg/L. The isotopic composition of methane in 6 of 8 samples with sufficient dissolved methane (about 1 mg/L) for isotopic analysis is consistent with a predominantly thermogenic methane source (sample carbon isotopic ratio δ13C CH4 values ranging from -56.36 to -45.97 parts per thousand (‰) and hydrogen isotopic ratio δD CH4 values ranging from -233.1 to -141.1 ‰). However, the low levels of ethane relative to methane indicate that the methane may be of microbial origin and subsequently underwent oxidation. Isotopic compositions indicated a possibly mixed thermogenic and microbial source (carbon dioxide reduction process) for the methane in 1 of the 8 samples (δ 13 C CH4 of -63.72 and δD CH4 of -192.3 ‰) and potential oxidation of microbial and (or) thermogenic methane in the remaining sample (δ 13 C CH4 of -46.56 and δD CH4 of -79.7 ‰). Groundwater samples with relatively elevated methane concentrations (near or greater than 1 mg/L) had a chemical composition that differed in some respects (pH, selected major ions, and inorganic trace constituents) from groundwater with relatively low methane concentrations (less than 0.75 mg/L). The seven well-water samples with the highest methane concentrations (from about 1 to 9.6 mg/L) also had among the highest pH values (8.1 to 9.3, respectively) and the highest concentrations of sodium, lithium, boron, fluoride, arsenic, and bromide. Relatively elevated concentrations of some other constituents, such as barium, strontium, and chloride, commonly were present in, but not limited to, those well-water samples with elevated methane. Groundwater samples with the highest methane concentrations had chloride/bromide ratios that indicate mixing with a small amount of brine (0.02 percent or less, by volume) similar in composition to that reported for gas and oil well brines in Pennsylvania. Most other samples with low methane concentrations (less than about 1 mg/L) had chloride/bromide ratios that indicate predominantly man-made sources of chloride, such as road salt, septic systems, and (or) animal waste. Although naturally occurring brines may originate from deeper parts of the aquifer system, the man-made sources are likely to affect shallow groundwater. Geochemical modeling showed that the water chemistry of samples with elevated pH, sodium, lithium, bromide, and alkalinity could result from dissolution of calcite (calcium carbonate) combined with cation exchange and mixing with a small amount of brine. Through cation exchange reactions (which are equivalent to processes in a water softener) calcium ions released by calcite dissolution are exchanged for sodium ions on clay minerals. The spatial distribution of groundwater compositions generally shows that (1) relatively dilute, slightly acidic, oxygenated, calcium-carbonate type waters tend to occur in the uplands along the western border of Wayne County; (2) waters of near neutral pH with the highest amounts of hardness (calcium and magnesium) generally occur in areas of intermediate altitudes; and (3) waters with pH values greater than 8, low oxygen concentrations, and the highest arsenic, sodium, lithium, bromide, and methane concentrations can occur in deep wells in uplands but most frequently occur in stream valleys, especially at low elevations (less than about 1,200 ft above North American Vertical Datum of 1988) where groundwater may be discharging regionally, such as to the Delaware River. Thus, the baseline assessment of groundwater quality in Wayne County prior to gas-well development shows that shallow (less than about 1,000 ft deep) groundwater is generally of good quality, but methane and some constituents present in high concentrations in brine (and produced waters from gas and oil wells) may be present at low to moderate concentrations in some parts of Wayne County.

Pennsylvania↗

USGS1999

Surface-water-quality assessment of the Yakima River basin, Washington: Overview of major findings, 1987-91

