Geology topics

Peter B. McMahon

Publications and source records attributed to Peter B. McMahon.

At least 37 records · Page 2Linked to original sources

USGS2019

Occurrence and sources of radium in groundwater associated with oil fields in the southern San Joaquin Valley, California

Geochemical data from 40 water wells were used to examine the occurrence and sources of radium (Ra) in groundwater associated with three oil fields in California (Fruitvale, Lost Hills, South Belridge). 226Ra+228Ra activities (range=0.010-0.51 Bq/L) exceeded the 0.185 Bq/L drinking-water standard in 18% of the wells (not drinking-water wells). Radium activities were correlated with TDS concentrations (p<0.001, ρ=0.90, range=145-15,900 mg/L), Mn+Fe concentrations (p<0.001, ρ=0.82, range=<0.005-18.5 mg/L), and pH (p<0.001, ρ=-0.67, range=6.2-9.2), indicating Ra in groundwater was influenced by salinity, redox, and pH. Ra-rich groundwater was mixed with up to 45% oil-field water at some locations, primarily infiltrating through unlined disposal ponds, based on Cl, Li, noble-gas, and other data. Yet 228Ra/226Ra ratios in pond-impacted groundwater (median=3.1) differed from those in oil-field water (median=0.51). PHREEQC mixing calculations and spatial geochemical variations suggest the Ra in oil-field water was removed by co-precipitation with secondary barite and adsorption on Mn-Fe precipitates in the near-pond environment. The saline, organic-rich oil-field water subsequently mobilized Ra from downgradient aquifer sediments via Ra-desorption and Mn/Fe-reduction processes. This study demonstrates that infiltration of oil-field water may leach Ra into groundwater by changing salinity and redox conditions in the subsurface rather than by mixing with a high-Ra source.

California↗

USGS2019

Groundwater quality and hydrology with emphasis on selenium mobilization and transport in the Lower Gunnison River Basin, Colorado, 2012–16

Dissolved selenium is a contaminant of concern in the lower Gunnison River Basin, Colorado. Selenium is naturally present in the Cretaceous Mancos Shale and is leached to groundwater and surface water by irrigation. The groundwater on the east side of the Uncompahgre River in Delta and Montrose Counties is one of the primary sources of selenium concentration and load to surface water in the lower Gunnison River Basin. Although little information about the contribution of groundwater to surface water has been historically available, groundwater has often been implicated as an appreciable source of selenium to surface water. From 2012 to 2016, the U.S. Geological Survey, in cooperation with the Bureau of Reclamation, the Colorado Water Conservation Board, and the Gunnison Basin Selenium Management Program, established a 30-well groundwater-monitoring network on irrigated land to characterize the hydrology and groundwater quality of the shallow groundwater system on the east side of the Uncompahgre River in the lower Gunnison River Basin. The installation of the 30-well network and the data collected allowed for the development of a conceptual model of selenium mobilization and transport in the shallow groundwater system. Monitoring wells were completed in surficial deposits and in weathered Mancos Shale, which generally exhibited unconfined and confined conditions, respectively. Groundwater-quality monitoring provides information on the distribution of selenium and the geochemical processes controlling selenium concentrations in shallow groundwater. Monitoring wells were sampled between August 2013 and March 2015 to understand groundwater quality, seasonality, sources of recharge, and groundwater age. Concentrations of dissolved selenium ranged from below the limit of detection to 4,100 micrograms per liter (µg/L), with a median concentration of 14 µg/L. Concentrations showed a high degree of spatial variability and no seasonal difference. Similarly, no seasonal pattern was observed in specific conductance values of groundwater despite the considerably lower specific conductance value of irrigation water. Reduction-oxidation processes are important controls on selenium mobility. Nitrate derived from geologic material was a primary control on reduction-oxidation conditions in groundwater and inhibited selenium reduction to less mobile forms. Nitrate was reduced by denitrification in groundwater, but it was not reduced to the extent necessary to allow for selenium reduction. Groundwater ages were determined for groundwater samples from eight wells and ranged from 6 to 20 years old. Isotopic data indicate groundwater was recharged by irrigation water; no information collected supported an older, deeper source of recharge to the shallow groundwater system. Data on water level in all wells showed response to irrigation practices, but the response was delayed in some wells, which may be an indication of distance from recharge source.

