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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 289 records · Page 16Linked to original sources
USGS1989

Risk perceptions and behavioral context: U.S. Forest Service fire management professionals

Fire managers from the U.S. Forest Service were surveyed to determine which decision factors most strongly influenced their fire‐risk decisions. Safety, the resources at risk, public opinion, and the reliability of information were important influences on these decisions. This research allowed direct comparison between fire managers’ perceptions of factor importance and how their fire‐risk decisions changed in response to those factors. These risk decisions were highly responsive to changes in context (an escaped wildfire decision versus a prescribed burning decision) as well as to changing factors. The results demonstrate the utility of using scenarios in risk research and the vital importance of context in studying risk‐taking behavior. Research which attempts to remove risk decisions from their real‐world context may well distort the nature of risk‐taking behavior.

Society & Natural Resources: An International Jour↗
USGS2023

Machine-learning predictions of groundwater specific conductance in the Mississippi Alluvial Plain, south-central United States, with evaluation of regional geophysical aerial electromagnetic data as explanatory variables

The Mississippi Alluvial Plain, located in the south-central United States, is undergoing long-term groundwater-level declines within the surficial Mississippi River Valley alluvial aquifer (hereinafter referred to as “alluvial aquifer”), which has raised concerns about future groundwater availability. In some parts of the alluvial aquifer, groundwater availability for common uses such as irrigation, public supply, and domestic use is limited by quality (for example, high salinity) rather than quantity of water stored in the aquifer. The Mississippi Alluvial Plain region has an abundance of water-quality measurements in the alluvial aquifer and deeper aquifers; however, large areas lack direct measurements of salinity to evaluate regional groundwater availability. Statistical models can interpolate between wells to fill in spatial data gaps. In 2021, the U.S. Geological Survey trained two boosted regression tree (BRT) machine-learning models on specific conductance data available between 1942 and 2020 to predict spatially continuous surfaces of groundwater salinity at multiple depths for the alluvial aquifer and deeper aquifers. Well construction information, water levels, and surficial variables such as geomorphology and soils were included as explanatory variables in this baseline model. Additionally, subsurface electrical resistivity data from the first aquifer-wide aerial electromagnetic (AEM) survey for the region were incorporated to create a geophysical model. This work expands on prior BRT salinity predictions of the alluvial aquifer and extends predictions south to the Gulf of Mexico, where groundwater salinity is high. AEM survey data were not available for the southern extent of the alluvial aquifer at the time of modeling. A BRT model was trained without (baseline) and with (geophysical) AEM variables to test the ability of the models to predict salinity where explanatory data are missing and response data are sparse. Additionally, model sensitivity to AEM survey data was evaluated to better understand how AEM variables influence specific conductance predictions. Model performance was improved with the addition of geophysical data, which added three-dimensional information, thereby improving salinity predictions at depth. Groundwater specific conductance predictions can help inform other geophysical investigations in the southern extent of the study area, where high groundwater specific conductance can obfuscate changes in aquifer sediment resistivity and could limit groundwater resources for agricultural, public supply, and domestic uses.

Scientific Investigations Report↗
USGS1998

Hydrogeology and sources of recharge to the Buffalo and Wahpeton aquifers in the southern part of the Red River of the North drainage basin, west-central Minnesota and southeastern North Dakota