Surface-water-quality conditions were assessed in the Yakima River Basin, which drains 6,155 square miles of mostly forested, range, and agricultural land in Washington. The Yakima River Basin is one of the most intensively farmed and irrigated areas in the United States, and is often referred to as the “Nation’s Fruitbowl.” Natural and anthropogenic sources of contaminants and flow regulation control water-quality conditions throughout the basin. This report summarizes the spatial and temporal distribution, sources, and implications of the dissolved oxygen, water temperature, pH, suspended sediment, nutrient, organic compound (pesticide), trace element, fecal indicator bacteria, radionuclide, and aquatic ecology data collected during the 1987–91 water years. The Yakima River descends from a water surface altitude of 2,449 feet at the foot of Keechelus Dam to 340 feet at its mouth downstream from Horn Rapids Dam near Richland. The basin can be divided into three distinct river reaches on the basis of its physical characteristics. The upper reach, which drains the Kittitas Valley, has a high gradient, with an average streambed slope of 14 feet per mile (ft/mi) over the 74 miles from the foot of Keechelus Dam (river mile [RM] 214.5) to just upstream from Umtanum. The middle reach, which drains the Mid Valley, extends a distance of 33 miles from Umtanum (RM 140.4) to just upstream from Union Gap and also has a high gradient, with an average streambed slope of 11 ft/mi. The lower reach of the Yakima River drains the Lower Valley and has an average streambed slope of 7 ft/mi over the 107 miles from Union Gap (RM 107.2) to the mouth of the Yakima River. These reaches exhibited differences in water-quality conditions related to the differences in geologic sources of contaminants and land use. Compared with the rest of the basin, the Kittitas Valley and headwaters of the Naches River Subbasin had relatively low concentrations and loads of suspended sediment, nutrients, organic compounds, and fecal indicator bacteria. There were very few failures to meet the Washington State dissolved oxygen standard or exceedances of the water temperature and pH standards in this reach. In general, these areas are considered to be areas of lessdegraded water quality in the basin. The preTertiary metamorphic and intrusive rocks of the Cle Elum and Teanaway River Subbasins, however, were found to be significant geologic sources of antimony, arsenic, chromium, copper, mercury, nickel, selenium, and zinc. As a result, the arsenic, chromium, and nickel concentrations measured in the streambed sediment of the Kittitas Valley were 13 to 74 times higher than those measured in the Lower Valley. The Mid and Lower Valleys had similar water-quality conditions, governed by the intensive agricultural and irrigation activities, highly erosive landscapes, and flow regulation. Most of the failures to meet the Washington State standards for dissolved oxygen and exceedances of the standards for water temperature and pH occurred in the Mid and Lower Valleys. Agricultural drains in the Mid and Lower Valleys were found to be significant sources of nutrients, suspended sediment, pesticides, and fecal indicator bacteria. Downstream from the irrigation diversions near Union Gap, summertime streamflow in the Yakima River was drastically reduced to only a few hundred cubic feet per second. In the lower Yakima River, agricultural return flow typically accounts for as much as 80 percent of the main stem summertime flow near the downstream terminus of the basin. Therefore, the water-quality characteristics of the lower Yakima River resemble those of the agricultural drains. The highest fecal bacteria concentrations (35,000 colonies of Escherichia coli per 100 milliliters of water) were measured in the Granger/Sunnyside area, the location of most of the livestock in the basin. The east side area of the Lower Valley (area east of the Yakima River) was the predominant source area for suspended sediment and pesticides in the basin. This area had the largest acreage of irrigated land and generally received the largest application of pesticides. Owing to the highly erosive soils of the area, the suspended sediment load from the east side in June 1989 (320 kilograms per day) was five or more times larger than from any other area, and the loads of several of the more hydrophobic organic compounds were four or more times larger. An ecological assessment of the Yakima River Basin ranked physical, chemical, and biological conditions at impaired (degraded) sites against reference sites in an effort to understand how land use changes physical and chemical site characteristics and how biota respond to these changes. For this assessment, the basin was divided into four natural ecological categories: (1) Cascades ecoregion, (2) Eastern Cascades Slopes and Foothills ecoregion, (3) Columbia Basin ecoregion, and (4) large rivers. Each of these categories has a unique combination of climate and landscape features that produces a distinctive terrestrial vegetation assemblage. In the combined Cascades and Eastern Cascades site group, which had the fewest impaired sites, the metals index was the only physical and chemical index that indicated any impairment. The moderate levels of impairment noted in the invertebrate and algal communities were not, however, associated with metals, and may have been related to the effects of logging, although the intensity of logging was not directly quantified in this study. Sites in the Columbia Basin site group were all moderately or severely impaired with the exception of the two reference sites (Umtanum Creek and Satus Creek below Dry Creek), which showed no physical, chemical, or biological impairment. Three sites were heavily affected by agriculture (Granger Drain, Moxee Drain, and Spring Creek) and were listed as severely impaired by most of the physical, chemical, and biological condition indices. Agriculture was the primary cause of the impairment of biological communities in this site group. The primary physical and chemical indicators of agricultural effects were nutrients, pesticides, dissolved solids, and substrate embeddedness, which all tended to increase with agricultural intensity. The biological effects of agriculture were manifested by a decrease in the abundance and number of native species of fish and invertebrates, a shift in algal communities to species indicative of eutrophic conditions, and higher abundances. There was also an increase in the abundance and number of nonnative fish species due to the prevalence of fish that are largely tolerant of nutrient-rich conditions. Main stem (large river) sites downstream from the city of Yakima exhibited severe impairment of fish communities associated with high levels of pesticides in fish tissues and the presence of external anomalies on fish.

Washington↗

USGS2012

Quality of water from crystalline rock aquifers in New England, New Jersey, and New York, 1995-2007