Colorado↗

USGS2019

Hydrocarbons in upland groundwater, Marcellus Shale Region, Northeastern Pennsylvania and Southern New York, USA

Water samples from 50 domestic wells located <1 km (proximal) and >1 km (distal) from shale-gas wells in upland areas of the Marcellus Shale region were analyzed for chemical, isotopic, and groundwater-age tracers. Uplands were targeted because natural mixing with brine and hydrocarbons from deep formations is less common in those areas compared to valleys. CH 4 -isotope, predrill CH 4 -concentration, and other data indicate that one proximal sample (5% of proximal samples) contains thermogenic CH 4 (2.6 mg/L) from a relatively shallow source (Catskill/Lock Haven Formations) that appears to have been mobilized by shale-gas production activities. Another proximal sample contains five other volatile hydrocarbons (0.03–0.4 μg/L), including benzene, more hydrocarbons than in any other sample. Modeled groundwater-age distributions, calibrated to 3 H, SF 6 , and 14 C concentrations, indicate that water in that sample recharged prior to shale-gas development, suggesting that land-surface releases associated with shale-gas production were not the source of those hydrocarbons, although subsurface leakage from a nearby gas well directly into the groundwater cannot be ruled out. Age distributions in the samples span ∼20 to >10000 years and have implications for relating occurrences of hydrocarbons in groundwater to land-surface releases associated with recent shale-gas production and for the time required to flush contaminants from the system.

New York, Pennsylvania↗

USGS2019

Methane emissions from groundwater pumping in the USA

Atmospheric methane accumulation contributes to climate change, hence quantifying methane emissions is essential to assess and model the impacts. Here we estimate methane emissions from groundwater pumping in the Los Angeles Basin (LAB), north-eastern Pennsylvania, and the Principal aquifers of the USA using the average concentrations of methane in groundwater and annual groundwater pumping volumes. High average methane concentrations, 44.1 mg/L, and extensive groundwater pumping, ~3.1 x 1011 L/a in the LAB, result in the annual emission of ~2.9 x 10-3 Tg of microbial methane. Ethane emissions in the LAB were 3.5 x 10-6 Tg/a. Lower methane emissions estimated for NE Pennsylvania, ~3.0 x 10-8 Tg/a, reflect lower methane concentrations and groundwater pumping, 0.7 mg/L and 4.67 x 107 L/a, respectively. Methane concentrations and groundwater withdrawals, 1.06 x 1014 L, across the USA enabled the estimation of the total emissions of methane from Principal aquifers (92% of total pumping) of 0.044 Tg/a in the year 2000, which represents a small percentage (~0.2%) of the total annual US methane emissions, but a previously unquantified flux in the global methane budget. Globally, groundwater-pumping methane emissions were estimated to be 0.53 Tg/a, 0.2% of global methane emissions, by adopting a global estimate for groundwater extraction, and an average methane concentration in older groundwater of 0.44 mg/L.

Climate and Atmospheric Science↗

USGS2019

Groundwater quality of a public supply aquifer in proximity to oil development, Fruitvale Oil Field, Bakersfield, California

