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USGS2017

Groundwater quality in the shallow aquifers of the Tulare, Kaweah, and Tule Groundwater Basins and adjacent highlands areas, Southern San Joaquin Valley, California

Groundwater provides more than 40 percent of California’s drinking water. To protect this vital resource, the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State’s groundwater quality and increases public access to groundwater-quality information. The shallow aquifers of the Tulare, Kaweah, and Tule groundwater basins and adjacent highlands areas of the southern San Joaquin Valley constitute one of the study units being evaluated.

California↗

USGS2016

Regional land subsidence caused by the compaction of susceptible aquifer systems accompanying groundwater extraction

Land subsidence includes both gentle downwarping and sudden sinking of segments of the land surface. Major anthropogenic causes of land subsidence are extraction of fluids including water, oil, and gas. Measurement and detec- tion of land subsidence include both ground-based and remotely sensed air- borne and space-based methods. Methods for measurement of subsidence at points include differential leveling, global positioning system surveys, and extensometers. Satellite-borne differential interferometric synthetic aperture radar and airborne LiDAR techniques can detect land-surface movement over wide areas of interest. Aquifer-system compaction and subsidence owing to groundwater extraction typically occurs in areas of unconsolidated alluvial or basin-fill aquifer systems comprising aquifers and aquitards. Approaches to analyzing and modeling deformation of aquifer systems follow from the basic relations between head, stress, compressibility, and groundwater flow. Analysis and simulation of aquifer-system compaction have been addressed primarily using either an approach based on conventional groundwater flow theory or an approach based on linear poroelasticity theory. Both approaches rely on the principle of effective stress outlined by Karl Terzaghi in 1925. In the approach based on conventional groundwater flow theory, an aquitard drainage model explains the compaction of fine grained material using the principle of effective stress and theory of hydrodynamic lag. Packages for the widely-used MODFLOW groundwater model are available to simulate aqui- fer-system compaction and land subsidence using the aquitard-drainage approach. Poroelasticity theory describes the more fully coupled processes of groundwater flow and three-dimensional deformation of aquifer systems. The general theory accounts for compressible fluid, porous matrix and solid grains. Simulation codes using the poroelastic theory include some commer- cial software products and a few research codes.

Book chapter↗

USGS2017

Groundwater quality in the glacial aquifer system, United States

Groundwater provides nearly 50 percent of the Nation’s drinking water. To help protect this vital resource, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water (Burow and Belitz, 2014). The glacial aquifer system constitutes one of the important areas being evaluated.

Glacial Aquifer System↗

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↗

USGS2025

Groundwater budget for the surficial aquifer surrounding Lake Nokomis, Minneapolis, Minnesota

During prolonged periods of above-average precipitation, rising groundwater levels have the potential to cause damage to and interfere with underground infrastructure and building foundations. To understand the relations between precipitation and groundwater in the vicinity of Lake Nokomis, the U.S. Geological Survey, in collaboration with the University of Minnesota, quantified five components of the groundwater budget: groundwater recharge, change in surficial aquifer storage, surficial aquifer groundwater discharge to Lake Nokomis, groundwater evapotranspiration, and groundwater discharge to underlying bedrock aquifers. Field data, geologic records, and empirical calculation methods were used to quantify groundwater budget components for April 2023 through April 2024. Lake water budget data indicate that Lake Nokomis is a flowthrough system during periods with no outflow through the weir, with groundwater inputs equal to outputs. Roughly 40 percent of precipitation that fell in the study area was added to the surficial aquifer as recharge. Uncertainty in the vertical hydraulic conductivity resulted in wide-ranging estimates (spanning three orders of magnitude) of water discharging from the surficial aquifer to the underlying bedrock aquifer. Drought conditions persisted for the duration of this study and were not representative of the conditions that motivated this study. This study is a start towards understanding relations between precipitation, Lake Nokomis levels, and groundwater levels that could affect local underground infrastructure.

