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USGS2014

A simple headspace equilibration method for measuring dissolved methane

Dissolved methane concentrations in the ocean are close to equilibrium with the atmosphere. Because methane is only sparingly soluble in seawater, measuring it without contamination is challenging for samples collected and processed in the presence of air. Several methods for analyzing dissolved methane are described in the literature, yet none has conducted a thorough assessment of the method yield, contamination issues during collection, transport and storage, and the effect of temperature changes and preservative. Previous extraction methods transfer methane from water to gas by either a "sparge and trap" or a "headspace equilibration" technique. The gas is then analyzed for methane by gas chromatography. Here, we revisit the headspace equilibration technique and describe a simple, inexpensive, and reliable method to measure methane in fresh and seawater, regardless of concentration. Within the range of concentrations typically found in surface seawaters (2-1000 nmol L -1 ), the yield of the method nears 100% of what is expected from solubility calculation following the addition of known amount of methane. In addition to being sensitive (detection limit of 0.1 ppmv, or 0.74 nmol L -1 ), this method requires less than 10 min per sample, and does not use highly toxic chemicals. It can be conducted with minimum materials and does not require the use of a gas chromatograph at the collection site. It can therefore be used in various remote working environments and conditions.

Limnology and Oceanography: Methods↗

USGS2025

Evaluating the potential to quantify salmon habitat via UAS-based particle image velocimetry

Continuous, high-resolution data for characterizing freshwater habitat conditions can support successful management of endangered salmonids. Uncrewed aircraft systems (UAS) make acquiring such fine-scale data along river channels more feasible, but workflows for quantifying reach-scale salmon habitats are lacking. We evaluated the potential for UAS-based mapping of hydraulic habitats using spectrally based depth retrieval and particle image velocimetry (PIV) by comparing these methods to a more well-established flow modeling approach. Our results indicated that estimates of water depth, depth-averaged velocity, and flow direction derived via remote sensing and modeling techniques were comparable and in good agreement with field measurements. Predictions of spring-run Chinook salmon ( Oncorhynchus tshawytscha ) juvenile rearing habitat produced from PIV and model output were similar, with small errors relative to direct field observations. Estimates of hydraulic heterogeneity based on kinetic energy gradients in the flow field were generally consistent between PIV and flow modeling, but errors relative to field measurements were larger. PIV results were sensitive to the velocity index ( α ) used to convert surface velocities to depth-averaged velocities. Sun glint precluded PIV analysis along the margins of some images and a large degree of overlap between frames was thus required to obtain continuous coverage of the reach. Similarly, shadows cast by riparian vegetation caused gaps in spectrally based bathymetric maps. Despite these limitations, our results suggest that for sites with sufficient water surface texture, UAS-based PIV can provide detailed hydraulic habitat information at the reach scale, with accuracies comparable to traditional field methods and multidimensional flow modeling.

Oregon↗

USGS2002

Rangeland health attributes and indicators for qualitative assessment

Panels of experts from the Society for Range Management and the National Research Council proposed that status of rangeland ecosystems could be ascertained by evaluating an ecological site's potential to conserve soil resources and by a series of indicators for ecosystem processes and site stability. Using these recommendations as a starting point, we developed a rapid, qualitative method for assessing a moment-in-time status of rangelands. Evaluators rate 17 indicators to assess 3 ecosystem attributes (soil and site stability, hydrologic function, and biotic integrity) for a given location. Indicators include rills, water flow patterns, pedestals and terracettes, bare ground, gullies, wind scour and depositional areas, litter movement, soil resistance to erosion, soil surface loss or degradation, plant composition relative to infiltration, soil compaction, plant functional/structural groups, plant mortality, litter amount, annual production, invasive plants, and reproductive capability. In this paper, we detail the development and evolution of the technique and introduce a modified ecological reference worksheet that documents the expected presence and amount of each indicator on the ecological site. In addition, we review the intended applications for this technique and clarify the differences between assessment and monitoring that lead us to recommend this technique be used for moment-in-time assessments and not be used for temporal monitoring of rangeland status. Lastly, we propose a mechanism for adapting and modifying this technique to reflect improvements in understanding of ecosystem processes. We support the need for quantitative measures for monitoring rangeland health and propose some measures that we believe may address some of the 17 indicators.

