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Groundwater & aquifers

Find USGS groundwater and aquifer publications. Search an aquifer name or place, compare source-supplied dates and open reports for hydrogeology, recharge and water-level studies.

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At least 289 records · Page 16Linked to original sources

USGS2017

Preliminary-assessment and upgrade of a groundwater flow model of the Seacoast Bedrock Aquifer, New Hampshire

In 2003 and 2004, the U.S. Geological Survey investigated the availability of groundwater resources in a 160-square mile area of coastal New Hampshire (Figure 1) using a regional groundwater flow model (Mack, 2009). At that time, population growth and increasing water demand prompted concern for the sustainability of the region’s groundwater resources in a fractured-crystalline bedrock-aquifer with little storage. The groundwater flow model developed for the previous study incorporated detailed water-use information for 2003-4 and simulated the effects of projected increases in water use. However, poor stream representation may reduce the effectiveness of the original model head simulations. Improvements to the model, made by incorporating the USGS’s MODLFOW-2005 Newton formulation (MODFLOW-NWT, Niswonger and others, 2011) and by more accurately representing stream characteristics, are presented in an example simulating approximate changes in water use. Groundwater heads in an area of relatively larger population change, near the center of the Seacoast’s fractured bedrock aquifer, were simulated with the upgraded model using published 2004, and approximated 2015, water use rates. This area is situated at a local topographic high point and near the junction of three towns, where drainages flow westward, toward Great Bay, and eastward, toward the Atlantic Ocean (Figure 1).

New Hampshire↗

USGS2014

Numerical simulation of groundwater flow in the Columbia Plateau Regional Aquifer System, Idaho, Oregon, and Washington

A three-dimensional numerical model of groundwater flow was constructed for the Columbia Plateau Regional Aquifer System (CPRAS), Idaho, Oregon, and Washington, to evaluate and test the conceptual model of the system and to evaluate groundwater availability. The model described in this report can be used as a tool by water-resource managers and other stakeholders to quantitatively evaluate proposed alternative management strategies and assess the long‑term availability of groundwater. The numerical simulation of groundwater flow in the CPRAS was completed with support from the Groundwater Resources Program of the U.S. Geological Survey Office of Groundwater. The model was constructed using the U.S. Geological Survey modular three-dimensional finite-difference groundwater-flow model, MODFLOW-NWT. The model uses 3-kilometer (9,842.5 feet) grid cells that subdivide the model domain by 126 rows and 131 columns. Vertically, the model domain was subdivided into six geologic model units. From youngest to oldest, the units are the Overburden, the Saddle Mountains Basalt, the Mabton Interbed, the Wanapum Basalt, the Vantage Interbed, and the Grande Ronde Basalt. Natural recharge was estimated using gridded historical estimates of annual precipitation for the period 1895–2007. Pre-development recharge was estimated to be the average natural recharge for this period. Irrigation recharge and irrigation pumping were estimated using a remote-sensing based soil-water balance model for the period 1985–2007. Pre-1985 irrigation recharge and pumping were estimated using previously published compilation maps and the history of large-scale irrigation projects. Pumping estimates for municipal, industrial, rural, residential, and all other uses were estimated using reported values and census data. Pumping was assumed to be negligible prior to 1920. Two models were constructed to simulate groundwater flow in the CPRAS: a steady-state predevelopment model representing conditions before large-scale pumping and irrigation altered the system, and a transient model representing the period 1900–2007. Automated parameter-estimation techniques (steady-state predevelopment model) and traditional trial-and-error (transient model) methods were used for calibration. To calibrate the steady-state and transient models, 10,525 and 46,460 water level measurements, respectively, and 50 base-flow estimates were used. The steady-state model simulated the shape, slope, and trends of a potentiometric surface that was generally consistent with mapped water levels. For the transient model, the mean and median difference between simulated and measured hydraulic heads is -10 and 4 ft, respectively, with a standard deviation of 164 ft over a 5,648 ft range of measured heads. The residuals for the simulation period show that 52 percent of the simulated heads exceeded measured heads with a median residual value of 43 ft, and 48 percent were less than measured heads with a median residual value of -76 ft. The CPRAS model was constructed to derive components of the groundwater budget and help understand the interactions of stresses, such as recharge, groundwater pumping, and commingling wells on the groundwater and surface-water system. Through these applications, the model can be used to identify trends in groundwater storage and use, and quantify groundwater availability. The annual groundwater budgets showed several patterns of change over the simulation period. Groundwater pumping was negligible until the 1950s and began to increase significantly during the 1970s and 1980s. Recharge was highly variable due to the interannual variability of precipitation, but began to increase in the late 1940s due to the increase in surface-water irrigation projects. Groundwater contributions to streamflow (base flow) followed recharge closely. However, in areas of significant groundwater-level decline, base flow is reduced. Groundwater pumping had the greatest effect on water levels, followed by irrigation enhanced recharge. Commingling was a larger factor in structurally complex upland areas where hydraulic-head gradients are naturally high. Groundwater pumping has increased substantially over the past 40–50 years; this increase resulted in declining water levels at depth and decreased base flows over much of the study area. The effects of pumping are mitigated somewhat by the increase of surface-water irrigation, especially in the shallow Overburden unit, and commingling wells in some areas. During dry to average years, groundwater pumping causes a net loss of groundwater in storage and current condition (2000–2007) groundwater pumping exceeds recharge in all but the wettest of years.