Declining hydraulic heads in the Buffalo and Wahpeton aquifers are of concern to the Minnesota Department of Natural Resources and local water managers because of limited groundwater resources in the southern part of the Red River of the North drainage basin. The U.S. Geological Survey, in cooperation with the Minnesota Department of Natural Resources and Moorhead Public Service, investigated the hydrogeology of and sources of recharge to the Buffalo and Wahpeton aquifers. The Buffalo aquifer is a complex, heterogeneous outwash deposit composed of medium to coarse sand and gravel. Part of the Buffalo aquifer is unconfined and part is confined. The direction of ground water flow in the Buffalo aquifer is from east to west. Water-level declines in observation wells near the Moorhead Public Service North Well Field extend beyond the eastern and western boundaries of the Buffalo aquifer. Transmissivity and storativity calculated from the drawdown part of an aquifer test ranged from 20,870 to 23,852 feet squared per day and from 3.0 x 10 -5 to 3.2 x 10 -2 , respectively. Transmissivity and hydraulic conductivity values of 29,090 and 28,450 feet squared per day and 272 and 266 feet per day were calculated from the recovery-phase data. Potential recharge from the Buffalo River and its tributaries to the Buffalo aquifer ranged from 5 to 14 cubic feet per second. Recharge from precipitation where the Buffalo aquifer is unconfined was about 1.49 x 10 5 cubic feet per day. Recharge per unit length of the Buffalo aquifer during an aquifer test near the Moorhead Public Service North Well Field ranged from 3.9 x 10 -4 to 2.0 x 10 -2 cubic feet per day. The Wahpeton Shallow Sand, the Wahpeton Sand Plain, and the Wahpeton Buried Valley aquifers comprise the Wahpeton aquifers in order of increasing depth. All the aquifers are composed of fine- to coarse-grained sand mixed with gravel. Confining units are interleaved with the Wahpeton aquifers. Ground-water-flow directions in the Wahpeton aquifers were changed by ground-water development. Before development, ground water flowed from the Wahpeton Buried Valley aquifer upward to the Wahpeton Sand Plain aquifer and the Wahpeton Shallow Sand aquifer. After development, ground water flowed from the Wahpeton Shallow Sand aquifer to the Wahpeton Sand Plain and the Wahpeton Buried Valley aquifers. The potential sources of recharge to the Wahpeton aquifers investigated were the Red River of the North, and adjacent hydro geologic units. The volume of ground water pumped from the Wahpeton aquifers provides an estimate of the upper limit for the volume of recharge to the aquifer. Based on pumpage from all of the Wapheton aquifers from 1990 to 1993, the upper limit is about 580 million gallons per year (2.4 x 10 5 cubic feet per day).

Minnesota, North Dakota↗
USGS2019

Monitoring the Riverine Pulse: Applying high-frequency nitrate data to advance integrative understanding of biogeochemical and hydrological processes

Widespread deployment of sensors that measure river nitrate (NO3-) concentrations has led to many recent publications in water resources journals including review papers focused on data quality assurance, improved load calculations, and better nutrient management. The principal objective of this paper is to review and synthesize studies of high-frequency NO3- data that have aimed to improve understanding of the hydrologic and biogeochemical processes underlying episodic, diel, and long-term stream NO3- dynamics. Investigations have provided unprecedented detail on hysteresis and flushing patterns during high flow, seasonal variation during baseflow, and responses to multi-year climate variation. Analyses of high-frequency data have led to notable advances in understanding how climate variation affects spatial and temporal NO3- patterns, especially dry-wet cycles and antecedent moisture. Further advances have been limited by few investigations that include high-frequency measurements outside the channel and the short duration of many records. High-frequency data for multiple constituents have provided new insight to the relative roles of hydrology and biogeochemistry as highlighted by studies of the roles of autotrophic uptake, denitrification, riparian evapotranspiration, and temperature-driven changes in viscosity as drivers of diel patterns. Comparisons of short-duration high-frequency data with long-duration low frequency data have described similarities and differences in concentration – discharge patterns and highlighted the role of legacy stores. Investigators have applied innovative analysis approaches not previously possible with low-frequency or temporally-irregular data. Future availability of long-duration high-frequency data will provide new insight to processes, resulting in improved conceptual models and a deeper understanding of the role of climate variation.

WIREs Water↗
USGS2021

The 4th paradigm in multiscale data representation: Modernizing the National Geospatial Data Infrastructure

The need of citizens in any nation to access geospatial data in readily usable form is critical to societal well-being, and in the United States (US), demands for information by scientists, students, professionals and citizens continue to grow. Areas such as public health, urbanization, resource management, economic development and environmental management require a variety of data collected from many sources to identify problems, monitor trends and propose solutions. Such information needs and demands have driven the coordination of federal and regional government agencies with respective private sector participation to develop national geospatial data infrastructures in many countries.