Journal of Range Management↗

USGS2010

Use of the azimuthal resistivity technique for determination of regional azimuth of transmissivity

Many bedrock units contain joint sets that commonly act as preferred paths for the movement of water, electrical charge, and possible contaminants associated with production or transit of crude oil or refined products. To facilitate the development of remediation programs, a need exists to reliably determine regional-scale properties of these joint sets: azimuth of transmissivity ellipse, dominant set, and trend(s). The surface azimuthal electrical resistivity survey method used for local in situ studies can be a noninvasive, reliable, efficient, and relatively cost-effective method for regional studies. The azimuthal resistivity survey method combines the use of standard resistivity equipment with a Wenner array rotated about a fixed center point, at selected degree intervals, which yields an apparent resistivity ellipse from which joint-set orientation can be determined. Regional application of the azimuthal survey method was tested at 17 sites in an approximately 500 km2 (193 mi2) area around Milwaukee, Wisconsin, with less than 15m (50 ft) overburden above the dolomite. Results of 26 azimuthal surveys were compared and determined to be consistent with the results of two other methods: direct observation of joint-set orientation and transmissivity ellipses from multiple-well-aquifer tests. The average of joint-set trend determined by azimuthal surveys is within 2.5?? of the average of joint-set trend determined by direct observation of major joint sets at 24 sites. The average of maximum of transmissivity trend determined by azimuthal surveys is within 5.7?? of the average of maximum of transmissivity trend determined for 14 multiple-well-aquifer tests. Copyright ?? 2010 The American Association of Petroleum Geologists/Division of Environmental Geosciences. All rights reserved.

Environmental Geosciences↗

USGS2006

Puhimau thermal area: a window into the upper east rift zone of Kilauea Volcano, Hawaii?

We report the results of two soil CO 2 efflux surveys by the closed chamber circulation method at the Puhimau thermal area in the upper East Rift Zone (ERZ) of Kilauea volcano, Hawaii. The surveys were undertaken in 1996 and 1998 to constrain how much CO 2 might be reaching the ERZ after degassing beneath the summit caldera and whether the Puhimau thermal area might be a significant contributor to the overall CO 2 budget of Kilauea. The area was revisited in 2001 to determine the effects of surface disturbance on efflux values by the collar emplacement technique utilized in the earlier surveys. Utilizing a cutoff value of 50 g m −2 d −1 for the surrounding forest background efflux, the CO 2 emission rates for the anomaly at Puhimau thermal area were 27 t d −1 in 1996 and 17 t d −1 in 1998. Water vapor was removed before analysis in all cases in order to obtain CO 2 values on a dry air basis and mitigate the effect of water vapor dilution on the measurements. It is clear that Puhimau thermal area is not a significant contributor to Kilauea's CO 2 output and that most of Kilauea's CO 2 (8500 t d −1 ) is degassed at the summit, leaving only magma with its remaining stored volatiles, such as SO 2 , for injection down the ERZ. Because of the low CO 2 emission rate and the presence of a shallow water table in the upper ERZ that effectively scrubs SO 2 and other acid gases, Puhimau thermal area currently does not appear to be generally well suited for observing temporal changes in degassing at Kilauea.

Hawaii↗

USGS1992

Simulation of soluble waste transport and buildup in surface waters using tracers

Soluble tracers can be used to simulate the transport and dispersion of soluble wastes that might have been introduced or are planned for introduction into surface waters. Measured tracer-response curves produced from the injection of a known quantity of soluble tracer can be used in conjunction with the superposition principle to simulate potential waste buildup in streams, lakes, and estuaries. Such information is particularly valuable to environmental and water-resource planners in determining the effects of proposed waste discharges. The theory, techniques, analysis, and presentation of results of tracer-waste simulation tests in rivers, lakes, and estuaries are described. This manual builds on other manuals on dye tracing with emphasis on the expanded use of time-of-travel type data.

Open-File Report↗

USGS1993

Simulation of soluble waste transport and buildup in surface waters using tracers

Soluble tracers can be used to simulate the transport and dispersion of soluble wastes that might have been introduced or are planned for introduction into surface waters. Measured tracer-response curves produced from the injection of a known quantity of soluble tracer can be used in conjunction with the superposition principle to simulate potential waste buildup in streams, lakes, and estuaries. Such information is particularly valuable to environmental and water-resource planners in determining the effects of proposed waste discharges. The theory, techniques, analysis, and presentation of results of tracer-waste simulation tests in rivers, lakes, and estuaries are described. This manual builds on other manuals dealing with dye tracing by emphasizing the expanded use of data from time-of-travel studies.