Due to concerns over the effects of oil production activities on groundwater quality in California, chemical, isotopic, dissolved gas and age-dating tracers were analyzed in samples collected from public-supply wells and produced-water sites in the Fruitvale oil field (FVOF). A combination of newly collected and historical data was used to determine whether oil formation fluids have mixed with groundwater used for public supply and what the potential pathways for the migration of oil formation fluids into groundwater may be. Stable isotopes of water (δ 2 H and δ 18 O) and age dating ( 3 H, 3 He trit , SF 6 and 14 C) tracers in groundwater samples were consistent with the Kern River being the main source of recharge to aquifers. The distribution of major ion concentrations and pH with distance from the Kern River indicate that natural processes were the primary controls on groundwater salinity. Two of 14 groundwater samples had δ 13 C-DIC values (−2.4 to +1.9 per mil) consistent with mixtures of <1 to about 9 percent oil-field water. Concentrations of TDS in groundwater samples were generally much lower (129–1,200 milligrams per liter (mg/l), median 216 mg/l) than produced water samples (586–24,930 mg/l, median 2,717 mg/l), suggesting that any mixing of oil-field water with groundwater has not significantly affected groundwater salinity. Trace concentrations of thermogenic methane were detected in three groundwater samples that did not have dissolved inorganic or isotopic indicators consistent with mixing of oil-field water, suggesting that stray gases may have migrated from the subsurface via preferential pathways such as leaky well bores into groundwater aquifers. Low concentrations of petroleum hydrocarbons were detected in samples that also contained anthropogenic VOCs and components of post- and pre-1950s recharge, indicating that petroleum hydrocarbons could have come from subsurface and/or surface sources. Overall, the results of this study indicated that groundwater currently used for public supply in the FVOF was of good quality with little, if any, effects from oil production activities. This may be due in part to the relatively rapid flushing of the aquifer system by recharge from the Kern River.

California↗

USGS2018

Elevated manganese concentrations in United States groundwater, role of land surface–soil–aquifer connections

Chemical data from 43 334 wells were used to examine the role of land surface–soil–aquifer connections in producing elevated manganese concentrations (>300 μg/L) in United States (U.S.) groundwater. Elevated concentrations of manganese and dissolved organic carbon (DOC) in groundwater are associated with shallow, anoxic water tables and soils enriched in organic carbon, suggesting soil-derived DOC supports manganese reduction and mobilization in shallow groundwater. Manganese and DOC concentrations are higher near rivers than farther from rivers, suggesting river-derived DOC also supports manganese mobilization. Anthropogenic nitrogen may also affect manganese concentrations in groundwater. In parts of the northeastern U.S. containing poorly buffered soils, ∼40% of the samples with elevated manganese concentrations have pH values < 6 and elevated concentrations of nitrate relative to samples with pH ≥ 6, suggesting acidic recharge produced by the oxidation of ammonium in fertilizer helps mobilize manganese. An estimated 2.6 million people potentially consume groundwater with elevated manganese concentrations, the highest densities of which occur near rivers and in areas with organic carbon rich soil. Results from this study indicate land surface–soil–aquifer connections play an important role in producing elevated manganese concentrations in groundwater used for human consumption.

Environmental Science & Technology↗

USGS2018

Groundwater chemistry and water-level elevations in bedrock aquifers of the Piceance and Yellow Creek watersheds, Rio Blanco County, Colorado, 2013–16