Crystalline bedrock aquifers in New England and parts of New Jersey and New York (NECR aquifers) are a major source of drinking water. Because the quality of water in these aquifers is highly variable, the U.S. Geological Survey (USGS) statistically analyzed chemical data on samples of untreated groundwater collected from 117 domestic bedrock wells in New England, New York, and New Jersey, and from 4,775 public-supply bedrock wells in New England to characterize the quality of the groundwater. The domestic-well data were from samples collected by the USGS National Water-Quality Assessment (NAWQA) Program from 1995 through 2007. The public-supply-well data were from samples collected for the U.S. Environmental Protection Agency (USEPA) Safe Drinking Water Act (SDWA) Program from 1997 through 2007. Chemical data compiled from the domestic wells include pH, specific conductance, dissolved oxygen, alkalinity, and turbidity; 6 nitrogen and phosphorus compounds, 14 major ions, 23 trace elements, 222 radon gas (radon), 48 pesticide compounds, and 82 volatile organic compounds (VOCs). Additional samples were collected from the domestic wells for the analysis of gross alpha- and gross beta-particle radioactivity, radium isotopes, chlorofluorocarbon isotopes, and the dissolved gases methane, carbon dioxide, nitrogen, and argon. Chemical data compiled from the public-supply wells include pH, specific conductance, nitrate, iron, manganese, sodium, chloride, fluoride, arsenic, uranium, radon, combined radium ( 226 radium plus 228 radium), gross alpha-particle radioactivity, and methyl tert -butyl ether (M t BE). Patterns in fluoride, arsenic, uranium, and radon distributions were discernable when the data were compared to lithology groupings of the bedrock, indicating that the type of bedrock has an effect on the quality of groundwater from NECR aquifers. Fluoride concentrations were significantly higher in groundwater samples from the alkali granite, peraluminous granite, and metaluminous granite lithology groups than from samples in the other lithology groups. Water samples from 1.4 percent of 2,167 studied wells had fluoride concentrations that were equal to or greater than the maximum contaminant level (MCL) of 4 milligrams per liter (mg/L) and 7.5 percent of the wells had fluoride concentrations that were equal to or greater than the secondary MCL of 2 mg/L. For arsenic, groundwater samples from the calcareous metasedimentary rocks in the New Hampshire-Maine geologic province, peraluminous granite, and pelitic rocks lithology groups had higher concentrations than did samples from the other lithology groups. Water samples from 13.3 percent of 2,054 studied wells had arsenic concentrations that were equal to or greater than the MCL of 10 micrograms per liter (μg/L), about double the national rate of occurrence in community-supply systems and in domestic wells of the United States. Uranium concentrations were significantly higher in groundwater samples from the peraluminous granite, alkali granite, and calcareous metasedimentary rocks in the New Hampshire-Maine geologic province lithology groups than from samples in the other lithology groups. Water samples from 14.2 percent of 556 studied wells had uranium concentrations equal to or greater than the MCL of 30 μg/L. Radon activities were equal to or greater than the proposed MCL of 300 picocuries per liter (pCi/L) in 95 percent of 943 studied wells, and 33 percent of the wells had radon activities were equal to or greater than the proposed alternative maximum contaminant level (AMCL) of 4,000 pCi/L. Radon activities exceeded the proposed AMCL in 20 percent or more of groundwater samples in each of the studied lithology groups with a minimum of 9 samples, but radon activities were significantly higher in groundwater samples from the alkali granite, peraluminous granite, and Narragansett basin metasedimentary rocks lithology groups. Water samples from 3.2 percent of 564 studied wells had combined radium activities equal to or greater than the MCL of 5 pCi/L; however, combined radium activities were not significantly different among the studied lithology groups. Land use and population density also were evaluated to explain patterns in water quality. Concentrations of nitrate, sodium, chloride, and MtBE from the studied wells were significantly greater in areas of high population density (≥50 persons per square kilometer) than in areas of low population density (<50 persons per square kilometer). Concentrations of sodium, chloride, and M t BE from the studied wells were significantly greater in areas classified as developed (urban lands) than in areas classified as undeveloped (forested), agricultural, or mixed (no dominant land use). Nitrate concentrations from the public-supply wells were not significantly different among the four land use categories, but nitrate concentrations from the domestic wells were significantly greater in areas classified as developed than in areas classified as undeveloped, agricultural, or mixed. Chloride to bromide mass ratios in the domestic well samples indicate that the groundwater was probably affected by at least three halogen sources: local precipitation and recharge waters, remnant seawater and connate waters evolved from seawater, and recharge waters affected by road salt. The groundwater in the NECR aquifers generally contained low concentrations of nitrate, VOCs, and pesticides. Less than 1 percent of water samples from 4,781 studied wells had concentrations of nitrate greater than the MCL of 10 mg/L. Less than 1 percent of water samples from 1,299 studied wells exceeded the USEPA advisory level of 20 to 40 μg/L for M t BE. None of the other studied VOCs exceeded a human health benchmark. M t BE (36 percent frequency detection) and chloroform (32.9 percent frequency detection) were the most frequently detected (>0.02 μg/L) VOCs in the domestic wells. M t BE was detected more often in water samples with apparent ages of less than 25 years than in water samples with apparent ages greater than 25 years. This finding is consistent with the time period of high M t BE use in areas in the United States where reformulated gasoline was mandated. The largest pesticide concentration was an estimated concentration of 0.06 μg/L for the herbicide metolachlor. Deethylatrazine, a degradate of atrazine, (18 percent frequency detection) and atrazine (8 percent frequency detection) were the only pesticide compounds detected (>0.001 μg/L) in more than 3 percent of the domestic wells. None of the detected pesticide compounds exceeded human health benchmarks. Concentrations of nitrate and gross alpha-particle activities were significantly greater in the water samples from the domestic wells than in samples from the public-supply wells. Concentrations of sodium, chloride, iron, manganese, and uranium were significantly greater in the water samples from the public-supply wells than in the samples from the domestic wells. One possible explanation may be related to differences in field processing (filtered samples from the domestic wells compared to unfiltered samples from the public-supply wells). The high frequency of detections for a wide variety of manmade and naturally occurring contaminants in both domestic and public-supply wells shows the vulnerability of NECR aquifers to contamination. The highly variable water quality and the association with highly variable lithology of crystalline bedrock underscores the importance of testing individual wells to determine if concentrations for the most commonly detected contaminants exceed human health benchmarks.