Book chapter↗
USGS2020

Identifying areas of degrading and improving groundwater-quality conditions in the State of California, USA, 1974-2014

Areas of improving and degrading groundwater-quality conditions in the State of California were assessed using spatial weighting of a new metric for scoring wells based on constituent concentrations and the direction and magnitude of a trend slope (Sen). Individual well scores were aggregated across 2135 equal-area grid cells covering the entire groundwater resource used for public supply in the state. Spatial weighting allows results to be aggregated locally (well or grid cell), regionally (groundwater basin), provincially, or statewide. Results differentiate degrading (increasing concentration trends) areas with low to moderate concentrations (unimpaired) from degrading areas with moderate to high concentrations (impaired). Results also differentiate improving areas (decreasing concentration trends) in the same manner. Multi-year to decadal groundwater-quality trends were computed from periodic, inorganic water-quality data for 38 constituents collected between 1974 and 2014 for compliance monitoring of nearly 13,000 public-supply wells (PSWs) in the State of California. Mann-Kendall (MK) rank correlations and Sen’s slope estimator were used to detect statistically significant trends for the entire period of recorded data (long-term trend), for the period since 2000 (recent trend), for different pumping seasons (seasonal trend), and for reversals of trends. Statewide, the most frequently detected trends since 2000 were for nitrate (36%), gross alpha/uranium (10%), arsenic (14%), total dissolved solids (TDS) (23%), and the major ions that contribute to TDS (19–28%). The Transverse and Selected Peninsular Ranges (TSPR) and the San Joaquin Valley (SJV) hydrogeologic provinces had the largest percentage of areas with moderate to high nitrate concentrations and groundwater quality trends. Improving nitrate concentrations in parts of the TSPR is associated with long-term managed aquifer recharge that has replaced historical, agriculturally affected groundwater with low-nitrate recharge in parts of the TSPR. This example suggests that application of dilute, excess surface water to agricultural fields during the winter could improve groundwater-quality in the SJV over the long term.

California↗
USGS2026

How to accelerate advances in ecological forecasting

Ecological forecasting offers critical insights for managing natural resources and safeguarding public well-being. Despite growing demand for these forecasts, progress is hindered by fragmented systems, redundant workflows, and limited interoperability. Drawing lessons from weather forecasting and recent successes like the NEON Ecological Forecasting Challenge, shared cyberinfrastructure is important for advancing ecological prediction. By adopting common standards, open-source tools, and scalable architectures, and fostering transdisciplinary collaboration, the ecological forecasting community can overcome technical and institutional barriers. Such investments could accelerate scientific understanding, improve forecast reliability, and empower decisionmakers to anticipate environmental change and respond effectively.

Eos, American Geophysical Union↗
USGS2021

Time to refine mercury mass balance models for fish

Mercury mass balance models (MMBMs) for fish are powerful tools for understanding factors affecting growth and food consumption by free-ranging fish in rivers, lakes, and oceans. Moreover, MMBMs can be used to predict the consequences of global mercury reductions, overfishing, and climate change on mercury (Hg) concentration in commercially and recreationally valuable species of fish. Such predictions are useful in decision-making by resource managers and public health policy makers, because mercury is a neurotoxin and the primary route of exposure of mercury to humans is via consumption of fish. Recent evidence has emerged to indicate that the current-day version of MMBMs overestimates the rate at which fish eliminate mercury from their bodies. Consequently, MMBMs overestimate food consumption by fish and underestimate Hg concentration in fish. In this perspective, we explore underlying reasons for this overestimation of Hg-elimination rate, as well as consequences and implications of this overestimation. We highlight emerging studies that distinguish species and sex as contributing factors, in addition to body weight and water temperature, that can play an important role in how quickly Hg is eliminated from fish. Future research directions for refining MMBMs are discussed.

Facets↗
USGS2009

Gene and antigen markers of Shiga-toxin producing E. coli from Michigan and Indiana river water: Occurrence and relation to recreational water quality criteria

The relation of bacterial pathogen occurrence to fecal indicator bacteria (FIB) concentrations used for recreational water quality criteria (RWQC) is poorly understood. This study determined the occurrence of Shiga-toxin producing Escherichia coli (STEC) markers and their relation to FIB concentrations in Michigan and Indiana river water. Using 67 fecal coliform (FC) bacteria cultures from 41 river sites in multiple watersheds, we evaluated the occurrence of five STEC markers: the Escherichia coli (EC) O157 antigen and gene, and the STEC virulence genes eae A, stx 1, and stx 2. Simple isolations from selected FC cultures yielded viable EC O157. By both antigen and gene assays, EC O157 was detected in a greater proportion of samples exceeding rather than meeting FC RWQC ( P < 0.05), but was unrelated to EC and enterococci RWQC. The occurrence of all other STEC markers was unrelated to any FIB RWQC. The eaeA , stx2, and stx1 genes were found in 93.3, 13.3, and in 46.7% of samples meeting FC RWQC and in 91.7, 0.0, and 37.5% of samples meeting the EC RWQC. Although not statistically significant, the percentage of samples positive for each STEC marker except stx1 was lower in samples that met, as opposed to exceeded, FIB RWQC. Viable STEC were common members of the FC communities in river water throughout southern Michigan and northern Indiana, regardless of FIB RWQC. Our study indicates that further information on the occurrence of pathogens in recreational waters, and research on alternative indicators of their occurrence, may help inform water-resource management and public health decision-making.