Idaho, Oregon, Washington↗

USGS2021

Oxygen-controlled recirculating seepage meter reveals extent of nitrogen transformation in discharging coastal groundwater at the aquifer–estuary interface

Nutrient loads delivered to estuaries via submarine groundwater discharge (SGD) play an important role in the nitrogen (N) budget and eutrophication status. However, accurate and reliable quantification of the chemical flux across the final decimeters and centimeters at the sediment–estuary interface remains a challenge, because there is significant potential for biogeochemical alteration due to contrasting conditions in the coastal aquifer and surface sediment. Here, a novel, oxygen- and light-regulated ultrasonic seepage meter, and a standard seepage meter, were used to measure SGD and calculate N species fluxes across the sediment–estuary interface. Coupling the measurements to an endmember approach based on subsurface N concentrations and an assumption of conservative transport enabled estimation of the extent of transformation occurring in discharging groundwater within the benthic zone. Biogeochemical transformation within reactive estuarine surface sediment was a dominant driver in modifying the N flux carried upward by SGD, and resulted in a similar percentage of N removal (~ 42–52%) as did transformations occurring deeper within the coastal aquifer salinity mixing zone (~ 42–47%). Seasonal shifts in the relative importance of biogeochemical processes including denitrification, nitrification, dissimilatory nitrate reduction, and assimilation altered the composition of the flux to estuarine surface water, which was dominated by ammonium in June and by nitrate in August, despite the endmember-based observation that fixed N in discharging groundwater was strongly dominated by nitrate. This may have important ramifications for the ecology and management of estuaries, since past N loading estimates have generally assumed conservative transport from the nearshore aquifer to estuary.

Limnology and Oceanography↗

USGS2013

Evaluation of the groundwater-flow model for the Ohio River alluvial aquifer near Carrollton, Kentucky, updated to conditions in September 2010

The Ohio River alluvial aquifer near Carrollton, Ky., is an important water resource for the cities of Carrollton and Ghent, as well as for several industries in the area. The groundwater of the aquifer is the primary source of drinking water in the region and a highly valued natural resource that attracts various water-dependent industries because of its quantity and quality. This report evaluates the performance of a numerical model of the groundwater-flow system in the Ohio River alluvial aquifer near Carrollton, Ky., published by the U.S. Geological Survey in 1999. The original model simulated conditions in November 1995 and was updated to simulate groundwater conditions estimated for September 2010. The files from the calibrated steady-state model of November 1995 conditions were imported into MODFLOW-2005 to update the model to conditions in September 2010. The model input files modified as part of this update were the well and recharge files. The design of the updated model and other input files are the same as the original model. The ability of the updated model to match hydrologic conditions for September 2010 was evaluated by comparing water levels measured in wells to those computed by the model. Water-level measurements were available for 48 wells in September 2010. Overall, the updated model underestimated the water levels at 36 of the 48 measured wells. The average difference between measured water levels and model-computed water levels was 3.4 feet and the maximum difference was 10.9 feet. The root-mean-square error of the simulation was 4.45 for all 48 measured water levels. The updated steady-state model could be improved by introducing more accurate and site-specific estimates of selected field parameters, refined model geometry, and additional numerical methods. Collection of field data to better estimate hydraulic parameters, together with continued review of available data and information from area well operators, could provide the model with revised estimates of conductance values for the riverbed and valley wall, hydraulic conductivities for the model layer, and target water levels for future simulations. Additional model layers, a redesigned model grid, and revised boundary conditions could provide a better framework for more accurate simulations. Additional numerical methods would identify possible parameter estimates and determine parameter sensitivities.

Kentucky↗

USGS2025

Hydrogeology and groundwater quality in the Snake River alluvial aquifer at Jackson Hole Airport, Wyoming, 2011–20

The Snake River alluvial aquifer underlying the Jackson Hole Airport (JHA) in northwest Wyoming is an important source of water used for domestic, commercial, and irrigation purposes by the airport and nearby residents. The U.S. Geological Survey, in response to previously identified water-quality concerns in the area, monitored and evaluated changes in hydrogeologic characteristics and groundwater-quality conditions of the alluvial aquifer during 2011–20. During that period, the Jackson Hole Airport made several changes that potentially improved water quality at and downgradient from the airport. Well, water level, and hydrogeologic data were collected from the alluvial aquifer to identify hydrogeologic characteristic and groundwater quality changes. Additionally, results of statistical tests were applied to water-quality results to evaluate trends in selected physical properties and constituent concentrations with time. The trends of those data show that water quality did improve overall during the study period compared to previously collected data. Presumably, these trends are in response to the changes in the aircraft deicing/anti-icing fluid (ADAF) formulation used by the JHA, the many JHA infrastructure improvements made during 2011–20, the degradation of existing ADAFs in subsurface soils and groundwater, or some combination of these possibilities.