Minnesota↗

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↗

USGS2013

A geochemical approach to determine sources and movement of saline groundwater in a coastal aquifer

Geochemical evaluation of the sources and movement of saline groundwater in coastal aquifers can aid in the initial mapping of the subsurface when geological information is unavailable. Chloride concentrations of groundwater in a coastal aquifer near San Diego, California, range from about 57 to 39,400 mg/L. On the basis of relative proportions of major-ions, the chemical composition is classified as Na-Ca-Cl-SO 4 , Na-Cl, or Na-Ca-Cl type water. δ 2 H and δ 18 O values range from −47.7‰ to −12.8‰ and from −7.0‰ to −1.2‰, respectively. The isotopically depleted groundwater occurs in the deeper part of the coastal aquifer, and the isotopically enriched groundwater occurs in zones of sea water intrusion. 87 Sr/ 86 Sr ratios range from about 0.7050 to 0.7090, and differ between shallower and deeper flow paths in the coastal aquifer. 3 H and 14 C analyses indicate that most of the groundwater was recharged many thousands of years ago. The analysis of multiple chemical and isotopic tracers indicates that the sources and movement of saline groundwater in the San Diego coastal aquifer are dominated by: (1) recharge of local precipitation in relatively shallow parts of the flow system; (2) regional flow of recharge of higher-elevation precipitation along deep flow paths that freshen a previously saline aquifer; and (3) intrusion of sea water that entered the aquifer primarily during premodern times. Two northwest-to-southeast trending sections show the spatial distribution of the different geochemical groups and suggest the subsurface in the coastal aquifer can be separated into two predominant hydrostratigraphic layers.

California↗

USGS2012

Vulnerability of recently recharged groundwater in principal aquifers of the United States to nitrate contamination

Recently recharged water (defined here as <60 years old) is generally the most vulnerable part of a groundwater resource to nonpoint-source nitrate contamination. Understanding at the appropriate scale the interactions of natural and anthropogenic controlling factors that influence nitrate occurrence in recently recharged groundwater is critical to support best management and policy decisions that are often made at the aquifer to subaquifer scale. New logistic regression models were developed using data from the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program and National Water Information System for 17 principal aquifers of the U.S. to identify important source, transport, and attenuation factors that control nonpoint source nitrate concentrations greater than relative background levels in recently recharged groundwater and were used to predict the probability of detecting elevated nitrate in areas beyond the sampling network. Results indicate that dissolved oxygen, crops and irrigated cropland, fertilizer application, seasonally high water table, and soil properties that affect infiltration and denitrification are among the most important factors in predicting elevated nitrate concentrations. Important differences in controlling factors and spatial predictions were identified in the principal aquifer and national-scale models and support the conclusion that similar spatial scales are needed between informed groundwater management and model development.

Environmental Science & Technology↗

USGS2016

Simulating groundwater flow in karst aquifers with distributed parameter models—Comparison of porous-equivalent media and hybrid flow approaches

Understanding karst aquifers, for purposes of their management and protection, poses unique challenges. Karst aquifers are characterized by groundwater flow through conduits (tertiary porosity), and (or) layers with interconnected pores (secondary porosity) and through intergranular porosity (primary or matrix porosity). Since the late 1960s, advances have been made in the development of numerical computer codes and the use of mathematical model applications towards the understanding of dual (primary [matrix] and secondary [fractures and conduits]) porosity groundwater flow processes, as well as characterization and management of karst aquifers. The Floridan aquifer system (FAS) in Florida and parts of Alabama, Georgia, and South Carolina is composed of a thick sequence of predominantly carbonate rocks. Karst features are present over much of its area, especially in Florida where more than 30 first-magnitude springs occur, numerous sinkholes and submerged conduits have been mapped, and numerous circular lakes within sinkhole depressions are present. Different types of mathematical models have been applied for simulation of the FAS. Most of these models are distributed parameter models based on the assumption that, like a sponge, water flows through connected pores within the aquifer system and can be simulated with the same mathematical methods applied to flow through sand and gravel aquifers; these models are usually referred to as porous-equivalent media models. The partial differential equation solved for groundwater flow is the potential flow equation of fluid mechanics, which is used when flow is dominated by potential energy and has been applied for many fluid problems in which kinetic energy terms are dropped from the differential equation solved. In many groundwater model codes (basic MODFLOW), it is assumed that the water has a constant temperature and density and that flow is laminar, such that kinetic energy has minimal impact on flow. Some models have been developed that incorporate the submerged conduits as a one-dimensional pipe network within the aquifer rather than as discrete, extremely transmissive features in a porous-equivalent medium; these submerged conduit models are usually referred to as hybrid models and may include the capability to simulate both laminar and turbulent flow in the one-dimensional pipe network. Comparisons of the application of a porous-equivalent media model with and without turbulence (MODFLOW-Conduit Flow Process mode 2 and basic MODFLOW, respectively) and a hybrid (MODFLOW-Conduit Flow Process mode 1) model to the Woodville Karst Plain near Tallahassee, Florida, indicated that for annual, monthly, or seasonal average hydrologic conditions, all methods met calibration criteria (matched observed groundwater levels and average flows). Thus, the increased effort required, such as the collection of data on conduit location, to develop a hybrid model and its increased computational burden, is not necessary for simulation of average hydrologic conditions (non-laminar flow effects on simulated head and spring discharge were minimal). However, simulation of a large storm event in the Woodville Karst Plain with daily stress periods indicated that turbulence is important for matching daily springflow hydrographs. Thus, if matching streamflow hydrographs over a storm event is required, the simulation of non-laminar flow and the location of conduits are required. The main challenge in application of the methods and approaches for developing hybrid models relates to the difficulty of mapping conduit networks or having high-quality datasets to calibrate these models. Additionally, hybrid models have long simulation times, which can preclude the use of parameter estimation for calibration. Simulation of contaminant transport that does not account for preferential flow through conduits or extremely permeable zones in any approach is ill-advised. Simulation results in other karst aquifers or other parts of the FAS may differ from the comparison demonstrated herein.