Journal of Environmental Quality↗
USGS2017

Groundwater-quality data in 12 GAMA study units: Results from the 2006–10 initial sampling period and the 2008–13 trend sampling period, California GAMA Priority Basin Project

The Priority Basin Project (PBP) of the Groundwater Ambient Monitoring and Assessment (GAMA) program was developed in response to the Groundwater Quality Monitoring Act of 2001 and is being conducted by the U.S. Geological Survey in cooperation with the California State Water Resources Control Board. From 2004 through 2012, the GAMA-PBP collected samples and assessed the quality of groundwater resources that supply public drinking water in 35 study units across the State. Selected sites in each study unit were sampled again approximately 3 years after initial sampling as part of an assessment of temporal trends in water quality by the GAMA-PBP. Twelve of the study units, initially sampled during 2006–11 (initial sampling period) and sampled a second time during 2008–13 (trend sampling period) to assess temporal trends, are the subject of this report. The initial sampling was designed to provide a spatially unbiased assessment of the quality of untreated groundwater used for public water supplies in the 12 study units. In these study units, 550 sampling sites were selected by using a spatially distributed, randomized, grid-based method to provide spatially unbiased representation of the areas assessed (grid sites, also called “status sites”). After the initial sampling period, 76 of the previously sampled status sites (approximately 10 percent in each study unit) were randomly selected for trend sampling (“trend sites”). The 12 study units sampled both during the initial sampling and during the trend sampling period were distributed among 6 hydrogeologic provinces: Coastal (Northern and Southern), Transverse Ranges and Selected Peninsular Ranges, Klamath, Modoc Plateau and Cascades, and Sierra Nevada Hydrogeologic Provinces. For the purposes of this trend report, the six hydrogeologic provinces were grouped into two hydrogeologic regions based on location: Coastal and Mountain. The groundwater samples were analyzed for a number of synthetic organic constituents (volatile organic compounds, pesticides, and pesticide degradates), constituents of special interest (perchlorate and 1,2,3-trichloropropane), and natural inorganic constituents (nutrients, major and minor ions, and trace elements). Isotopic tracers (tritium, carbon-14, and stable isotopes of hydrogen and oxygen in water) also were measured to help identify processes affecting groundwater quality and the sources and ages of the sampled groundwater. More than 200 constituents and water-quality indicators were measured during the trend sampling period. Quality-control samples (blanks, replicates, matrix-spikes, and surrogate compounds) were collected at about one-third of the trend sites, and the results for these samples were used to evaluate the quality of the data for the groundwater samples. On the basis of detections in laboratory and field blank samples collected by GAMA-PBP study units, including the 12 study units presented here, reporting levels for some groundwater results were adjusted in this report. Differences between replicate samples were mostly within acceptable ranges, indicating low variability in analytical results. Matrix-spike recoveries were largely within the acceptable range (70 to 130 percent). This study did not attempt to evaluate the quality of water delivered to consumers. After withdrawal, groundwater used for drinking water typically is treated, disinfected, and blended with other waters to achieve acceptable water quality. The comparison benchmarks used in this report apply to treated water that is served to the consumer, not to untreated groundwater. To provide some context for the results, however, concentrations of constituents measured in these groundwater samples were compared with benchmarks established by the U.S. Environmental Protection Agency and the State of California. Comparisons between data collected for this study and benchmarks for drinking water are for illustrative purposes only and are not indicative of compliance or non-compliance with those benchmarks. Most organic constituents that were detected in groundwater samples from the trend sites were found at concentrations less than health-based benchmarks. One volatile organic compound—perchloroethene—was detected at a concentration greater than the health-based benchmark in samples from one trend site during the initial and trend sampling periods. Chloroform was detected in at least 10 percent of the samples at trend sites in both sampling periods. Methyl tert -butyl ether was detected in samples from more than 10 percent of the trend sites during the initial sampling period. No pesticide or pesticide degradate was detected in greater than 10 percent of the samples from trend sites or at concentrations greater than their health-based benchmarks during either sampling period. Nutrients were not detected at concentrations greater than their health-based benchmarks during either sampling period. Most detections of major ions and trace elements in samples from trend sites were less than health-based benchmarks during both sampling periods. Arsenic and boron each were detected at concentrations greater than the health-based benchmark in samples from four trend sites during the initial and trend sampling periods. Molybdenum was detected in samples from four trend sites at concentrations greater than the health-based benchmark during both sampling periods. Samples from two of these trend sites had similar molybdenum concentrations, and two had substantially different concentrations during the initial and trend sampling periods. Uranium was detected at a concentration greater than the health-based benchmark only at two trend sites.