Techniques of Water-Resources Investigations↗

USGS1986

Percentage change in saturated thickness of the High Plains Aquifer, west-central Kansas, 1950 to average 1984-86

Continuing studies are being made in west-central Kansas to provide up-to-date information that will aid in the management of groundwater for irrigation. The results are presented of the sixth in a series of studies that used a statistical technique called kriging, to produce hydrologic maps. The kriging technique interpolates water level altitudes at the center of each 1 sq mi section in the study area on the basis of water level measurements from 169 observation wells. For this study, measurements made at each site during the winter months of 1984, 1985, and 1986 were averaged. These interpolated altitudes (1 ,859 in all), along with bedrock-surface and base yr water level altitudes, were used to prepare a hydrologic map that illustrates percentage change in saturated thickness. Saturated thickness, as used in this report, is the thickness of the High Plains aquifer between the groundwater surface, as indicated by water level altitudes, and the bedrock surface. Because irrigation development in west-central Kansas was minimal prior to 1950, the saturated thickness during 1950 represented a nearly static condition in the aquifer. Thus, the decrease in saturated thickness of the aquifer since 1950 is related to the effects of irrigation withdrawals on the volume of water in storage.

Kansas↗

USGS1999

Geology and ground-water resources of the Lawrenceville area, Georgia

The population of the Atlanta Metropolitan area continues to grow at a rapid pace and the demand for water supplies steadily increases. Exploration for ground-water resources, as a supplement to surface-water supplies, is being undertaken by many city and county governments. The application of effective investigative methods to characterization of the complex igneous and metamorphic fractured bedrock aquifers of the Piedmont physiographic province is essential to the success of these ground-water exploration programs. The U.S. Geological Survey, in cooperation with the City of Lawrenceville, Ga., began a study in December 1994 to apply various investigative techniques for field characterization of fractured crystalline-bedrock aquifers near Lawrenceville. Five major lithologic units were mapped in the Lawrenceville, Ga., area as part of an ongoing study of ground-water resources-amphibolite, biotite gneiss, button schist, granite gneiss, and quartzite/aluminous schist. These units generally are thin in outcrop width, have low angles of dip (nearly 0 to 20 degrees, dip reversals occur over short distances), and exhibit some shearing characteristics. The most productive unit for ground-water resources, on the basis of subsurface data collected through 1997, is the amphibolite. Historically, two wells drilled into this unit are recognized as having possibly the highest yields in the Piedmont region of northern Georgia. The City of Lawrenceville refurbished one well at the Rhodes Jordan Wellfield in 1990, and has pumped this well at an average rate of about 230 gallons per minute since 1995. In general, the composition of water collected from the bedrock wells, regolith wells, and City Lake is similar; calcium and bicarbonate are the dominant cation and anion, respectively. Water from the regolith wells and the lake have lower concentrations of major ions than does water from the bedrock wells. Many of the ground-water samples collected from the Rhodes Jordan Wellfield during October-November 1995, and from the wellfield and three additional observation well sites during August 1996, contain volatile organic compounds. Volatile organic compounds were detected in ground-water samples collected from several bedrock and regolith wells located in urban areas. Trace concentrations of tetrachloroethylene, trichloroethylene, 1,1-dichloroethane, trichlorofluoromethane, 1,1,1-trichloroethane, and cis-1,2-dichloroethene were detected. Methyl-tert-butyl ether (MTBE)-a compound used to increase the octane level in gasoline-was detected at concentrations above expected urban background levels in bedrock wells in the Rhodes Jordan Wellfield. Concentrations of MTBE ranged from 0.6 to 12 micrograms per liter in October-November 1995, and from 0.6 to 26 micrograms per liter in August 1996. Continuous ground-water-level data suggest that the fractured crystalline-bedrock aquifer (amphibolite unit) at the Rhodes Jordan Wellfield, generally is dewatered to a depth near a productive fracture during the regular pumping cycle of 18 hours per day, 5 days on and 2 days off per week. However, when the stress on the aquifer is increased by extending the pumping period up to as much as 18 days, or by pumping longer that 18 hours per day, the aquifer exhibits an unusual condition of recovery. Areal effects of pumping have been observed at distances of as much as one mile, extending across surface-water drainage divides.