The Piceance and Yellow Creek watersheds in Rio Blanco County, Colorado, are known to contain important energy resources (oil shale and natural gas) and mineral resources (nahcolite). The primary sources of fresh groundwater in the Piceance and Yellow Creek watersheds are bedrock aquifers in the Uinta and Green River Formations. The aquifers are divided into an upper and lower aquifer separated by a regionally extensive semiconfining layer. These aquifers provide water to streams and springs in the watersheds and are an important resource to people living and working in the area. Development of energy and mineral resources has the potential to affect the quality of groundwater in several ways. The Bureau of Land Management and the U.S. Geological Survey began groundwater monitoring in 2010 to characterize the groundwater quality and water-level elevations of shallow bedrock aquifers in the Piceance and Yellow Creek watersheds. The purpose of this report is to present ground-water chemistry and water-level elevations collected during 2013–16. Comparisons are made to data that were collected from the bedrock aquifers from 2010 to 2012 to identify the potential for changes in water quality and water-level elevations. Appreciable changes in water-level elevations and hydraulic gradient were observed in early April 2015 in two wells completed in the upper and lower aquifers. The hydraulic gradient between the two wells was consistently downward from the upper aquifer to the lower aquifer during 2010–15; however, in early April 2015, the gradient changed from downward to upward between the two aquifers. Overall, water-level elevations declined by about 14 and 11 feet in the upper and lower aquifers, respectively, from 2013 to 2016. Previously published data estimated groundwater ages at 1,200 years old in the upper aquifer and 9,600 years old in the lower aquifer. These groundwater ages indicate that ground-water was recharged over thousands of years. With such long periods of time for aquifer recharge, declines in water-level elevation over short time steps (a few months) have important implications for sustainable management of this resource. Solution mining activities or drilling for oil and natural gas in the area could be related to the changes observed in water-level elevations in these wells; however, further investigation would be needed to evaluate causation. Changes in major-ion chemistry were evaluated in the bedrock aquifer using time series plots of select major-ion data from 2010 to 2016. Major-ion chemistry was variable for a single well from 2010 to 2016 where alkalinity and sulfate were the most variable constituents. One possible explanation for the observed changes in major-ion chemistry may be that the sample depth for that well no longer represents the most appreciable flow in the borehole. On a larger scale, potential changes in flow within the borehole may indicate changes in the regional flow system. Methane and volatile organic compound concentrations were evaluated using a similar approach to that of major ions and had similar findings. Methane concentrations in wells sampled from 2010 to 2016 were generally constant. The only exception was observed at a single well where the range of methane concentrations was from 57.4 (2010) to 4.02 milligrams per liter (2013). This is the same well where changes in water-level elevation, hydraulic gradient, and major-ion chemistry were observed, providing multiple lines of evidence to indicate change in the bedrock aquifers. Sampling of a well located in an area with little energy development but where faults or fractures could provide a path for the migration of fluids indicate mixing of groundwater between the upper and lower aquifers.

Colorado↗

USGS2018

Origin of methane and sources of high concentrations in Los Angeles groundwater

In 2014, samples from 37 monitoring wells at 17 locations, within or near oil fields, and one site >5 km from oil fields, in the Los Angeles Basin, California, were analyzed for dissolved hydrocarbon gas isotopes and abundances. The wells sample a variety of depths of an aquifer system composed of unconsolidated and semiconsolidated sediments under various conditions of confinement. Concentrations of methane in groundwater samples ranged from 0.002 to 150 mg/L—some of the highest concentrations reported in a densely populated urban area. The δ13C and δ2H of the methane ranged from −80.8 to −45.5 per mil (‰) and −249.8 to −134.9‰, respectively, and, along with oxidation‐reduction processes, helped to identify the origin of methane as microbial methanogenesis and CO2 reduction as its main formation pathway. The distribution of methane concentrations and isotopes is consistent with the high concentrations of methane in Los Angeles Basin groundwater originating from relatively shallow microbial production in anoxic or suboxic conditions. Source of the methane is the aquifer sediments rather than the upward migration or leakage of thermogenic methane associated with oil fields in the basin.

California↗

USGS2018

Regional patterns in the geochemistry of oil-field water, southern San Joaquin Valley, California, USA

Chemical and isotopic data for water co-extracted with hydrocarbons in oil and gas fields are commonly used to examine the source of the formation water and possible impacts on groundwater in areas of oil and gas development. Understanding the geochemical variability of oil-field water could help to evaluate its origin and delineate possible contamination of shallow aquifers in cases where oil-field water is released to the environment. Here we report geochemical and multiple isotope (H, C, O, Sr, Ra) data from 22 oil wells, three sources of produced water that are disposed of in injection wells, and two surface disposal ponds in four oil fields in the southern San Joaquin Valley, California (Fruitvale, Lost Hills, North and South Belridge). Correlations between Cl and δ 18 O, as well as other ions, and gradual increases in salinity with depth, indicate dilution of one or more saline end-members by meteoric water. The saline end-members, represented by deep samples (610 m–2621 m) in three oil-bearing zones, are characterized by Na Cl composition, near-seawater Cl concentrations (median 20,000 mg/L), enriched δ 18 O H 2 O (median 3.4‰), high ammonium(up to 460 mg-N/L), and relatively high radium activity ( 226 Ra+ 228 Ra = 12.3 Bq/L). The deepest sample has low Na/Cl (0.74), high Ca/Mg (5.0), and low 87 Sr/ 86 Sr (0.7063), whereas the shallower samples have higher Na/Cl (0.86–1.2), Ca/Mg near 1, and higher 87 Sr/ 86 Sr (∼0.7083). The data are consistent with an original seawater source being modified by various depth and lithology dependent diagenetic processes. Dilution by meteoric water occurs naturally on the east side of the valley, and in association with water-injectionactivities on the west side. Meteoric-water flushing, particularly on the east side, results in lower solute concentrations (minimum total dissolved solids 2730 mg/L) and total radium (minimum 0.27 Bq/L) in oil-field water, and promotes biodegradation of dissolved organic carbon and hydrocarbon gases like propane. Acetate concentrations and δ 13 C of dissolved inorganic carbon indicate biogenic methane production occurs in some shallow oil zones. Natural and human processes produce substantial variability in the geochemistry of oil-field water that should be considered when evaluating mixing between oil-field waters and groundwater. The variability could result in uncertainty as to detecting the potential source and impact of oil-field water on groundwater.