California↗
USGS2005

Mercury contamination from historical gold mining in California

Mercury contamination from historical gold mines represents a potential risk to human health and the environment. This fact sheet provides background information on the use of mercury in historical gold mining and processing operations in California, with emphasis on historical hydraulic mining areas. It also describes results of recent USGS projects that address the potential risks associated with mercury contamination. Miners used mercury (quicksilver) to recover gold throughout the western United States. Gold deposits were either hardrock (lode, gold-quartz veins) or placer (alluvial, unconsolidated gravels). Underground methods (adits and shafts) were used to mine hardrock gold deposits. Hydraulic, drift, or dredging methods were used to mine the placer gold deposits. Mercury was used to enhance gold recovery in all the various types of mining operations; historical records indicate that more mercury was used and lost at hydraulic mines than at other types of mines. On the basis of USGS studies and other recent work, a better understanding is emerging of mercury distribution, ongoing transport, transformation processes, and the extent of biological uptake in areas affected by historical gold mining. This information has been used extensively by federal, state, and local agencies responsible for resource management and public health in California.

Fact Sheet↗
USGS2017

Monitoring the southwestern Wyoming landscape—A foundation for management and science

Natural resource monitoring involves repeated collections of resource condition data and analyses to detect possible changes and identify underlying causes of changes. For natural resource agencies, monitoring provides the foundation for management and science. Specifically, analyses of monitoring data allow managers to better understand effects of land-use and other changes on important natural resources and to achieve their conservation and management goals. Examples of natural resources monitored on public lands include wildlife habitats, plant productivity, animal movements and population trends, soil chemistry, and water quality and quantity. Broader definitions of monitoring also recognize the need for scientifically valid data to help support planning efforts and informed decisions, to develop adaptive management strategies, and to provide the means for evaluating management outcomes.

Wyoming↗
USGS2019

The U.S. Geological Survey Southwest Biological Science Center—Sound science to serve the American Southwest

Home to Arches, Grand Canyon, and Saguaro National Parks, among others, the American Southwest’s landscapes are as fragile as they are iconic. Energy development, water security, and grassland restoration are important to the region as it experiences population growth and increased demand for resources. The U.S. Geological Survey’s Southwest Biological Science Center provides sound scientific information to help identify effective management strategies for the Southwest’s abundant natural resources and vast public lands. Research is focused on two key areas—dryland ecology and river science.

Arizona, California, Nevada, Utah↗
USGS2017

Land change monitoring, assessment, and projection (LCMAP) revolutionizes land cover and land change research

When nature and humanity change Earth’s landscapes - through flood or fire, public policy, natural resources management, or economic development - the results are often dramatic and lasting. Wildfires can reshape ecosystems. Hurricanes with names like Sandy or Katrina will howl for days while altering the landscape for years. One growing season in the evolution of drought-resistant genetics can transform semiarid landscapes into farm fields. In the past, valuable land cover maps created for understanding the effects of those events - whether changes in wildlife habitat, water-quality impacts, or the role land use and land cover play in affecting weather and climate - came out at best every 5 to 7 years. Those high quality, high resolution maps were good, but users always craved more: even higher quality data, additional land cover and land change variables, more detailed legends, and most importantly, more frequent land change information. Now a bold new initiative called Land Change Monitoring, Assessment, and Projection (LCMAP) promises to fulfill that demand. Developed at the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center in Sioux Falls, South Dakota, LCMAP provides definitive, timely information on how, why, and where the planet is changing. LCMAP’s continuous monitoring process can detect changes as they happen every day that Landsat satellites acquire clear observations. The result will be to place near real-time information in the hands of land and resource managers who need to understand the effects these changes have on landscapes.