Florida↗

USGS2013

Chemistry and age of groundwater in bedrock aquifers of the Piceance and Yellow Creek watersheds, Rio Blanco County, Colorado, 2010-12

Fourteen monitoring wells completed in the Uinta and Green River Formations in the Piceance Creek and Yellow Creek watersheds in Rio Blanco County, Colorado, were sampled for chemical, isotopic, and groundwater-age tracers to provide information on the overall groundwater quality, the occurrence and distribution of chemicals that could be related to the development of underlying natural-gas reservoirs, and to better understand groundwater residence times in the flow system. Methane concentrations in groundwater ranged from less than 0.0005 to 387 milligrams per liter. The methane was predominantly biogenic in origin, although the biogenic methane was mixed with thermogenic methane in water from seven wells. Three BTEX compounds (benzene, toluene, and ethylbenzene) were detected in water from six of the wells, but none of the concentrations exceeded Federal drinking-water standards. The presence of thermogenic methane in the aquifers indicates a connection and vulnerability to chemicals in deeper geologic units. Helium-4 data indicate that groundwater had ages ranging from less than 1,000 years to greater than 50,000 years. The presence of old groundwater in parts of the aquifers indicates that these aquifers may not be useful for large-scale water supply because of low recharge rates.

Colorado↗

USGS2014

Quality of groundwater in the Denver Basin aquifer system, Colorado, 2003-5

Groundwater resources from alluvial and bedrock aquifers of the Denver Basin are critical for municipal, domestic, and agricultural uses in Colorado along the eastern front of the Rocky Mountains. Rapid and widespread urban development, primarily along the western boundary of the Denver Basin, has approximately doubled the population since about 1970, and much of the population depends on groundwater for water supply. As part of the National Water-Quality Assessment Program, the U.S. Geological Survey conducted groundwater-quality studies during 2003–5 in the Denver Basin aquifer system to characterize water quality of shallow groundwater at the water table and of the bedrock aquifers, which are important drinking-water resources. For the Denver Basin, water-quality constituents of concern for human health or because they might otherwise limit use of water include total dissolved solids, fluoride, sulfate, nitrate, iron, manganese, selenium, radon, uranium, arsenic, pesticides, and volatile organic compounds. For the water-table studies, two monitoring-well networks were installed and sampled beneath agricultural (31 wells) and urban (29 wells) land uses at or just below the water table in either alluvial material or near-surface bedrock. For the bedrock-aquifer studies, domestic- and municipal-supply wells completed in the bedrock aquifers were sampled. The bedrock aquifers, stratigraphically from youngest (shallowest) to oldest (deepest), are the Dawson, Denver, Arapahoe, and Laramie-Fox Hills aquifers. The extensive dataset collected from wells completed in the bedrock aquifers (79 samples) provides the opportunity to evaluate factors and processes affecting water quality and to establish a baseline that can be used to characterize future changes in groundwater quality. Groundwater samples were analyzed for inorganic, organic, isotopic, and age-dating constituents and tracers. This report discusses spatial and statistical distributions of chemical constituents and evaluates natural and human-related processes that affect water quality. Findings are synthesized to assess the vulnerability of the Denver Basin aquifer system to groundwater contamination. The chemistry of groundwater samples collected from the water-table wells was generally different from that of samples collected from the bedrock-aquifer wells. Samples from the water-table wells tended to have higher concentrations of total dissolved solids and most major ions. Concentrations of several constituents with potential human-health concerns, including nitrate, selenium, uranium, and arsenic, decreased with depth and were highest in samples from the water-table wells. Exceedances of drinking-water standards and water-quality benchmarks were more frequently associated with shallow groundwater samples; concentrations of total dissolved solids and sulfate exceeded water-quality benchmarks for about half or more of samples from the water-table wells. The sediments and rocks of the Denver Basin are natural sources of the trace elements