Connecticut, Maine, Massachusetts, New Hampshire, ↗

USGS1957

Geology of possible petroleum provinces in Alaska

The history of petroleum exploration in Alaska and the geology of possible petroleum provinces in Alaska are reviewed. Maps showing Alaska's major Mesozoic and Tertiary tectonic elements, possible petroleum provinces, and indications of petrol, are included in this report. Annotated references in Geological Survey publications relating to petroleum and oil shale in Alaska are given at the end of the report. For the purpose of appraising its petroleum possibilities, Alaska is divided into the southern, central, and northern major geologic-physiographic regions. Southern Alaska includes the arcuate mountain chain formed by the Alaska and Aleutian Ranges and the Mentasta- Nutzotin Mountains, the coastal range and valley area to the south, and the southeastern Alaska "panhandle" -- an area of 185,000 square miles. Oil seeps on the west shore of Cook Inlet in southern Alaska were known as early as 1853, and claims were staked in this region in 1882. Drilling began near the oil seeps in the Katalla district about 1901, and this started Alaska's first period of oil activity. From 1902 to 1933 the Katalla field produced 154,000 barrels of oil from fractured shale and sandstone of Tertiary age the- first and only commercial production in Alaska. On the basis of geology, surficial indications of petroleum, and test wells drilled, six possible petroleum provinces are indicated in southern Alaska. They are Heceta Island area, Keku Islands area, Cook Inlet Mesozoic province, Gulf of Alaska Tertiary province, Cook Inlet Tertiary province, and Copper River basin. The exposed rocks in the Heceta Island area include lower Paleozoic graywacke-type sandstone, sandstone, conglomerate, and massive limestones with reeflike structures; igneous rocks are rare or lacking in much of the area. The Kosciusko-Tuxekan-Heceta synclinorium, the main structural feature, is modified by minor folds and faults. Some of the minor folds are reported to be broad and open, with flanks dipping 20°-145°. As far as known, the Heceta Island area has not heretofore been seriously considered as a possible petroleum province. Rocks of Silurian to Cretaceous age are exposed in the Keku Island area and include moderately folded and relatively unaltered limestone and other marine sedimentary rocks. The Cook Inlet Mesozoic province, a land area of approximately 18,500 square miles, includes a great thickness of unmetamorphosed marine sedimentary rocks of Jurassic and Cretaceous age. At least 23 test wells were drilled or started in this province by the end of 1955. Shows of oil and gas were encountered in many of these wells. During 1955 at least ten oil companies were active in this area and by the end of 1955 about 1 1/2 million acres were included in oil and gas leases applied for or granted. The Gulf of Alaska Tertiary province includes about 5,200 square miles in which rocks of Tertiary age are exposed or are believed to underlie Quaternary deposits. Between 1901 and the end of 1955 about 47 wells were drilled or started in this province. The Cook Inlet Tertiary province embraces an area of about 9,500 square miles, of which about 4,100 is covered by the shallow waters of Cook Inlet. Petroleum exploration has been in that part of the area which overlaps the Cook Inlet Mesozoic province. Eocene or younger Tertiary nonmarine sedimentary rocks are believed to underlie much of the province, and marine rocks of Tertiary age may also be present. The Copper River basin is a topographic basin underlain by unconsolidated deposits of Quaternary age. Tertiary rocks favorable for the accumulation of petroleum may underlie part of the basin but this is not believed likely. Except for some leasing activity no petroleum exploration has been recorded in the Copper River Basin to the end of 1955. Central Alaska is a region of about 275,000 square miles and consists of an irregular assemblage of intricately dissected uplands and alluvium-floored lowland basins. Scattered peaks of resistant intrusive igneous rocks surmount most of the upland areas. In the vast region of central Alaska only six test wells are known to have been drilled for the purpose of finding oil and gas. The maximum depth reached was 350 feet and the holes were mostly or entirely in Quaternary deposits. In recent years several oil companies have investigated some parts of the region and large areas in the Yukon-Koyukuk province are now under lease. Oil seeps, gas seeps, and other indications of petroleum have been reported from many localities; samples from two localities have been analyzed and reported to be petroleum. The geology of central Alaska is similar in a general way to that of the area between the Rocky Mountains and Sierra-Cascade belts of the United States. Sedimentary rocks, probably equivalent to the Precambrian Belt series, and rocks of the Cambrian and all younger geologic systems have been recognized in central Alaska. The structure of the region is known to be complex, but except in local mineral districts, it has not been mapped in detail. Based on the limited amount of available information, the region cannot be regarded as distinctly favorable for significant accumulations of petroleum. However, three pre-Cenozoic provinces, the Yukon-Koyukuk, the Kobuk, and the Kandik, and several large Cenozoic basin provinces may be worthy of further investigation. Northern Alaska includes the Brooks Range and all the treeless tundra north to the Arctic Coast, an area of about 125,000 square miles. The presence of oil seeps along the Arctic Coast has been known at least since 1900 and a description of the Cape Simpson oil seeps vas published in 1909. Since then oil and gas seeps have been described from nine localities, and oil shales and oil-bearing sandstones are known from many localities in the Arctic Foothills province. Oil and gas deposits have been discovered and geologic conditions are favorable for oil and gas accumulations in approximately half of the region. In 1923 approximately 37,000 square miles in northern Alaska was reserved by Executive order as Naval Petroleum Reserve No. 4. In 1944 the U. S. Navy began a vast petroleum exploration program which was suspended in 1953. In the years 1945 through 1955, 37 test wells and 45 core tests were drilled on 18 structures. Three oil fields, Umiat, Simpson, and Fish Creek, and two gas fields, South Barrow and Gubik, were discovered. Total reserve estimates for all discoveries of oil to 1955 range from 30 to 100 million barrels, and for gas, from 370 billion to 900 billion cubic feet. All northern Alaska, with the exception of the Brooks Range, can be considered a possible petroleum province, but the region can be subdivided into provinces of somewhat different potentialities. These subdivisions roughly correspond with the geomorphic provinces and sections, which in turn reflect differences in geology. The known oil-bearing beds are of Mesozoic age, primarily Cretaceous, and thus the possible petroleum provinces could be designated as Mesozoic. However, Paleozoic and Cenozoic rocks with favorable reservoir characteristics are exposed in the region and possibly underlie, in favorable structural situations, some of the areas as yet not tested. The Arctic Coastal Plain province includes gently folded and flat-lying Mesozoic beds that overlie a basement complex of Paleozoic and early Mesozoic age. Near the southern edge of this province the basement rocks are at depths of at least 20,000 feet, and to the north these rocks rise to within 2,500 feet of the surface. The Teshukpuk Lake section of the Arctic Coastal plain includes many of the known oil seeps; it is the most accessible to sea transportation, and lies almost completely within NPR 4. Thirteen test wells and 35 core tests have been drilled here; one gas field and two (at present, noncommercial) oil fields have been discovered. The possibility of further discoveries may depend largely on locating porous sandstones in stratigraphic rather than anticlinal traps. The White Hills section is distinguished topographically from the Teshukpruk section by its white-gravel-covered hills and fever lakes, and geologically by the presence of Tertiary rocks, including 2,000 feet of nonmarine beds in the west and at least 7,000 feet of marine beds to the east, in the vicinity of Carter Creek. This section appears to be more complex structurally. No test wells have been drilled in the White Hills section. The Northern Foothills section includes many closed anticlines. Twenty-four test wells and ten core tests have been drilled on 11 structures and two discoveries have been made -the Umiat oil field and the Gubik gas field. All these tests have been drilled in Cretaceous rocks. The Southern Foothills section is structurally similar to the Alberta Foothills and to the northern part of the Brooks Range. Great thicknesses of marine shale of Lover Cretaceous, Jurassic, and Triassic age are exposed. The outcropping Mesozoic sandstones are generally poorly sorted, nonporous, and impermeable. To the south the section is bordered by mountainous exposures of Mississippian limestone, which probably underlie at least part of this section. The rocks that underlie the deeply eroded complex structures of the Brooks Range include schist, slate, argillite, and limestone. Some exposed limestones have a strong petroleum-like odor and contain traces of petroleum residues.