General Information Product↗
USGS1987

Operation of hydrologic data collection stations by the U.S. Geological Survey in 1987

The U.S. Geological Survey operates hydrologic data collection stations nationwide which serve the needs of all levels of government, the private sector, and the general public, for water resources information. During fiscal year 1987, surface water discharge was determined at 10,624 stations; stage data on streams, reservoirs, and lakes were recorded at 1,806 stations; and various surface water quality characteristics were determined at 2,901 stations. In addition, groundwater levels were measured at 32,588 stations, and the quality of groundwater was determined at 9,120 stations. Data on sediment were collected daily at 174 stations and on a periodic basis at 878 stations. Information on precipitation quantity was collected at 909 stations, and the quality of precipitation was analyzed at 78 stations. Data collection platforms for satellite telemetry of hydrologic information were used at 2,292 Geological Survey stations. Funding for the hydrologic stations was derived, either solely or from a combination, from three major sources - the Geological Survey 's Federal Program appropriation, the Federal-State Cooperative Program, and reimbursements from other Federal agencies. The number of hydrologic stations operated by the Geological Survey declined from fiscal year 1983 to 1987. The number of surface water discharge stations were reduced by 452 stations; surface water quality stations declined by 925 stations; groundwater level stations declined by 1,051 stations; while groundwater quality stations increased by 1,472 stations. (Author 's abstract)

Open-File Report↗
USGS2013

Estimated rates of groundwater recharge to the Chicot, Evangeline and Jasper aquifers by using environmental tracers in Montgomery and adjacent counties, Texas, 2008 and 2011

Montgomery County is in the northern part of the Houston, Texas, metropolitan area, the fourth most populous metropolitan area in the United States. As populations have increased since the 1980s, groundwater has become an important resource for public-water supply and industry in the rapidly growing area of Montgomery County. Groundwater availability from the Gulf Coast aquifer system is a primary concern for water managers and community planners in Montgomery County and requires a better understanding of the rate of recharge to the system. The Gulf Coast aquifer system in Montgomery County consists of the Chicot, Evangeline, and Jasper aquifers, the Burkeville confining unit, and underlying Catahoula confining system. The individual sand and clay sequences of the aquifers composing the Gulf Coast aquifer system are not laterally or vertically continuous on a regional scale; however, on a local scale, individual sand and clay lenses can extend over several miles. The U.S. Geological Survey, in cooperation with the Lone Star Groundwater Conservation District, collected groundwater-quality samples from selected wells within or near Montgomery County in 2008 and analyzed these samples for concentrations of chlorofluorocarbons (CFCs), sulfur hexafluoride (SF 6 ), tritium (3H), helium-3/tritium ( 3 He/ 3 H), helium-4 ( 4 He), and dissolved gases (DG) that include argon, carbon dioxide, methane, nitrogen and oxygen. Groundwater ages, or apparent age, representing residence times since time of recharge, were determined by using the assumption of a piston-flow transport model. Most of the environmental tracer data indicated the groundwater was recharged prior to the 1950s, limiting the usefulness of CFCs, SF 6 , and 3 H concentrations as tracers. In many cases, no tracer was usable at a well for the purpose of estimating an apparent age. Wells not usable for estimating an apparent age were resampled in 2011 and analyzed for concentrations of major ions and carbon-14 ( 14 C). At six of these wells, additional 4 He and DG samples were collected and analyzed. Recharge rates estimated from environmental tracer data are dependent upon several hydrogeologic variables and have inherent uncertainties. By using the recharge estimates derived from samples collected from 14 wells completed in the Chicot aquifer for which apparent groundwater ages could be determined, recharge to the Chicot aquifer ranged from 0.2 to 7.2 inches (in.) per year (yr). Based on data from one well, estimated recharge to the unconfined zone of the Evangeline aquifer (outcrop) was 0.1 in./yr. Based on data collected from eight wells, estimated rates of recharge to the confined zone of the Evangeline aquifer ranged from less than 0.1 to 2.8 in./yr. Based on data from one well, estimated recharge to the unconfined zone of the Jasper aquifer (outcrop) was 0.5 in./yr. Based on data collected from nine wells, estimated rates of recharge to the confined zone of the Jasper aquifer ranged from less than 0.1 to 0.1 in./yr. The complexity of the hydrogeology in the area, uncertainty in the conceptual model, and numerical assumptions required in the determination of the recharge rates all pose limitations and need to be considered when evaluating these data on a countywide or regional scale. The estimated recharge rates calculated for this study are specific to each well location and should not be extrapolated or inferred as a countywide average. Local variations in the hydrogeology and surficial conditions can affect the recharge rate at a local scale.