California↗

USGS2018

Methane in groundwater from a leaking gas well, Piceance Basin, Colorado, USA

Site-specific and regional analysis of time-series hydrologic and geochemical data collected from 15 monitoring wells in the Piceance Basin indicated that a leaking gas well contaminated shallow groundwater with thermogenic methane. The gas well was drilled in 1956 and plugged and abandoned in 1990. Chemical and isotopic data showed the thermogenic methane was not from mixing of gas-rich formation water with shallow groundwater or natural migration of a free-gas phase. Water-level and methane-isotopic data, and video logs from a deep monitoring well, indicated that a shale confining layer ~125 m below the zone of contamination was an effective barrier to upward migration of water and gas. The gas well, located 27 m from the contaminated monitoring well, had ~1000 m of uncemented annular space behind production casing that was the likely pathway through which deep gas migrated into the shallow aquifer. Measurements of soil gas near the gas well showed no evidence of methane emissions from the soil to the atmosphere even though methane concentrations in shallow groundwater (16 to 20 mg/L) were above air-saturation levels. Methane degassing from the water table was likely oxidized in the relatively thick unsaturated zone (~18 m), thus rendering the leak undetectable at land surface. Drilling and plugging records for oil and gas wells in Colorado and proxies for depth to groundwater indicated thousands of oil and gas wells were drilled and plugged in the same timeframe as the implicated gas well, and the majority of those wells were in areas with relatively large depths to groundwater. This study represents one of the few detailed subsurface investigations of methane leakage from a plugged and abandoned gas well. As such, it could provide a useful template for prioritizing and assessing potentially leaking wells, particularly in cases where the leakage does not manifest itself at land surface.

Colorado↗

USGS2017

Methane and benzene in drinking-water wells overlying the Eagle Ford, Fayetteville, and Haynesville Shale hydrocarbon production areas

Water wells ( n = 116) overlying the Eagle Ford, Fayetteville, and Haynesville Shale hydrocarbon production areas were sampled for chemical, isotopic, and groundwater-age tracers to investigate the occurrence and sources of selected hydrocarbons in groundwater. Methane isotopes and hydrocarbon gas compositions indicate most of the methane in the wells was biogenic and produced by the CO 2 reduction pathway, not from thermogenic shale gas. Two samples contained methane from the fermentation pathway that could be associated with hydrocarbon degradation based on their co-occurrence with hydrocarbons such as ethylbenzene and butane. Benzene was detected at low concentrations (<0.15 μg/L), but relatively high frequencies (2.4–13.3% of samples), in the study areas. Eight of nine samples containing benzene had groundwater ages >2500 years, indicating the benzene was from subsurface sources such as natural hydrocarbon migration or leaking hydrocarbon wells. One sample contained benzene that could be from a surface release associated with hydrocarbon production activities based on its age (10 ± 2.4 years) and proximity to hydrocarbon wells. Groundwater travel times inferred from the age-data indicate decades or longer may be needed to fully assess the effects of potential subsurface and surface releases of hydrocarbons on the wells.