Wyoming↗

USGS2007

Effect of an offshore sinkhole perforation in a coastal confined aquifer on submarine groundwater discharge

In order to explore submarine groundwater discharge in the vicinity of karst features that penetrate the confining layer of an offshore, partially confined aquifer, we constructed a three-dimensional groundwater model using the SUTRA (Saturated-Unsaturated TRAnsport) variable-density groundwater flow model. We ran a parameter sensitivity analysis, testing the effects of recharge rates, permeabilities of the aquifer and confining layer, and thickness of the confining layer. In all simulations, less than 20% of the freshwater recharge for the entire model exits through the sinkhole. Recirculated seawater usually accounts for 10-30% of the total outflow from the model. Often, the sinkhole lies seaward of the transition zone and acts as a recharge feature for recirculating seawater. The permeability ratio between aquifer and confining layer influences the configuration of the freshwater wedge the most; as confining layer permeability decreases, the wedge lengthens and the fraction of total discharge exiting through the sinkhole increases. Copyright ?? 2007 IAHS Press.

Conference Paper↗

USGS2026

Streamflow, base-flow, and precipitation trends and simulated effects of groundwater withdrawals from the North Fork Red River aquifer on base flows upgradient from Lake Altus, western Oklahoma, 1980–2022

The U.S. Geological Survey, in cooperation with the Bureau of Reclamation, used five scenarios created from a previously published numerical groundwater-flow model (1980–2013) and historical streamflow records (1980–2022) to investigate the relation between groundwater withdrawals from the North Fork Red River aquifer and inflows to Lake Altus from the North Fork Red River in western Oklahoma. The five scenarios were (1) a scaled-equal-proportionate-share (EPS) groundwater-withdrawal scenario, (2) a study-area-scaled-reported groundwater-withdrawal scenario, (3) a zonal-scaled-reported groundwater-withdrawal scenario, (4) a historical drought-threshold scenario, and (5) a base-flow and evapotranspiration depletion scenario. For the scaled-EPS groundwater-withdrawal scenario, EPS groundwater withdrawals were often much higher than reported groundwater withdrawals and greatly decreased base flows for most scale factors. For the study-area-scaled-reported groundwater-withdrawal scenario, base flows were reduced more but by smaller percentages during wet periods than during dry periods when scaling simulated reported groundwater withdrawals. For the zonal-scaled-reported groundwater-withdrawal scenario, scaling simulated reported groundwater withdrawals within selected zones with more groundwater withdrawals did not always affect base flows more than scaling reported groundwater withdrawals within zones with less groundwater withdrawals. For the historical drought-threshold scenario, curtailing groundwater withdrawals at the drought thresholds increased annual base flows to Lake Altus by about 1,169 to 3,665 acre-feet. For the base-flow and evapotranspiration depletion scenario, the distance between a groundwater well and a stream was a major factor affecting base flow to the North Fork Red River when increasing groundwater withdrawals; however, spatially variable hydrologic properties and saturated-zone evapotranspiration could also affect the relation between base flows and groundwater withdrawals.

Oklahoma↗

USGS1998

Use of 3H/3He Ages to evaluate and improve groundwater flow models in a complex buried-valley aquifer

Combined use of the tritium/helium 3 ( 3 H/ 3 He) dating technique and particle-tracking analysis can improve flow-model calibration. As shown at two sites in the Great Miami buried-valley aquifer in southwestern Ohio, the combined use of 3 H/ 3 He age dating and particle tracking led to a lower mean absolute error between measured heads and simulated heads than in the original calibrated models and/or between simulated travel times and 3 H/ 3 He ages. Apparent groundwater ages were obtained for water samples collected from 44 wells at two locations where previously constructed finite difference models of groundwater flow were available (Mound Plant and Wright-Patterson Air Force Base (WPAFB)). The two-layer Mound Plant model covers 11 km 2 within the buried-valley aquifer. The WPAFB model has three layers and covers 262 km 2 within the buried-valley aquifer and adjacent bedrock uplands. Sampled wells were chosen along flow paths determined from potentiometric maps or particle-tracking analyses. Water samples were collected at various depths within the aquifer. In the Mound Plant area, samples used for comparison of 3 H/ 3 He ages with simulated travel times were from wells completed in the uppermost model layer. Simulated travel times agreed well with 3 H/ 3 He ages. The mean absolute error (MAE) was 3.5 years. Agreement in ages at WPAFB decreased with increasing depth in the system. The MAEs were 1.63, 17.2, and 255 years for model layers 1, 2, and 3, respectively. Discrepancies between the simulated travel times and 3 H/ 3 He ages were assumed to be due to improper conceptualization or incorrect parameterization of the flow models. Selected conceptual and parameter modifications to the models resulted in improved agreement between 3 H/ 3 He ages and simulated travel times and between measured and simulated heads and flows.