Georgia↗

USGS2008

Guide to the Revised Ground-Water Flow and Heat Transport Simulator: HYDROTHERM - Version 3

The HYDROTHERM computer program simulates multi-phase ground-water flow and associated thermal energy transport in three dimensions. It can handle high fluid pressures, up to 1 ? 109 pascals (104 atmospheres), and high temperatures, up to 1,200 degrees Celsius. This report documents the release of Version 3, which includes various additions, modifications, and corrections that have been made to the original simulator. Primary changes to the simulator include: (1) the ability to simulate unconfined ground-water flow, (2) a precipitation-recharge boundary condition, (3) a seepage-surface boundary condition at the land surface, (4) the removal of the limitation that a specified-pressure boundary also have a specified temperature, (5) a new iterative solver for the linear equations based on a generalized minimum-residual method, (6) the ability to use time- or depth-dependent functions for permeability, (7) the conversion of the program code to Fortran 90 to employ dynamic allocation of arrays, and (8) the incorporation of a graphical user interface (GUI) for input and output. The graphical user interface has been developed for defining a simulation, running the HYDROTHERM simulator interactively, and displaying the results. The combination of the graphical user interface and the HYDROTHERM simulator forms the HYDROTHERM INTERACTIVE (HTI) program. HTI can be used for two-dimensional simulations only. New features in Version 3 of the HYDROTHERM simulator have been verified using four test problems. Three problems come from the published literature and one problem was simulated by another partially saturated flow and thermal transport simulator. The test problems include: transient partially saturated vertical infiltration, transient one-dimensional horizontal infiltration, two-dimensional steady-state drainage with a seepage surface, and two-dimensional drainage with coupled heat transport. An example application to a hypothetical stratovolcano system with unconfined ground-water flow is presented in detail. It illustrates the use of HTI with the combination precipitation-recharge and seepage-surface boundary condition, and functions as a tutorial example problem for the new user.

Techniques and Methods↗

USGS2019

Differentiating sediment sources using sediment fingerprinting techniques, in the Sprague River Basin, South-Central Oregon

Identifying sources of sediment to streams in the Sprague River Basin, in south-central Oregon, is important for restoration efforts that are focused on reducing sediment erosion and transport. Reducing sediment loads in these streams also contributes to compliance with the total maximum daily load reduction requirements for total phosphorus in this basin. In the Sprague River Basin, phosphorus occurs in surface waters in both dissolved phase and particulate phase, and particulate phosphorus is readily transported in streams on fine-grained suspended sediments, which eventually deposit in Upper Klamath Lake. The lake has seasonal blooms of cyanobacteria that require phosphorus for growth and degrade water-quality conditions, violating State water-quality standards and creating conditions that are stressful to two endangered suckers that reside in the lake. Identifying sources of sediment to the Sprague River could help inform restoration actions by determining the principal locations in the basin contributing fine sediment to the river. The U.S. Geological Survey, in cooperation with the U.S. Fish and Wildlife Service, conducted a proof-of-concept study to determine if sediment fingerprinting can differentiate sources of bank erosion by source material, basin, river reach, and soil horizon. The sediment fingerprinting approach uses properties of streambank and streambed sediment to differentiate between multiple sediment sources by determining a composite signature, or fingerprint. The composite fingerprint is established by combining fingerprint properties from laboratory results of elemental analysis, stable isotopes, and total carbon and nitrogen. The methods for differentiating sediment samples for this study include grouping bank and bed samples by basin, river reach, and soil horizon, and using non-parametric statistics to determine which fingerprint properties could be used to differentiate the sample groups. Results indicate that fingerprint properties differentiated source material, river reach, and basin, and were more successful at differentiating samples grouped by geographic location (basin and reach) compared to source material. Source material (banks, bed, levees) were differentiated with three fingerprint properties—Antimony (Sb), copper (Cu), and manganese (Mn). The basin category (South Fork and main-stem Sprague River) differentiated the South Fork and main stem with stable nitrogen isotopes (δ 15 N), aluminum (Al), silicon (Si), and vanadium (V). Specific river reaches within the study area were differentiated with 11 different fingerprint properties. These results can be used for apportionment studies using suspended sediment samples and mixing models to determine sediment source contributions within the basin.