Alaska↗

USGS2016

Water-quality trends and constituent-transport analysis for selected sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site in the upper Clark Fork Basin, Montana, water years 1996–2015

During the extended history of mining in the upper Clark Fork Basin in Montana, large amounts of waste materials enriched with metallic contaminants (cadmium, copper, lead, and zinc) and the metalloid trace element arsenic were generated from mining operations near Butte and milling and smelting operations near Anaconda. Extensive deposition of mining wastes in the Silver Bow Creek and Clark Fork channels and flood plains had substantial effects on water quality. Federal Superfund remediation activities in the upper Clark Fork Basin began in 1983 and have included substantial remediation near Butte and removal of the former Milltown Dam near Missoula. To aid in evaluating the effects of remediation activities on water quality, the U.S. Geological Survey began collecting streamflow and water-quality data in the upper Clark Fork Basin in the 1980s. Trend analysis was done on specific conductance, selected trace elements (arsenic, copper, and zinc), and suspended sediment for seven sampling sites in the Milltown Reservoir/Clark Fork River Superfund Site for water years 1996–2015. The most upstream site included in trend analysis is Silver Bow Creek at Warm Springs, Montana (sampling site 8), and the most downstream site is Clark Fork above Missoula, Montana (sampling site 22), which is just downstream from the former Milltown Dam. Water year is the 12-month period from October 1 through September 30 and is designated by the year in which it ends. Trend analysis was done by using a joint time-series model for concentration and streamflow. To provide temporal resolution of changes in water quality, trend analysis was conducted for four sequential 5-year periods: period 1 (water years 1996–2000), period 2 (water years 2001–5), period 3 (water years 2006–10), and period 4 (water years 2011–15). Because of the substantial effect of the intentional breach of Milltown Dam on March 28, 2008, period 3 was subdivided into period 3A (October 1, 2005–March 27, 2008) and period 3B (March 28, 2008–September 30, 2010) for the Clark Fork above Missoula (sampling site 22). Trend results were considered statistically significant when the statistical probability level was less than 0.01. In conjunction with the trend analysis, estimated normalized constituent loads (hereinafter referred to as “loads”) were calculated and presented within the framework of a constituent-transport analysis to assess the temporal trends in flow-adjusted concentrations (FACs) in the context of sources and transport. The transport analysis allows assessment of temporal changes in relative contributions from upstream source areas to loads transported past each reach outflow. Trend results indicate that FACs of unfiltered-recoverable copper decreased at the sampling sites from the start of period 1 through the end of period 4; the decreases ranged from large for one sampling site (Silver Bow Creek at Warm Springs [sampling site 8]) to moderate for two sampling sites (Clark Fork near Galen, Montana [sampling site 11] and Clark Fork above Missoula [sampling site 22]) to small for four sampling sites (Clark Fork at Deer Lodge, Montana [sampling site 14], Clark Fork at Goldcreek, Montana [sampling site 16], Clark Fork near Drummond, Montana [sampling site 18], and Clark Fork at Turah Bridge near Bonner, Montana [sampling site 20]). For period 4 (water years 2011–15), the most notable changes indicated for the Milltown Reservoir/Clark Fork River Superfund Site were statistically significant decreases in FACs and loads of unfiltered-recoverable copper for sampling sites 8 and 22. The period 4 changes in FACs of unfiltered-recoverable copper for all other sampling sites were not statistically significant. Trend results indicate that FACs of unfiltered-recoverable arsenic decreased at the sampling sites from period 1 through period 4 (water years 1996–2015); the decreases ranged from minor (sampling sites 8–20) to small (sampling site 22). For period 4 (water years 2011–15), the most notable changes indicated for the Milltown Reservoir/Clark Fork River Superfund Site were statistically significant decreases in FACs and loads of unfiltered-recoverable arsenic for sampling site 8 and near statistically significant decreases for sampling site 22. The period 4 changes in FACs of unfiltered-recoverable arsenic for all other sampling sites were not statistically significant. Trend results indicate that FACs of suspended sediment decreased at the sampling sites from period 1 through period 4 (water years 1996–2015); the decreases ranged from moderate (sampling site 8) to small (sampling sites 11–22). For period 4 (water years 2011–15), the changes in FACs of suspended sediment were not statistically significant for any sampling sites. The reach of the Clark Fork from Galen to Deer Lodge is a large source of metallic contaminants and suspended sediment, which strongly affects downstream transport of those constituents. Mobilization of copper and suspended sediment from flood-plain tailings and the streambed of the Clark Fork and its tributaries within the reach results in a contribution of those constituents that is proportionally much larger than the contribution of streamflow from within the reach. Within the reach from Galen to Deer Lodge, unfiltered-recoverable copper loads increased by a factor of about 4 and suspended-sediment loads increased by a factor of about 5, whereas streamflow increased by a factor of slightly less than 2. For period 4 (water years 2011–15), unfiltered-recoverable copper and suspended-sediment loads sourced from within the reach accounted for about 41 and 14 percent, respectively, of the loads at Clark Fork above Missoula (sampling site 22), whereas streamflow sourced from within the reach accounted for about 4 percent of the streamflow at sampling site 22. During water years 1996–2015, decreases in FACs and loads of unfiltered-recoverable copper and suspended sediment for the reach generally were proportionally smaller than for most other reaches. Unfiltered-recoverable copper loads sourced within the reaches of the Clark Fork between Deer Lodge and Turah Bridge near Bonner (just upstream from the former Milltown Dam) were proportionally smaller than contributions of streamflow sourced from within the reaches; these reaches contributed proportionally much less to copper loading in the Clark Fork than the reach between Galen and Deer Lodge. Although substantial decreases in FACs and loads of unfiltered-recoverable copper and suspended sediment were indicated for Silver Bow Creek at Warm Springs (sampling site 8), those substantial decreases were not translated to downstream reaches between Deer Lodge and Turah Bridge near Bonner. The effect of the reach of the Clark Fork from Galen to Deer Lodge as a large source of copper and suspended sediment, in combination with little temporal change in those constituents for the reach, contributes to this pattern. With the removal of the former Milltown Dam in 2008, substantial amounts of contaminated sediments that remained in the Clark Fork channel and flood plain in reach 9 (downstream from Turah Bridge near Bonner) became more available for mobilization and transport than before the dam removal. After the removal of the former Milltown Dam, the Clark Fork above Missoula (sampling site 22) had statistically significant decreases in FACs of unfiltered-recoverable copper in period 3B (March 28, 2008, through water year 2010) that continued in period 4 (water years 2011–15). Also, decreases in FACs of unfiltered-recoverable arsenic and suspended sediment were indicated for period 4 at this site. The decrease in FACs of unfiltered-recoverable copper for sampling site 22 during period 4 was proportionally much larger than the decrease for the Clark Fork at Turah Bridge near Bonner (sampling site 20). Net mobilization of unfiltered-recoverable copper and arsenic from sources within reach 9 are smaller for period 4 than for period 1 when the former Milltown Dam was in place, providing evidence that contaminant source materials have been substantially reduced in reach 9.