selenium, uranium, and arsenic, which affect their concentrations in groundwater. Detections of organic contaminants, which are typically indicative of human sources of contamination to groundwater, were more frequent in samples from the water-table wells. Pesticide compounds and volatile organic compounds were detected in 33 and 62 percent, respectively, of water-table well samples. Detected organic contaminant concentrations were much less than the associated drinking-water standards. Samples collected from the bedrock aquifers had lower concentrations of total dissolved solids than did samples collected from the water-table wells, although within the bedrock-aquifer samples, concentrations increased from the Dawson to Denver to Arapahoe to Laramie-Fox Hills aquifers. Concentrations of total dissolved solids and many constituents varied spatially and with depth in the bedrock aquifers, likely as a result of ion-exchange and oxidation-reduction reactions, which are important processes affecting water quality. Major-ion chemistry generally evolved from a calcium-bicarbonate to calcium-sulfate composition, with some sodium-bicarbonate and sodium-sulfate facies in the deeper bedrock aquifers, likely resulting from longer residence times and more extensive water-rock interaction. Oxidation-reduction conditions generally evolved from oxic at the water table to anoxic with increasing depth in the bedrock aquifers. Most samples from the bedrock aquifers were anoxic. Exceedances of drinking-water standards and water-quality benchmarks for the bedrock aquifers occurred in 1 percent or less of samples for nitrate, selenium, or arsenic; there were no exceedances for uranium. Exceedances for total dissolved solids, sulfate, manganese, and iron were generally between about 10 and 20 percent for the bedrock-aquifer samples. Radon concentrations, which were only measured in samples collected from two of the bedrock aquifers, exceeded the lower proposed drinking-water standard for more than 90 percent of samples but exceeded the higher alternative standard for less than 5 percent of samples. Pesticide compounds and volatile organic compounds were detected in 3 and 22 percent, respectively, of bedrock-aquifer samples, all at concentrations that were that were much less than drinking-water standards. Water-quality data were synthesized to evaluate factors that affect spatial and depth variability in water quality and to assess aquifer vulnerability to contaminants from geologic materials and those of human origin. The quality of shallow groundwater in the alluvial aquifer and shallow bedrock aquifer system has been adversely affected by development of agricultural and urban areas. Land use has altered the pattern and composition of recharge. Increased recharge from irrigation water has mobilized dissolved constituents and increased concentrations in the shallow groundwater. Concentrations of most constituents associated with poor or degraded water quality in shallow groundwater decreased with depth; many of these constituents are not geochemically conservative and are affected by geochemical reactions such as oxidation-reduction reactions. Groundwater age tracers provide additional insight into aquifer vulnerability and help determine if young groundwater of potentially poor quality has migrated to deeper parts of the bedrock aquifers used for drinking-water supply. Age-tracer results were used to group samples into categories of young, mixed, and old groundwater. Groundwater ages transitioned from mostly young in the water-table wells to mostly mixed in the shallowest bedrock aquifer, the Dawson aquifer, to mostly old in the deeper bedrock aquifers. Although the bedrock aquifers are mostly old groundwater of good water quality, several lines of evidence indicate that young, contaminant-bearing recharge has reached shallow to moderate depths in some areas of the bedrock aquifers. The Dawson aquifer is the most vulnerable of the bedrock aquifers to contamination, but results indicate that the older (deeper) bedrock aquifers are also vulnerable to groundwater contamination and that mixing with young recharge has occurred in some areas. Heavy pumping has caused water-level declines in the bedrock aquifers in some parts of the Denver Basin, which has the potential to enhance the transport of contaminants from overlying units. Results of this study are consistent with the existing conceptual understanding of aquifer processes and groundwater issues in the Denver Basin and add new insight into the vulnerability of the bedrock aquifers to groundwater contamination.