Oregon↗

USGS1986

Influence of size-fractioning techniques on concentrations of selected trace metals in bottom materials from two streams in northeastern Ohio

Identical stream-bottom material samples, when fractioned to the same size by different techniques, may contain significantly different trace-metal concentrations. Precision of techniques also may differ, which could affect the ability to discriminate between size-fractioned bottom-material samples having different metal concentrations. Bottom-material samples fractioned to less than 0.020 millimeters by means of three common techniques (air elutriation, sieving, and settling) were analyzed for six trace metals to determine whether the technique used to obtain the desired particle-size fraction affects the ability to discriminate between bottom materials having different trace-metal concentrations. In addition, this study attempts to assess whether median trace-metal concentrations in size-fractioned bottom materials of identical origin differ depending on the size-fractioning technique used. Finally, this study evaluates the efficiency of the three size-fractioning techniques in terms of time, expense, and effort involved. Bottom-material samples were collected at two sites in northeastern Ohio: One is located in an undeveloped forested basin, and the other is located in a basin having a mixture of industrial and surface-mining land uses. The sites were selected for their close physical proximity, similar contributing drainage areas, and the likelihood that trace-metal concentrations in the bottom materials would be significantly different. Statistically significant differences in the concentrations of trace metals were detected between bottom-material samples collected at the two sites when the samples had been size-fractioned by means of air elutriation or sieving. Statistical analyses of samples that had been size fractioned by settling in native water were not measurably different in any of the six trace metals analyzed. Results of multiple comparison tests suggest that differences related to size-fractioning technique were evident in median copper, lead, and iron concentrations. Technique-related differences in copper concentrations most likely resulted from contamination of air-elutriated samples by a feed tip on the elutriator apparatus. No technique-related differences were observed in chromium, manganese, or zinc concentrations. Although air elutriation was the most expensive sizefractioning technique investigated, samples fractioned by this technique appeared to provide a superior level of discrimination between metal concentrations present in the bottom materials of the two sites. Sieving was an adequate lower-cost but more laborintensive alternative.

Ohio↗

USGS2020

Quantifying background nitrate removal mechanisms in an agricultural watershed with contrasting subcatchment baseflow concentrations

Numerous studies have documented the linkages between agricultural nitrogen loads and surface water degradation. In contrast, potential water quality improvements due to agricultural best management practices are difficult to detect because of the confounding effect of background nitrate removal rates, as well as the groundwater-driven delay between land surface action and stream response. To characterize background controls on nitrate removal in two agricultural catchments, we calibrated groundwater travel time distributions with subsurface environmental tracer data to quantify the lag time between historic agricultural inputs and measured baseflow nitrate. We then estimated spatially distributed loading to the water table from nitrate measurements at monitoring wells, using machine learning techniques to extrapolate the loading to unmonitored portions of the catchment to subsequently estimate catchment removal controls. Multiple models agree that in-stream processes remove as much as 75% of incoming loads for one subcatchment while removing <20% of incoming loads for the other. The use of a spatially variable loading field did not result in meaningfully different optimized parameter estimates or model performance when compared with spatially constant loading derived directly from a county-scale agricultural nitrogen budget. Although previous studies using individual well measurements have shown that subsurface denitrification due to contact with a reducing argillaceous confining unit plays an important role in nitrate removal, the catchment-scale contribution of this process is difficult to quantify given the available data. Nonetheless, the study provides a baseline characterization of nitrate transport timescales and removal mechanisms that will support future efforts to detect water quality benefits from ongoing best management practice implementation.

Maryland↗

USGS2014

Caution on the use of NBS 30 biotite for hydrogen-isotope measurements with on-line high-temperature conversion systems