Montana↗

USGS2019

Water-quality and geochemical variability in the Little Arkansas River and Equus aquifer, south-central Kansas, 2001–16

The city of Wichita’s water supply currently (2019) comes from two primary sources: Cheney Reservoir and the Equus Beds aquifer. The Equus Beds aquifer storage and recovery project was developed to help the city of Wichita meet increasing future water demands. Source water for artificial recharge comes from the Little Arkansas River during above-base-flow conditions, is treated using National Primary Drinking Water Regulations as a guideline, and is injected into the Equus Beds aquifer through recharge wells or surface spreading basins for later use. The Equus Beds aquifer storage and recovery project currently (2019) consists of two coexisting phases. Phase I began in 2007 and captures Little Arkansas River water and indirect streambank diversion well water for aquifer recharge using 4 wells and 2 recharge basins. Phase II began in 2013 and currently (2019) includes a surface-water treatment facility, a river intake facility, eight recharge injection wells, and a third recharge basin. The U.S. Geological Survey, in cooperation with the City of Wichita, completed this study to summarize water-quality and geochemical variability of the Equus Beds aquifer. Data in this report can be used to establish baseline conditions before implementing artificial aquifer recharge further, document groundwater quality, evaluate changing conditions, identify environmental factors affecting groundwater, provide science-based information for decision making, and help meet regulatory monitoring requirements. Physicochemical properties were measured and water-quality data were collected from 2 Little Arkansas River surface-water sites and 63 Equus Beds aquifer groundwater sites, including 38 areal assessment index wells (IWs) during 2001 through 2016. Data collection included discrete samples and additional continuous measurements at selected sites. Discretely collected samples were analyzed for physicochemical properties, dissolved solids, primary ions, nutrients (nitrogen and phosphorus species), organic carbon, indicator bacteria, trace elements, arsenic species, organic compounds, and radioactivity. This report focuses discussion on aquifer water quality. Federal drinking-water criteria were used to evaluate aquifer water quality. Primary drinking-water criteria are those that are enforceable for public drinking water. Secondary criteria are those that can cause aesthetics or tastes that are unpleasant. Continuously collected data at a subset of sites included streamflow, groundwater levels, water temperature, specific conductance, pH, oxidation-reduction potential (ORP), dissolved oxygen, turbidity, nitrate plus nitrite, and fluorescent dissolved organic matter. Continuous measurement of physicochemical properties in near-real time allowed characterization of Little Arkansas River surface water and Equus Beds aquifer groundwater during conditions and time scales that would not have been possible otherwise and served as a complement to discrete water-quality sampling. During 2001 through 2016, less than 1 percent of chloride and nitrate plus nitrite, 7 percent of dissolved iron, 48 percent of dissolved manganese, 12 percent of dissolved arsenic, and 39 percent of atrazine detections in surface-water samples exceeded their respective Federal primary or secondary drinking-water criteria. None of the surface-water samples collected exceeded the Federal sulfate criterion, and every sample had detections of total coliform bacteria during the study. Constituents of concern in the Equus Beds aquifer exceeded their respective Federal criteria throughout the study period and included chloride, sulfate, nitrate plus nitrite, Escherichia coli ( E. coli ), total coliforms, and dissolved iron and arsenic species. About 5 percent of shallow (less than 80 feet) and 7 percent of deep (greater than 80 feet) IW chloride sample concentrations exceeded the secondary Federal criterion of 250 milligrams per liter (mg/L). Chloride tended to exceed its criterion in shallow and deep wells along the Arkansas River and near Burrton, Kansas, an area with past oil and gas activities. Chloride concentrations near Burrton were larger in the deep parts of the aquifer. About 18 percent of shallow and 13 percent of deep IW sulfate sample concentrations exceeded the secondary Federal criterion of 250 mg/L. Mean sulfate concentrations tended to exceed the criterion in the central part of the study area. Shallow IW mean nitrate plus nitrite (hereafter referred to as “nitrate”) was substantially larger than mean deep IW nitrate. Geochemical conditions in the deeper aquifer reduced forms of nitrogen to species such as ammonia. About 15 percent of shallow and less than 1 percent of deep IW nitrate sample concentrations exceeded the Federal criterion of 10 mg/L. Mean shallow IW nitrate concentrations exceeded the criterion in the northeastern and southeastern parts of the study area; on average, deep IW nitrate concentrations did not exceed the criterion. E. coli and fecal coliform bacteria detections were usually at or near the detection limit. E. coli was detected in 3 percent of shallow and deep IWs, and fecal coliform bacteria were detected in 8 percent of shallow and 6 percent of deep IWs. Total coliforms were detected in 24 percent of shallow and 12 percent of deep IWs. E. coli coliphage was detected in two shallow IW samples (1 percent of samples) at the detection limit and was not detected in deep IW samples. Dissolved iron was detected in 51 percent of shallow and 62 percent of deep IW samples. Dissolved iron concentrations exceeded the secondary Federal criterion of 0.3 mg/L in 38 percent of shallow and 46 percent of deep IW samples. Mean dissolved iron concentrations were largest mostly in the central and northwest part of the study area corresponding to an area of the aquifer where aquifer material is more clay-rich. The distribution of large dissolved iron concentrations was similar to that of large sulfate concentrations. About 55 percent of shallow and 92 percent of deep IW dissolved manganese samples exceeded the secondary Federal criterion of 0.05 mg/L. Almost all samples from the central and northern parts of the study area had mean dissolved manganese concentrations that exceeded the Federal criterion in the shallow part of the aquifer. Mean dissolved manganese concentrations in the shallow part of the aquifer were substantially large (greater than 1,000 micrograms per liter [μg/L]) in wells near the Little Arkansas River and in the central part of the study area because of chemically reducing conditions in the aquifer that likely related to larger percentages of clay in the aquifer material. Concentrations of dissolved arsenic species generally were larger in the deep parts of the aquifer. Arsenite was the dominant form of arsenic on average in shallow (52 percent) and deep (55 percent) IWs. About 12 percent of shallow and 34 percent of deep IW dissolved arsenic sample concentrations exceeded the Federal primary drinking criterion of 10 μg/L. Shallow IW dissolved arsenic concentrations were larger near the Little Arkansas River and the center of the study area; large shallow IW dissolved arsenic concentrations (10–50 μg/L) in the center of the study area correspond to areas that have had the most water-level recovery since the historical low in 1993. Mean ORP in shallow IWs generally decreased with increasing water-level depths and were inversely related to mean dissolved arsenic concentrations because of more reducing conditions (smaller ORP) at larger depths below the land surface. Larger dissolved arsenic concentrations in the shallow parts of the aquifer were associated with decreases in water levels and a subsequent decrease in ORP and thus more reducing conditions. Atrazine was detected in about 58 percent of shallow and 28 percent of deep IWs and did not exceed the primary Federal criterion of 3 μg/L in any groundwater samples. Atrazine concentrations in shallow IWs generally were largest in the northwest part of the study area near the North Branch Kisiwa Creek, and atrazine concentrations in deep IWs generally were largest most often in the southern part of the study area. Gross α radioactivity concentrations exceeded the primary Federal criterion of 15 picocuries per liter in 4 percent of shallow IW samples. Gross α and gross β radioactivity concentrations generally were larger in the southern third of the aquifer. Most groundwater-sample-simulated minerals saturation indices (SIs) were consistently negative (undersaturated). Minerals that had SI values that were consistently or typically positive (oversaturated) included iron oxide, hydroxide, and quartz-group minerals. Several SI values for arsenic- and manganese-bearing minerals were consistently negative. Some manganese-bearing mineral SI values ranged from undersaturated to oversaturated in shallow and deep IWs during the study. Several carbonate minerals in shallow and deep IWs varied across their equilibrium state. Calcite SI values were larger more often in the deep parts of the aquifer and did not show a clear distributional pattern. Mean and median calcite SI values for shallow and deep IWs were negative (undersaturated) indicating the potential for calcite dissolution if calcite is present for a substantial part of the study period. However, some individual calcite SI values in this study indicated saturation and subsequent calcite precipitation may occur in the study area, potentially resulting in formation of calcite mineral deposits that may reduce efficiency of injection wells. SI values with respect to iron hydroxide varied across their equilibrium states. Mean and median SI values with respect to iron hydroxide were undersaturated in shallow and deep IWs; however, some samples had positive SI values indicating there is potential for iron hydroxide precipitation, possibly caused by leaching and oxidation of iron-containing minerals, like pyrite, in the aquifer material.