Ohio↗

USGS1998

Size-selective predation on groundwater bacteria by nanoflagellates in an organic-contaminated aquifer

Time series incubations were conducted to provide estimates for the size selectivities and rates of protistan grazing that may be occurring in a sandy, contaminated aquifer. The experiments involved four size classes of fluorescently labeled groundwater bacteria (FLB) and 2- to 3-μm-long nanoflagellates, primarily Spumella guttula (Ehrenberg) Kent, that were isolated from contaminated aquifer sediments (Cape Cod, Mass.). The greatest uptake and clearance rates (0.77 bacteria · flagellate −1 · h −1 and 1.4 nl · flagellate −1 · h −1 , respectively) were observed for 0.8- to 1.5-μm-long FLB (0.21-μm 3 average cell volume), which represent the fastest growing bacteria within the pore fluids of the contaminated aquifer sediments. The 19:1 to 67:1 volume ratios of nanoflagellate predators to preferred bacterial prey were in the lower end of the range commonly reported for other aquatic habitats. The grazing data suggest that the aquifer nanoflagellates can consume as much as 12 to 74% of the unattached bacterial community in 1 day and are likely to have a substantive effect upon bacterial degradation of organic groundwater contaminants.

Massachusetts↗

USGS2009

Hydrogeology and Groundwater Resources of the Coastal Aquifers of Southeastern Massachusetts

The glacially derived aquifer systems of southeastern Massachusetts compose the largest groundwater reservoir in the State. Population increases, land-use changes, and climate change in this area could lead to three primary environmental effects that relate directly to groundwater resources - (1) increases in pumping that could adversely affect environmentally sensitive groundwater-fed surface waters, such as ponds, streams, and wetlands; (2) changes in land use that could affect the quality of water in the aquifer; and (3) changes in precipitation and mean sea level that can affect water levels, streamflow, and the position of the freshwater/saltwater boundary. Therefore, understanding groundwater flow and the factors that can affect it is critical to managing and protecting this vital resource.

Circular↗

USGS2018

Extraction and development of inset models in support of groundwater age calculations for glacial aquifers

The U.S. Geological Survey developed a regional model of Lake Michigan Basin (LMB). This report describes the construction of five MODFLOW inset models extracted from the LMB regional model and their application using the particle-tracking code MODPATH to simulate the groundwater age distribution of discharge to wells pumping from glacial deposits. The five study areas of the inset model correspond to 8-digit hydrologic unit code (HUC8) basins. Two of the basins are tributary to Lake Michigan from the east, two are tributary to the lake from the west, and one is just west of the western boundary of the Lake Michigan topographic basin. The inset models inherited many of the inputs to the parent LMB model, including the hydrostratigraphy and layering scheme, the hydraulic conductivity assigned to bedrock layers, recharge distribution, and water use in the form of pumping rates from glacial and bedrock wells. The construction of the inset models entailed modifying some inputs, most notably the grid spacing (reduced from cells 5,000 feet on a side in the parent LMB model to 500 feet on a side in the inset models). The refined grid spacing allowed for more precise location of pumped wells and more detailed simulation of groundwater/surface-water interactions. The glacial hydraulic conductivity values, the top bedrock surface elevation, and the surface-water network input to the inset models also were modified. The inset models are solved using the MODFLOW–NWT code, which allows for more robust handling of conditions in unconfined aquifers than previous versions of MODFLOW. Comparison of the MODFLOW inset models reveals that they incorporate a range of hydrogeologic conditions relative to the glacial part of the flow system, demonstrated by visualization and analysis of model inputs and outputs and reflected in the range of ages generated by MODPATH for existing and hypothetical glacial wells. Certain inputs and outputs are judged to be candidate predictors that, if treated statistically, may be capable of explaining much of the variance in the simulated age metrics. One example of a predictor that model results indicate strongly affects simulated age is the depth of the well open interval below the simulated water table. The strength of this example variable as an overall predictor of groundwater age and its relation to other predictors can be statistically tested through the metamodeling process. In this way the inset models are designed to serve as a training area for metamodels that estimate groundwater age in glacial wells, which in turn will contribute to ongoing studies, under the direction of the U.S. Geological Survey National Water Quality Assessment, of contaminant susceptibility of shallow groundwater across the glacial aquifer system.