Environmental Science & Technology↗

USGS2017

Fraction of young water as an indicator of aquifer vulnerability along two regional flow paths in the Mississippi embayment aquifer system, southeastern USA

Wells along two regional flow paths were sampled to characterize changes in water quality and the vulnerability to contamination of the Memphis aquifer across a range of hydrologic and land-use conditions in the southeastern United States. The flow paths begin in the aquifer outcrop area and end at public supply wells in the confined parts of the aquifer at Memphis, Tennessee. Age-date tracer (e.g. SF 6 , 3 H, 14 C) data indicate that a component of young water is present in the aquifer at most locations along both flow paths, which is consistent with previous studies at Memphis that documented leakage of shallow water into the Memphis aquifer locally where the overlying confining unit is thin or absent. Mixtures of young and old water were most prevalent where long-term pumping for public supply has lowered groundwater levels and induced downward movement of young water. The occurrence of nitrate, chloride and synthetic organic compounds was correlated to the fraction of young water along the flow paths. Oxic conditions persisted for 10 km or more down dip of the confining unit, and the presence of young water in confined parts of the aquifer suggest that contaminants such as nitrate-N have the potential for transport. Long-term monitoring data for one of the flow-path wells screened in the confined part of the aquifer suggest that the vulnerability of the aquifer as indicated by the fraction of young water is increasing over time.

Hydrogeology Journal↗

USGS2017

Brackish groundwater in the United States

For some parts of the Nation, large-scale development of groundwater has caused decreases in the amount of groundwater that is present in aquifer storage and that discharges to surface-water bodies. Water supply in some areas, particularly in arid and semiarid regions, is not adequate to meet demand, and severe drought is affecting large parts of the United States. Future water demand is projected to heighten the current stress on groundwater resources. This combination of factors has led to concerns about the availability of freshwater to meet domestic, agricultural, industrial, mining, and environmental needs. To ensure the water security of the Nation, currently [2016] untapped water sources may need to be developed. Brackish groundwater is an unconventional water source that may offer a partial solution to current and future water demands. In support of the national census of water resources, the U.S. Geological Survey completed the national brackish groundwater assessment to better understand the occurrence and characteristics of brackish groundwater in the United States as a potential water resource. Analyses completed as part of this assessment relied on previously collected data from multiple sources; no new data were collected. Compiled data included readily available information about groundwater chemistry, horizontal and vertical extents and hydrogeologic characteristics of principal aquifers (regionally extensive aquifers or aquifer systems that have the potential to be used as a source of potable water), and groundwater use. Although these data were obtained from a wide variety of sources, the compiled data are biased toward shallow and fresh groundwater resources; data representing groundwater that is at great depths and is saline were not as readily available. One of the most important contributions of this assessment is the creation of a database containing chemical characteristics and aquifer information for the known areas with brackish groundwater in the United States. Previously published digital data relating to brackish groundwater resources were limited to a small number of State- and regional-level studies. Data sources for this assessment ranged from single publications to large datasets and from local studies to national assessments. Geochemical data included concentrations of dissolved solids, major ions, trace elements, nutrients, and radionuclides as well as physical properties of the water (pH, temperature, and specific conductance). Additionally, the database provides selected well information (location, yield, depth, and contributing aquifer) necessary for evaluating the water resource. The assessment was divided into national-, regional-, and aquifer-scale analyses. National-scale analyses included evaluation of the three-dimensional distribution of observed dissolved-solids concentrations in groundwater, the three-dimensional probability of brackish groundwater occurrence, and the geochemical characteristics of saline (greater than or equal to 1,000 mg/L of dissolved solids) groundwater resources. Regional-scale analyses included a summary of the percentage of observed grid cell volume in the region that was occupied by brackish groundwater within the mixture of air, water, and rock for multiple depth intervals. Aquifer-scale analyses focused primarily on four regions that contained the largest amounts of observed brackish groundwater and included a generalized description of hydrogeologic characteristics from previously published work; the distribution of dissolved-solids concentrations; considerations for developing brackish groundwater resources, including a summary of other chemical characteristics that may limit the use of brackish groundwater and the ability of sampled wells producing brackish groundwater to yield useful amounts of water; and the amount of saline groundwater being used in 2010.