RATIONALE The supply of NBS 30 biotite is nearly exhausted. During measurements of NBS 30 and potential replacements, reproducible δ 2 H VSMOW-SLAP values could not be obtained by three laboratories using high-temperature conversion (HTC) systems. The cause of this issue has been investigated using the silver-tube technique for hydrogen-isotope measurements of water. METHODS The δ 2 H VSMOW-SLAP values of NBS 30 biotite, other biotites, muscovites, and kaolinite with different particle sizes, along with IAEA-CH-7 polyethylene, and reference waters and NBS 22 oil that were sealed in silver-tube segments, were measured. The effect of absorbed water on mineral surfaces was investigated with waters both enriched and depleted in 2 H. The quantitative conversion of hydrogen from biotite into gaseous hydrogen as a function of mass and particle size was also investigated. RESULTS The δ 2 H VSMOW-SLAP values of NBS 30 obtained by three laboratories were as much as 21 ‰ too high compared with the accepted value of −65.7 ‰, determined by conventional off-line measurements. The experiments showed a strong correlation between grain size and the δ 2 H VSMOW-SLAP value of NBS 30 biotite, but not of biotites with lower iron content. The δ 2 H VSMOW-SLAP values of NBS 30 as a function of particle size show a clear trend toward −65.7 ‰ with finer grain size. CONCLUSIONS Determination of the δ 2 H VSMOW-SLAP values of hydrous minerals and of NBS 30 biotite by on-line HTC systems coupled to isotope-ratio mass spectrometers may be unreliable because hydrogen in this biotite may not be converted quantitatively into molecular hydrogen. Extreme caution in the use and interpretation of δ 2 H VSMOW-SLAP on-line measurements of hydrous minerals is recommended.

Rapid Communications in Mass Spectrometry↗

USGS2003

Modeling hyporheic zone processes

Stream biogeochemistry is influenced by the physical and chemical processes that occur in the surrounding watershed. These processes include the mass loading of solutes from terrestrial and atmospheric sources, the physical transport of solutes within the watershed, and the transformation of solutes due to biogeochemical reactions. Research over the last two decades has identified the hyporheic zone as an important part of the stream system in which these processes occur. The hyporheic zone may be loosely defined as the porous areas of the stream bed and stream bank in which stream water mixes with shallow groundwater. Exchange of water and solutes between the stream proper and the hyporheic zone has many biogeochemical implications, due to differences in the chemical composition of surface and groundwater. For example, surface waters are typically oxidized environments with relatively high dissolved oxygen concentrations. In contrast, reducing conditions are often present in groundwater systems leading to low dissolved oxygen concentrations. Further, microbial oxidation of organic materials in groundwater leads to supersaturated concentrations of dissolved carbon dioxide relative to the atmosphere. Differences in surface and groundwater pH and temperature are also common. The hyporheic zone is therefore a mixing zone in which there are gradients in the concentrations of dissolved gasses, the concentrations of oxidized and reduced species, pH, and temperature. These gradients lead to biogeochemical reactions that ultimately affect stream water quality. Due to the complexity of these natural systems, modeling techniques are frequently employed to quantify process dynamics.

Advances in Water Resources↗

USGS1994

Adsorption and desorption of atrazine and deethylatrazine by low organic carbon geologic materials

The adsorption and desorption of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and a primary metabolite, deethylatrazine (2-amino-4-chloro-6-isopropylamino-s-triazine; DEA), by low organic C (≤ 3.3 g kg −1 ) materials were measured by batch-equilibrium techniques. The adsorbents were samples of glacial outwash sand, till, and stream sediments. The adsorption of both atrazine and DEA conformed to linear isotherms. The adsorption of atrazine by most of the adsorbents yielded apparent K oc values that were in excess of those based on surface agricultural soils. Adsorption correlated with only the pH of the sand-water suspensions. The desorption of atrazine was hysteretic under the conditions of the measurement. DEA had a lower affinity for the same adsorbents; the mean ratio of K d values of DEA to those of atrazine was 0.37 ± 0.20. DEA adsorption did not correlate with organic C, surface area, clay content of the adsorbents, or with the pH of the suspensions. DEA adsorption, unlike atrazine, tended to be reversible. There was a linear relationship between the adsorption constants of atrazine and those of DEA.

Journal of Environmental Quality↗

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↗

USGS1980

A comparison of analog and digital modeling techniques for simulating three-dimensional ground-water flow on Long Island, New York

A three-dimensional electric-analog model of the Long Island, NY , groundwater system constructed by the U.S. Geological Survey in the early 1970 's was used as the basis for developing a digital, three-dimensional finite-difference model. The digital model was needed to provide faster modifications and more rapid solutions to water-management questions. Results generated by the two models are depicted as potentiometric-surface maps of the upper glacial and Magothy aquifers. Results compare favorably for all parts of Long Island except the northwestern part, where hydrologic discontinuities are most prevalent and which the two models represent somewhat differently. The mathematical and hydrologic principles used in development of ground-water models, and the procedures for calibration and acceptance, are presented in nontechnical terms. (USGS)

Water-Resources Investigations Report↗