Colorado↗

USGS2010

Effects of Changes in Irrigation Practices and Aquifer Development on Groundwater Discharge to the Jobos Bay National Estuarine Research Reserve near Salinas, Puerto Rico

Since 1990, about 75 acres of black mangroves have died in the Jobos Bay National Estuarine Research Reserve near Salinas, Puerto Rico. Although many factors can contribute to the mortality of mangroves, changes in irrigation practices, rainfall, and water use resulted in as much as 25 feet of drawdown in the potentiometric surface of the aquifer in the vicinity of the reserve between 1986 and 2002. To clarify the issue, the U.S. Geological Survey, in cooperation with the Puerto Rico Department of Natural and Environmental Resources, conducted a study to ascertain how aquifer development and changes in irrigation practices have affected groundwater levels and groundwater flow to the Mar Negro area of the reserve. Changes in groundwater flow to the mangrove swamp and bay from 1986 to 2004 were estimated in this study by developing and calibrating a numerical groundwater flow model. The transient simulations indicate that prior to 1994, high irrigation return flows more than offset the effect of reduced groundwater withdrawals. In this case, the simulated discharge to the coast in the modeled area was 19 million gallons per day. From 1994 through 2004, furrow irrigation was completely replaced by micro-drip irrigation, thus eliminating return flows and the simulated average coastal discharge was 7 million gallons per day, a reduction of 63 percent. The simulated average groundwater discharge to the coastal mangrove swamps in the reserve from 1986 to 1993 was 2 million gallons per day, compared to an average simulated discharge of 0.2 million gallons per day from 1994 to 2004. The average annual rainfall for each of these periods was 38 inches. The groundwater discharge to the coastal mangrove swamps in the Jobos Bay National Estuarine Research Reserve was estimated at about 0.5 million gallons per day for 2003-2004 because of higher than average annual rainfall during these 2 years. The groundwater flow model was used to test five alternatives for increasing groundwater discharge to the coastal mangrove swamps to approximately 1.4 million gallons per day: (1) artificially recharging the aquifer with injection wells or (2) by increasing irrigation return flow by going back to furrow irrigation; (3) termination of groundwater withdrawals near the mangroves; (4) reduction of groundwater withdrawals at irrigation wells by 50 percent; and (5) a combination of alternatives 2 and 4 increasing irrigation return flows and decreasing irrigation withdrawals. Each alternative assumed average climatic conditions and groundwater withdrawals at 2004 rates. Alternative 1 required 1.5 million gallons per day of injected water. Alternative 2 required flooding 958 acres with a rate of 1.84 million gallons per day if no crops are grown. Alternative 3 required the termination of 2.44 million gallons per day of withdrawals to achieve 1.34 million gallons per day of discharge to the mangroves. Alternative 4 did not achieve the objective with only 0.80 million gallons per day simulated discharge to the mangroves, while requiring a 1.26 million gallon per day reduction in groundwater withdrawals. Alternative 5 required flooding fields with additional 1.13 million gallons of day and the same reduction in groundwater withdrawals, but did achieve the objective of about 1.4 million gallons per day discharge to the mangroves. Alternative 1, incorporating injection wells near the reserve required the least amount of water to raise groundwater levels and maintain discharge of 1.4 million gallons per day through the mangroves.

Scientific Investigations Report↗

USGS2008

Pathogen and chemical transport in the karst limestone of the Biscayne aquifer: 1. Revised conceptualization of groundwater flow

The Biscayne aquifer is a highly transmissive karst limestone that serves as the sole source of drinking water to over two million residents in south Florida. The aquifer is characterized by eogenetic karst, where the most transmissive void space can be an interconnected, touching‐vug, biogenically influenced porosity of biogenic origin. Public supply wells in the aquifer are in close proximity to lakes established by surface mining. The mining of the limestone has occurred to the same depths as the production wells, which has raised concerns about pathogen and chemical transport from these surface water bodies. Hydraulic and forced gradient tracer tests were conducted to augment geologic and geophysical studies and to develop a hydrogeologic conceptual model of groundwater flow and chemical transport in the Biscayne aquifer. Geologic and geophysical data indicate multiple, areally extensive subhorizontal preferential flow zones of vuggy limestone separated by rock with a matrix pore system. The hydraulic response from an aquifer test suggests that the Biscayne aquifer behaves as a dual‐porosity medium; however, the results of the tracer test showed rapid transport similar to other types of karst. The tracer test and concurrent temperature logging revealed that only one of the touching‐vug flow zones dominates transport near the production wells. On the basis of the rising limb of the breakthrough curve, the dispersivity is estimated to be less than 3% of the tracer travel distance, which suggests that the fastest flow paths in the formation are likely to yield limited dilution of chemical constituents.