Illinois, Indiana, Michigan, Wisconsin↗

USGS2011

Groundwater modeling to evaluate interaquifer leakage in the Floridan aquifer system near Hunter Army Airfield and Fort Stewart

Simulations using a modified regional groundwater- flow model were used to determine the amount of leakage from the Upper Floridan aquifer (UFA) through the Lower Floridan confining unit (LFC) into the Lower Floridan aquifer (LFA) resulting from pumping about 1 million gallons per day at newly constructed LFA production wells at Hunter Army Airfield and Fort Stewart in coastal Georgia. Simulated steadystate drawdown at each of the LFA production wells closely matched observed drawdown during a 72-hour aquifer test with the observed water levels reaching steady-state by the end of the test period. However, simulated drawdown was greater than observed drawdown in the UFA because of the short duration of the aquifer test and the time required for groundwater movement through the LFC into the LFA. Steadystate simulations provide an estimate of leakage based on the long-term continuous operation of each production well. Results of model simulations indicate that interaquifer leakage accounts for 48 percent of the flow to the well at Hunter Army Airfield, and 98 percent of the flow to the well at Fort Stewart. Simulated results near the Hunter Army Airfield production well indicated that 65 percent of the leakage from the UFA to the LFA occurs within a 1-mile radius, whereas simulated results near the Fort Stewart production well indicated 80-percent leakage from the UFA to the LFA within the same radius. The greater amount of leakage to the production well near Fort Stewart can be attributed to the higher transmissivity of the UFA and higher vertical hydraulic conductivity in the LFC near the well.

Georgia↗

USGS2024

Simulation of groundwater-flow dynamics in the U.S. Northern High Plains driven by multi-model estimates of surficial aquifer recharge

There is growing interest in incorporating higher-resolution groundwater modeling within the framework of large-scale land surface models (LSMs), including processes such as three-dimensional flow, variable soil saturation, and surface water/groundwater interactions. Conversely, complex groundwater models (e.g., the U.S. Geological Survey Groundwater-Flow Model, MODFLOW) often use simpler representations of land surface dynamics (e.g., surface vegetation, evapotranspiration, recharge) and may benefit from higher process fidelity and temporal resolutions in these inputs. This study investigates the potential of improving groundwater representation in LSMs and land surface dynamics in MODFLOW through forcing MODFLOW with recharge from LSMs. Groundwater simulations build on an existing and well-calibrated MODFLOW model of the U.S. Northern High Plains aquifer, a hydrologically complex basin under the dual impacts of conversion of native vegetation to intense irrigated agricultural fields and climate change. Simulated groundwater recharge from four different land models are used to drive MODFLOW groundwater simulations. Results show relatively large discrepancies between recharge estimates among simulations. Forcing MODFLOW using recharge simulated by some of the LSMs in place of a simple water balance model marginally improves MODFLOW groundwater simulation. Further, our results support the efficacy of coupling LSMs to a sophisticated groundwater model such as MODFLOW. The coupling results in notable improvements in matching the historical groundwater levels through reduction of the skewness coefficient in percent bias histogram (from 1.50 and 1.41 in original LSMs to 0.44 and 0.27, respectively, when MODFLOW is forced by groundwater recharge from LSMs) and reduction of bias. This modeling effort seeks to identify the best compromise between comprehensive land surface processes from global LSMs and advanced representation of groundwater from regional models.

Journal of Hydrology↗

USGS2010

Status of groundwater levels and storage volume in the Equus Beds aquifer near Wichita, Kansas, January 2006 to January 2010

A part of the Equus Beds aquifer in southwestern Harvey County and northwestern Sedgwick County was developed to supply water to residents of Wichita and for irrigation in south-central Kansas. Groundwater pumping for city and agricultural use caused water levels to decline in a large part of the aquifer northwest of Wichita. In 1965, the city of Wichita began using water from Cheney Reservoir in addition to water from the Equus Beds aquifer to meet the city's increasing demand for water. Irrigation pumpage in the area increased substantially during the 1970s and 1980s and contributed to the water-level declines. Water-level declines reached their maximum to date in October 1992. Proposals to artificially recharge the aquifer have been made since the 1950s to meet future water-supply needs and to protect the aquifer from the intrusion of saltwater from sources to the south and west. In 2007, Wichita implemented Phase 1 of the Equus Beds Aquifer Storage and Recovery project for large-scale artificial recharge of the aquifer. A period of water-level rises associated with greater-than-average precipitation and decreased city pumpage from the area began in 1993 and continued through January 2010. During January 2010, the direction of ground-water flow in the Equus Beds aquifer in the area was generally from west to east, similar to the direction prior to development of the aquifer. Water-level changes since 1940 for the period January 2006 to January 2010 ranged from a decline of more than 30 feet to a rise of more than 4 feet. Almost all wells in the area had cumulative water-level rises from October 1992 (period of maximum storage loss) to January 2010 and from January 2007 (beginning of large-scale artificial recharge) to January 2010. The average cumulative water-level change from October 1992 to January 2010 was a rise of about 8.7 feet and from January 2007 to January 2010 was a rise of about 3.2 feet. The storage-volume change in the study area for the period October 1992 to January 2010 represented a recovery of about 183,000 acre-feet, or about 65 percent of the storage volume previously lost from August 1940 to October 1992, and was the largest recovery since October 1992 to date. Decreased city pumpage and artificial recharge during 1993-2009 and 2007-09 contributed to the recovery of storage volume in both periods, but artificial recharge's contribution was much smaller. Irrigation pumpage, because it increased during 1993-2009, did not contribute to the recovery of storage volume from October 1992 to January 2010. Recharge from excess precipitation contributed to the recovery of storage volume in both periods because precipitation averaged about 2 and 6 inches per year more than the annual long-term average of 31.52 inches during 1993-2009 and 2007-09, respectively. Sustainable yield for the Equus Beds aquifer in the study area was estimated to be about 57,000 acre-feet per year using two different methods. The sum of permitted annual irrigation (about 45,600 acre-feet) and city (about 31,400 acre-feet) pumpage of 77,000 acre-feet per year greatly exceeds the estimated sustainable yield. Effective water management, including additions to the water budget such as those from the Equus Beds Aquifer Storage and Recovery project, can help produce the most water for beneficial use in a more sustainable way.