Kansas↗

USGS2017

Geophysics- and geochemistry-based assessment of the geochemical characteristics and groundwater-flow system of the U.S. part of the Mesilla Basin/Conejos-Médanos aquifer system in Doña Ana County, New Mexico, and El Paso County, Texas, 2010–12

One of the largest rechargeable groundwater systems by total available volume in the Rio Grande/Río Bravo Basin (hereinafter referred to as the “Rio Grande”) region of the United States and Mexico, the Mesilla Basin/Conejos-Médanos aquifer system, supplies water for irrigation as well as for cities of El Paso, Texas; Las Cruces, New Mexico; and Ciudad Juárez, Chihuahua, Mexico. The U.S. Geological Survey in cooperation with the Bureau of Reclamation assessed the groundwater resources in the Mesilla Basin and surrounding areas in Doña Ana County, N. Mex., and El Paso County, Tex., by using a combination of geophysical and geochemical methods. The study area consists of approximately 1,400 square miles in Doña Ana County, N. Mex., and 100 square miles in El Paso County, Tex. The Mesilla Basin composes most of the study area and can be divided into three parts: the Mesilla Valley, the West Mesa, and the East Bench. The Mesilla Valley is the part of the Mesilla Basin that was incised by the Rio Grande between Selden Canyon to the north and by a narrow valley (about 4 miles wide) to the southeast near El Paso, Tex., named the Paso del Norte, which is sometimes referred to in the literature as the “El Paso Narrows.” Previously published geophysical data for the study area were compiled and these data were augmented by collecting additional geophysical and geochemical data. Geophysical resistivity measurements from previously published helicopter frequency domain electromagnetic data, previously published direct-current resistivity soundings, and newly collected (2012) time-domain electromagnetic soundings were used in the study to detect spatial changes in the electrical properties of the subsurface, which reflect changes that occur within the hydrogeology. The geochemistry of the groundwater system was evaluated by analyzing groundwater samples collected in November 2010 for physicochemical properties, major ions, trace elements, nutrients, pesticides (reported but not used in the assessment), and environmental tracers. The data obtained from these samples (with the exception of the pesticide data) were used to gain insights into processes controlling the groundwater movement through the groundwater system in the study area. Results from the geophysical and geochemical assessments facilitated the interpretation of the geochemical characteristics of the groundwater sources and geochemical groups within the groundwater system. The groundwater-flow system in the study area consists primarily of the Mesilla Basin aquifer system, which can be divided into four hydrogeologic units by using an informal classification scheme based on basin-fill stratigraphy and sedimentology with an emphasis on aquifer characteristics. The four hydrogeologic units are (1) the Rio Grande alluvium, which is the shallow aquifer of the Mesilla Basin within the confines of the Mesilla Valley, and the three hydrogeologic units that compose the Santa Fe Group: (2) the lower part of the Santa Fe Group, which is the least productive zone, (3) the middle part of the Santa Fe Group, which is the primary water-bearing hydrogeologic unit in the basin and is generally saturated, and (4) the upper part of the Santa Fe Group, which is the most productive water-bearing unit within the Santa Fe Group but is only partially saturated in the north and largely unsaturated in the south and western parts of the Mesilla Basin. The helicopter frequency domain electromagnetic survey results indicated that approximately half of the resistivity values were less than 10 ohm-meters at depths of 50 and 100 feet with a transition where the resistivity values changed from relatively high values (greater than 20 ohm-meters) to relatively low resistivity values (less than 10 ohm-meters) near Vado, New Mexico. Slightly more than 25 percent of the gridded resistivity values from the three-dimensional grid of the combined inverse modeling results of the direct-current resistivity and time-domain electromagnetic soundings were equal to or less than 10 ohm-meters with large regions of low resistivity becoming apparent in the southernmost part of the study area near the Paso Del Norte where these low resistivity features are spatially the widest at or below the top of the bedrock. These low resistivity values might represent clayey deposits, sediments composed largely of sand and gravel saturated with saline water, or both. Historical dissolved-solids-concentration data within the surface geophysical subset area of the study area were compiled and compared to the inverse modeling results of the combined direct-current resistivity and time-domain soundings; this comparison was done to strengthen the interpretation made from the combined inverse modeling results that the low resistivity features were representative of sand and gravel deposits saturated with saline water and not clayey deposits. Water-level altitudes within the Rio Grande alluvium generally decreased from north to south, with a west to east decrease in water-level altitudes near Las Cruces, New Mexico, as a result of groundwater pumping. Groundwater flow within the Santa Fe Group is more complex than the groundwater flow within the Rio Grande alluvium because of the larger lateral and vertical extent of the Santa Fe Group compared to the Rio Grande alluvium. Groundwater from the Organ Mountains flows directly south towards the Paso del Norte. Groundwater from the Robledo Mountains, the Rough and Ready Hills, and the Sleeping Lady Hills generally flows to the