Texas↗
USGS2020

Updated study reporting levels (SRLs) for trace-element data collected for the California Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project, October 2009–October 2018

Groundwater samples have been collected in California as part of statewide investigations of groundwater quality conducted by the U.S. Geological Survey for the Groundwater Ambient Monitoring and Assessment (GAMA) Priority Basin Project (PBP) since 2004. The GAMA-PBP is being conducted in cooperation with the California State Water Resources Control Board to assess and monitor the quality of groundwater resources used for public and domestic drinking-water supply and to improve public knowledge of groundwater quality in California. Quality-control samples (including but not limited to field, equipment, and source-solution blanks) were collected to evaluate and quantify the quality of the groundwater sample results. The GAMA-PBP previously determined study reporting levels (SRLs) for trace-element results based primarily on field blanks collected in California from May 2004 through March 2013. SRLs are raised reporting levels used to reduce the likelihood of reporting false detections attributable to contamination bias. The purpose of this report is to identify any changes in the pattern or magnitude of concentrations or detections in field blanks since the last evaluation that would require changing or ending the use of SRLs implemented in October 2009. Constituents analyzed were aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, chromium, hexavalent chromium, cobalt, copper, iron, lead, lithium, manganese, molybdenum, nickel, selenium, silver, strontium, thallium, uranium, vanadium, and zinc. For this review, data from 167 field blanks collected from October 2009 through October 2018 by the GAMA-PBP for trace elements were compiled. Based on a consistent pattern of decreasing cobalt and manganese concentrations in field blanks from 2009 to 2013, the GAMA-PBP decided to reevaluate all trace-element SRLs, effectively setting an end date for previously defined SRLs. Beginning October 2013, SRLs would be determined from field-blank data collected through October 2018. The detection frequency and upper limit of potential contamination bias (BD-90/90) were determined from field blanks for each trace element. The BD-90/90, that is, the upper 90-percent confidence limit of the 90th percentile concentration of potential extrinsic contamination, was calculated by assuming the binomial probability distribution. These results were compared to each constituent’s detection limit to determine whether an SRL was necessary to minimize the potential for detections in the groundwater samples, attributed principally to contamination bias. Results of the evaluation were used to set SRLs for trace-element data collected by the GAMA-PBP between October 2013 and October 2018. Trace elements prescribed an SRL based on this review were hexavalent chromium, cobalt, copper, lead, and zinc. This review also resulted in the removal of SRLs from iron, manganese, molybdenum, and nickel. Although an SRL for hexavalent chromium could not be evaluated in the earlier reviews because the data were not collected regularly until 2015, one was established herein as 0.34 micrograms per liter (µg/L). The SRL for cobalt, as previously implemented, had been to reject all results; it was changed to 0.16 µg/L following a reduction in cobalt field-blank detection frequency resulting from mitigation steps, starting in 2014, aimed at reducing contamination bias introduced by high-capacity capsule filters used during sample collection. The SRL for copper did not change, and the SRL for lead changed very little based on this review. Lastly, the SRL for zinc was lowered from 6.2 µg/L to 3.9 µg/L.

California↗
USGS2010

Groundwater sampling

Groundwater is protected in most areas as it is a primary source of drinking water. In the United States, 50% of the population relies on groundwater supplies (Reilly et al., 2008). Groundwater sampling in the United States became commonplace in the 20th century as contaminated water resources became apparent and a growing public concern emerged to protect water resources. In response to this concern, the U.S. government mandated a study in which scientists identified six categories of groundwater contaminant sources (OTA, 1984): Category 1-sources designed to discharge substances (e.g., injection well) Category 2-sources designed to store, treat, and/or dispose of substances; discharge through unplanned release (e.g., landfills) Category 3-sources designed to retain substances during transport or trans-mission (e.g., pipelines) Category 4-sources discharging as consequence of other planned activities (e.g., pesticide application) Category 5-sources providing conduit or inducing discharge through altered flow patterns (e.g., construction excavation) Category 6-naturally occurring sources whose discharge is created and/or exacerbated by human activity (e.g., salt water intrusion).

Book chapter↗