Scientific Investigations Report↗

USGS2014

Steady-state numerical groundwater flow model of the Great Basin carbonate and alluvial aquifer system

This report describes the construction, calibration, evaluation, and results of a steady-state numerical groundwater flow model of the Great Basin carbonate and alluvial aquifer system that was developed as part of the U.S. Geological Survey National Water Census Initiative to evaluate the nation’s groundwater availability. The study area spans 110,000 square miles across five states. The numerical model uses MODFLOW-2005, and incorporates and tests complex hydrogeologic and hydrologic elements of a conceptual understanding of an interconnected groundwater system throughout the region, including mountains, basins, consolidated rocks, and basin fill. The level of discretization in this model has not been previously available throughout the study area. Observations used to calibrate the model are those of water levels and discharge to evapotranspiration, springs, rivers, and lakes. Composite scaled sensitivities indicate the simulated values of discharge to springs, rivers, and lakes provide as much information about model parameters as do simulated water-level values. The model has 176 parameters and little parameter correlation. The simulated equivalents to observations provide enough information to constrain most parameters to smaller ranges than the conceptual constraints, and most parameter values are within reasonable ranges. Model fit to observations, comparison of simulated to conceptual water-level contours, and comparison of simulated to conceptual water budgets indicate this model provides a reasonable representation of the regional groundwater system. Eighty-six percent of the simulated values of water levels in wells are within 119 feet (one standard deviation of the error) of the observed values. Ninety percent of the simulated discharges are within 30 percent of the observed values. Total simulated recharge in the study area is within 10 percent of the conceptual amount; total simulated discharge is the same as conceptual discharge. Comparison of simulated hydraulic heads with the conceptual potentiometric surface indicates that the model accurately depicts major features of the hydraulic-head distribution. The incorporation of new recharge estimates and of mountain springs and streams as model observations creates higher simulated recharge mounds under many mountain ranges and highlights that in many cases, the regional flow paths go around, not through (or under) mountain ranges. Results from the model show that much of the flow in the groundwater system occurs in deeper layers, even though about 86 percent of the discharge occurs in layer 1. Over 95 percent of the recharge moves down from layer 1, and about 25 percent moves down to layer 8. The model was used to delineate six simulated groundwater flow regions that connect recharge areas to discharge areas. The eastern Great Salt Lake and Great Salt Lake Desert model regions contain 75 percent of the groundwater budget, but only 42 percent of the study area. In contrast, the more southern Death Valley and Colorado model regions contain only 12 percent of the groundwater budget, but 37 percent of the study area. Examples of potential use of the model to investigate the groundwater system include (1) the effects of different recharge, (2) different interpretations of the extent or offset of long faults or fault zones, and (3) different conceptual models of the spatial variation of hydraulic properties. The model can also be used to examine the ultimate effects of groundwater withdrawals on a regional scale, to provide boundary conditions for local-scale models, and to guide data collection.