southeast. Groundwater flowing near the north end of the midbasin uplift generally continues east towards the Rio Grande and then flows south on the east side of the midbasin uplift. Groundwater flowing near the west side of the midbasin uplift generally continues south parallel to the faults that make up the midbasin uplift and then flows east towards the Paso del Norte when it reaches the south end of the midbasin uplift. Groundwater from the Aden Hills and the East and West Potrillo Mountains flows to the south end of the midbasin uplift and then continues east towards the Paso del Norte. Throughout most of the Mesilla Valley, the vertical hydraulic gradient was downward because the water-level altitude in the Rio Grande alluvium was higher than it was in the Santa Fe Group, but in some areas (typically in the middle and southern parts of the Mesilla Valley), the vertical hydraulic gradient was substantially reduced or even reversed to an upward hydraulic gradient. The geochemistry data indicate that there was a complex system of multiple geochemical endmembers and mixing between these endmembers with recharge to the Rio Grande alluvium and Santa Fe Group composed mostly of seepage from the Rio Grande, inflows from deeper or neighboring water systems, and mountain-front recharge. Five distinct geochemical groups were identified in the Mesilla Basin study area: (1) ancestral Rio Grande (pre-Pleistocene) geochemical group, (2) modern Rio Grande (Pleistocene to present) geochemical group, (3) mountain-front geochemical group, (4) deep groundwater upwelling geochemical group, and (5) unknown freshwater geochemical group. The ancestral Rio Grande groundwater was water that recharged into the system as seepage losses from the ancestral Rio Grande; this groundwater generally flows from north to south-southeast towards the Paso del Norte. Groundwater on the west side of the midbasin uplift generally flows south until it reaches the southern part of the study area; from the southern part of the study area, the groundwater flows east towards the Paso del Norte. Groundwater on the east side of the midbasin uplift flows south-southeast towards the Paso del Norte where it mixes with groundwater from the modern Rio Grande, uplifted areas in the west, and the deep saline source. The water type of the modern Rio Grande geochemical group ranged from calcium-sulfate water type in the northern part of the study area to sodium-chloride-sulfate water type in the southern part of the study area; from north to south there was a substantial increase in specific conductance, strontium-87/strontium-86 ratio, potassium, and the trace metals of iron and lithium, changing the water chemistry such that it became similar to the water chemistry of the deep groundwater upwelling geochemical group. From age-dating results, water in the modern Rio Grande geochemical group was recharged to the Rio Grande alluvium within the past 10 years. The mountain-front geochemical group was generally old water (apparent age was greater than 10,000 carbon-14 years before present) that was somewhat mineralized and has relatively high concentrations of fluoride and silica, which might indicate longer exposure to volcanic and siliciclastic rocks or aluminosilicate minerals. There were five different locations of recharge determined from the groundwater geochemistry within the mountain-front geochemical group, all having a slightly different geochemical signature: (1) the Rough and Ready Hills, Robledo Mountains, and the Sleeping Lady Hills, (2) the Doña Ana Mountains, (3) the Aden Hills and West Potrillo Mountains, (4) the East Potrillo Mountains, and (5) the Sierra Juárez in Mexico. The groundwater from the Rough and Ready Hills, Robledo Mountains, the Sleeping Lady Hills, and the Doña Ana Mountains generally flows toward the Rio Grande and eventually mixes together and with the modern Rio Grande groundwater. The groundwater originating from the Aden Hills and East and West Potrillo Mountains generally flows east to southeast at a slow rate and eventually mixes and continues east, where it mixes with groundwater from the ancestral Rio Grande geochemical group and with the groundwater from the Sierra Juárez. The groundwater from the Sierra Juárez flows north and then east towards the Paso del Norte where it mixes with groundwater from the uplifted areas in the west, ancestral and modern Rio Grande groundwater, and the upwelling groundwater from a deep saline source. The deep groundwater upwelling geochemical group had the highest concentrations of bicarbonate, potassium, silica, aluminum, iron, and lithium within the study area, indicating that it had been in contact with carbonate and siliciclastic rocks for a much longer period of time and at higher temperatures compared to the other geochemical groups, and was most likely ancient marine groundwater originating from the Paleozoic and Cretaceous carbonate rocks which was upwelling into the Mesilla Basin aquifer system in the southeastern part of the study area through the extensive fault systems. Direct-current resistivity and time-domain electromagnetic soundings support the interpretation of ancient marine groundwater upwelling into the Mesilla Basin aquifer system, as do the analytical results from wells, and the helicopter frequency domain electromagnetic data collected along the Rio Grande. The hydrogen-2/hydrogen-1 ratio and oxygen-18/oxygen-16 ratio isotopic results for samples in the unknown freshwater geochemical group did not plot on the Rio Grande evaporation line, indicating this group did not have a Rio Grande signature (that is, there was no isotopic evidence of a component of Rio Grande water) and it also had the lowest mineralized content of any geochemical group in the study area.

New Mexico, Texas↗