Water Resources Research↗

USGS2010

Vulnerability of deep groundwater in the Bengal Aquifer System to contamination by arsenic

Shallow groundwater, the primary water source in the Bengal Basin, contains up to 100 times the World Health Organization (WHO) drinking-water guideline of 10g l 1 arsenic (As), threatening the health of 70 million people. Groundwater from a depth greater than 150m, which almost uniformly meets the WHO guideline, has become the preferred alternative source. The vulnerability of deep wells to contamination by As is governed by the geometry of induced groundwater flow paths and the geochemical conditions encountered between the shallow and deep regions of the aquifer. Stratification of flow separates deep groundwater from shallow sources of As in some areas. Oxidized sediments also protect deep groundwater through the ability of ferric oxyhydroxides to adsorb As. Basin-scale groundwater flow modelling suggests that, over large regions, deep hand-pumped wells for domestic supply may be secure against As invasion for hundreds of years. By contrast, widespread deep irrigation pumping might effectively eliminate deep groundwater as an As-free resource within decades. Finer-scale models, incorporating spatial heterogeneity, are needed to investigate the security of deep municipal abstraction at specific urban locations. ?? 2010 Macmillan Publishers Limited. All rights reserved.

Nature Geoscience↗

USGS2014

Groundwater studies: principal aquifer surveys

In 1991, the U.S. Congress established the National Water-Quality Assessment (NAWQA) program within the U.S. Geological Survey (USGS) to develop nationally consistent long-term datasets and provide information about the quality of the Nation’s streams and groundwater. The USGS uses objective and reliable data, water-quality models, and systematic scientific studies to assess current water-quality conditions, to identify changes in water quality over time, and to determine how natural factors and human activities affect the quality of streams and groundwater. NAWQA is the only non-regulatory Federal program to perform these types of studies; participation is voluntary. In the third decade (Cycle 3) of the NAWQA program (2013–2023), the USGS will evaluate the quality and availability of groundwater for drinking supply, improve our understanding of where and why water quality is degraded, and assess how groundwater quality could respond to changes in climate and land use. These goals will be addressed through the implementation of a new monitoring component in Cycle 3: Principal Aquifer Surveys.

Fact Sheet↗

USGS2022

Composite regional groundwater hydrographs for selected principal aquifers in New Mexico, 1980–2019

Groundwater is an important source of water for New Mexico. An estimated 48 percent of the total water used comes from groundwater sources, and groundwater levels generally are declining over large areas of New Mexico. Groundwater levels are affected by local and regional recharge or discharge processes. Groundwater hydrographs show the history of groundwater-level changes at a well. A single hydrograph is not necessarily representative of the larger regional area; however, individual hydrographs from several wells can be combined into a composite hydrograph to show average groundwater changes for a regional area. The U.S. Geological Survey, in cooperation with the New Mexico Office of the State Engineer, has been measuring groundwater levels in a network of wells since about 1925. Although groundwater levels in the statewide well network have been measured at various frequencies, most wells have been measured in 5-year cycles since about 1980. The composite hydrographs in this report were developed to show groundwater-level changes for selected principal aquifers in New Mexico. Composite hydrographs were developed using wells in the Colorado Plateaus aquifers, the High Plains aquifer, the Pecos River Basin alluvial aquifer, the Rio Grande aquifer system, and the Roswell Basin aquifer system. Statewide, groundwater levels generally have declined or remained steady over the time period in aquifers analyzed for this study. The largest water-level declines occurred in the Colorado Plateaus and High Plains aquifers and in the Rio Grande aquifer system (north-central New Mexico), where median water-level declines ranged from 17 to 40 feet and mean water-level declines ranged from 3.8 to 32 feet. Groundwater-level declines (or rises) were generally smaller in other areas of New Mexico.

New Mexico↗

USGS2018

Groundwater quality in the Floridan aquifer system, Southeastern United States

Groundwater provides nearly 50 percent of the Nation’s drinking water. To help protect this vital resource, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water (Burow and Belitz, 2014). The Floridan aquifer system constitutes one of the important aquifer systems being evaluated.

Alabama, Florida, Georgia, South Carolina↗

USGS2022

Groundwater quality in the Surficial Aquifer System, Southeastern United States:

Groundwater provides nearly 50 percent of the Nation’s drinking water. To help protect this vital resource, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water (Burow and Belitz, 2014). The surficial aquifer system constitutes one of the important aquifer systems being evaluated.

Alabama, Florida, Georgia, South Carolina↗