Great Basin↗

USGS2012

Groundwater, surface-water, and water-chemistry data from C-aquifer monitoring program, northeastern Arizona, 2005-11

The C aquifer is a regionally extensive multiple-aquifer system supplying water for municipal, agricultural, and industrial use in northeastern Arizona, northwestern New Mexico, and southeastern Utah. An increase in groundwater withdrawals from the C aquifer coupled with ongoing drought conditions in the study area increase the potential for drawdown within the aquifer. A decrease in the water table and potentiometric surface of C aquifer is illustrated locally by the drying up of Obed Meadows, a natural peat deposit, and Hugo Meadows, a natural wetland, both south of Joseph City, Arizona. Continual increase in water use from the C aquifer, including a planned increase in pumpage by the City of Flagstaff, is justification for continued monitoring of the C-aquifer system in order to quantify physical and chemical responses to pumping stresses. Fifteen of the 35 C-aquifer wells analyzed had water-level data sufficient for percentage difference calculation for 2005–11. Change in water level as a percentage of the initial water-level measurement for these 15 wells ranged from about -0.2 to about -0.5 percent. For historical water-level data, changes in water levels were greatest around pumping centers, as indicated by a -97.0 feet (percentage difference of -16.5 percent) change over the period of record (1962–2005) for the Lake Mary 1 Well near Flagstaff, Arizona. In more rural areas of the C aquifer, water levels showed less change for both the temporal focus of this report (2005–11) and for historical values. Continuous records of surface-water discharge from 2005 to 2007 for three discontinued streamflow-gaging stations (Clear Creek near Winslow, AZ, 09399000; Clear Creek below McHood Lake near Winslow, AZ, 09399100; and Chevelon Creek near Winslow, AZ, 09398000) were tabulated. For the period of record, Clear Creek near Winslow, AZ, and Chevelon Creek near Winslow, AZ, showed seasonal discharge distributions indicative of natural streams in the southwestern United States. Clear Creek below McHood Lake near Winslow, AZ, showed discharge distribution indicative of perennial spring flow with little variation annually. Physical and chemical data collected during four baseflow investigations (summer 2005, summer 2006, summer 2008, and winter 2010) conducted on Clear Creek, Chevelon Creek, and a portion of the Little Colorado River were compiled and analyzed. Data from 7 sampling sites established on the Little Colorado River, 11 sites along Chevelon Creek, and 14 sites along Clear Creek were included. For the four baseflow investigations presented, a 2,000–3,000 microsiemens per centimeter increase in specific conductance was measured in Chevelon Creek from near its headwaters to the confluence with the Little Colorado River because of the contribution of highly conductive spring discharge. Clear Creek showed a less consistent pattern of increase in specific conductance with distance, but still exhibited changes on the order of 5,000 microsiemens per centimeter over just a few river miles. Water-chemistry data for selected wells and baseflow investigations sites are presented. No well samples analyzed exceeded the U.S. Environmental Protection Agency Maximum Contaminant Level standards for drinking water, but several samples exceeded Secondary Maximum Contaminant Level standards for chloride, fluoride, sulfate, iron, and total dissolved solids.

Arizona↗

USGS2014

A reconnaissance spatial and temporal assessment of methane and inorganic constituents in groundwater in bedrock aquifers, Pike County, Pennsylvania, 2012-13

Pike County in northeastern Pennsylvania is underlain by the Devonian-age Marcellus Shale and other shales, formations that have potential for natural gas development. During 2012–13, the U.S. Geological Survey in cooperation with the Pike County Conservation District conducted a reconnaissance study to assess baseline shallow groundwater quality in bedrock aquifers prior to possible shale-gas development in the county. For the spatial component of the assessment, 20 wells were sampled in summer 2012 to provide data on the occurrence 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. For the temporal component of the assessment, 4 of the 20 wells sampled in summer 2012 were sampled monthly from July 2012 through June 2013 to provide data on seasonal variability in groundwater quality. All water samples were analyzed for major ions, nutrients, selected inorganic trace constituents (including metals and other elements), stable isotopes of water, radon-222, gross alpha- and gross beta-particle activity, dissolved gases (methane, ethane, and ethene), and, if possible, isotopic composition of methane. Additional analyses for boron and strontium isotopes, age-dating of water, and radium-226 were done on water samples collected from six wells in June 2013. Results of the summer 2012 sampling show that water from 16 (80 percent) of 20 wells had detectable concentrations of methane, but concentrations were less than 0.1 milligram per liter (mg/L) in most well-water samples; only two well-water samples had concentrations greater than 1 mg/L. The groundwater with elevated methane also had a chemical composition that differed in some respects (pH, selected major ions, and inorganic trace constituents) from groundwater with low methane concentrations. The two well-water samples with the highest methane concentrations (about 3.7 and 5.8 mg/L) also had the highest pH values (8.7 and 8.3, respectively) and the highest concentrations of sodium, lithium, boron, fluoride, and bromide. Elevated concentrations of some other constituents, such as barium, strontium, and chloride, were not limited to well-water samples with elevated methane, although the two samples with elevated methane also had among the highest concentrations of these constituents. One sample with elevated methane concentrations also had elevated arsenic concentrations, with the arsenic concentration of 30 micrograms per liter (μg/L) exceeding the drinking-water standard of 10 µg/L for arsenic. No other sample from the 20 wells sampled in summer 2012 had concentrations of constituents that exceeded any established primary drinking-water standards. However, radon-222 activities ranging up to 4,500 picocuries per liter (pCi/L) exceeded the proposed drinking-water standard of 300 pCi/L in 85 percent of the 20 well-water samples. The isotopic composition methane in the two high-methane samples (δC CH4 values of -64.55 and -64.41 per mil and δD CH4 values of -216.9 and -201.8 per mil, respectively) indicates a predominantly microbial source for the methane formed by a carbon dioxide reduction process. The stable isotopic composition of water (δD H20 and δ 18 O H20 ) in samples from all 20 wells falls on the local meteoric line, indicating water in the wells was of relatively recent meteoric origin (modern precipitation), including samples with elevated methane concentrations. Analytical results for 4 of the 20 wells sampled monthly for 1 year ending June 2013 in order to assess temporal variability in groundwater quality show that concentrations of major ions generally varied by less than 20 percent, with most differences less than 4 mg/L. Concentrations of methane varied by less than 1 μg/L (0.001 mg/L) in samples from three wells with low methane and by as much as 1 mg/L (1,000 μg/L) in samples from one well with relatively high methane. The isotopic composition of methane in the one well with relatively high methane varied slightly in the monthly samples, ranging from about -64.5 to -64.8 per mil for δ 13 C CH4 and from about -217 to -228 per mil for δD CH4 . The δ 13 C values for dissolved inorganic carbon (DIC) in water from this well were consistent with microbial methane formation by carbon dioxide reduction (drift-type methane) and varied little in the temporal samples, ranging from -10.5 to -10.1 per mil. Additional analyses of samples collected in late June 2013 from six wells with a range of methane and trace constituent concentrations provided baseline data on strontium and boron isotopic compositions ( 87 Sr/ 86 Sr ratios and δ 11 B, respectively) that potentially may be used to differentiate among sources of these constituents. The strontium and boron isotopic composition determined in the six shallow Pike County groundwater samples had 87 Sr/ 86 Sr ratios of 0.71426 to 0.71531 and δ 11 B values of 11.7 to 27.0 per mil, which differ from those reported for brines in Devonian-age formations in Pennsylvania. The June 2013 samples were also analyzed for radium-226 and age-dating dissolved gases. Activities of radium-226 ranged from 0.041 to 0.29 pCi/L in water samples from the six wells and were less than the drinking-water standard of 5 pCi/L for combined radium-226 and radium-228. Age-dating of groundwater using a method based on the presence of anthropogenic gases (chlorofluorocarbons and sulfur hexafluoride) released into the atmosphere yielded estimated recharge dates for water from these six wells that ranged from the 1940s to early 2000s. The oldest water was in samples from wells that had the highest methane concentrations and the youngest water was in samples from a continuously pumped 300-foot deep production well.

Pennsylvania↗

USGS2023

Groundwater residence times in glacial aquifers—A new general simulation-model approach compared to conventional inset models

Groundwater is important as a drinking-water source and for maintaining base flow in rivers, streams, and lakes. Groundwater quality can be predicted, in part, by its residence time in the subsurface, but the residence-time distribution cannot be measured directly and must be inferred from models. This report compares residence-time distributions from four areas where groundwater flow and travel time were simulated with conventional simulation-inset models (IMs) and with a new automated model-construction method called general simulation models (GSMs). The comparison provides an opportunity to explore controls on travel time and improve the methods used in the creation of GSMs. These models can be useful for three main-use cases: (1) rapid testing of relationships that govern groundwater flow and age, (2) generation of consistent examples for training a machine-learning metamodel, and (3) serving as a starting point for more detailed models. Comparison of the GSMs to IMs indicated a qualified pattern of agreement for residence-time distributions as indicated by the Nash-Sutcliffe efficiency and Spearman’s correlation coefficient. The agreement was best for the median values of the simulated residence times in young fractions of groundwater (defined as the fractions of groundwater in samples less than 65 years old) at the scale of the eight-digit hydrologic-unit code. Generally, the median values of the young fractions in the IMs were correlated with the median values from the GSMs. The relative trends across the four areas also were similar for the other residence-time metrics. The medians of residence-time metrics at finer scales show a fair degree of scatter. The GSM results compared most poorly for median travel times in the older fraction of groundwater (older than 65 years). The GSM approach is intended as a flexible framework for developing models that can be useful individually as screening tools or collectively to support projects in statistical learning. Although one set of GSM algorithms was presented here, the approach can accommodate many types of data and also different categories of prior information. Comparison of GSMs and IMs suggests ways in which the GSMs, while remaining easy to construct and calibrate, can be improved for estimating groundwater travel times. IMs do not yield exact travel times, and matching GSMs to IMs does not guarantee an improvement; however, IMs provide a convenient benchmark against which to explore relations between physical characteristics of watersheds and the distribution of travel times within them. This effort was undertaken as part of the National Water Quality Program of the U.S. Geological Survey to assist in determining the susceptibility of groundwater in glacial aquifers to a variety of natural and anthropogenic contaminants.

Illinois, Indiana, Michigan, Wisconsin↗