Professional Paper↗

USGS2017

Methane in aquifers used for public supply in the United States

In 2013 to 2015, 833 public supply wells in 15 Principal aquifers in the U.S. were sampled to identify which aquifers contained high methane concentrations (>1 mg/L) and determine the geologic, hydrologic, and geochemical conditions associated with high concentrations. This study represents the first national assessment of methane in aquifers used for public supply in the U.S. and, as such, advances the understanding of the occurrence and distribution of methane in groundwater nationally. Methane concentrations >1 and > 10 mg/L occurred in 6.7 and 1.1% of the samples, respectively. Most high concentrations occurred in aquifers in the Atlantic and Gulf Coastal Plain regions and upper Midwest. High methane concentrations were most commonly associated with Tertiary and younger aquifer sediments, old groundwater (>60 years), and concentrations of oxygen, nitrate-N, and sulfate <0.5 mg/L. Concentrations of methane were also positively correlated (p < 0.05) with dissolved organic carbon and ammonium. Case studies in Florida, Texas, and Iowa were used to explore how regional context from this data set could aid our understanding of local occurrences of methane in groundwater. Regional data for methane, Br/Cl ratios, sulfate, and other parameters helped identify mixing processes involving end members such as wastewater effluent-impacted groundwater, saline formation water, and pore water in glacial till that contributed methane to groundwater in some cases and supported methane oxidation in others.

Applied Geochemistry↗

USGS2016

Preliminary results from exploratory sampling of wells for the California oil, gas, and groundwater program, 2014–15

Introduction In 2014 and 2015, the U.S. Geological Survey (USGS) sampled water wells in the Los Angeles Basin and southern San Joaquin Valley, California, and oil wells in the San Joaquin Valley for analysis of multiple chemical, isotopic, and groundwater-age tracers. The purpose of this reconnaissance sampling was to evaluate the utility of tracers for assessing the effects of oil and gas production activities on groundwater quality in California. The study was done in cooperation with the California State Water Resources Control Board. Results of the study are intended to help design a regional groundwater-monitoring program to be implemented as part of California Senate Bill 4 (SB 4 statutes of 2013). The regional monitoring program plans to assess the effects of oil and gas production activities on groundwater quality and to provide a regional context for local monitoring of the groundwater-quality effects from well-stimulation treatments, which are techniques used to improve oil and gas production by increasing their rate of flow to the well. California SB 4 mandates that this local monitoring is to be done by oil-well operators in accordance with monitoring criteria established by the State Water Board. This report evaluates the utility of the chemical, isotopic, and groundwater-age tracers for assessing sources of salinity, methane, and petroleum hydrocarbons in groundwater overlying or near several California oil fields. Tracers of dissolved organic carbon in oil-field-formation water are also discussed. Tracer data for samples collected from 51 water wells and 4 oil wells are examined.

California↗

USGS2015

Chemical considerations for an updated National assessment of brackish groundwater resources

Brackish groundwater (BGW) is increasingly used for water supplies where fresh water is scarce, but the distribution and availability of such resources have not been characterized at the national scale in the United States since the 1960s. Apart from its distribution and accessibility, BGW usability is a function of the chemical requirements of the intended use, chemical characteristics of the resource, and treatment options to make the resource compatible with the use. Here, we discuss relations between these three chemical factors using national-scale examples and local case studies. In a preliminary compilation of BGW data in the United States, five water types accounted for the major-ion composition of 70% of samples. PHREEQC calculations indicate that 57–77% of samples were oversaturated with respect to barite, calcite, or chalcedony. In the study, 5–14% of samples had concentrations of arsenic, fluoride, nitrate, or uranium that exceeded drinking-water standards. In case studies of the potential use of BGW for drinking water, irrigation, and hydraulic fracturing, PHREEQC simulations of a hypothetical treatment process resembling reverse osmosis (RO) showed that BGW had the potential to form various assemblages of mineral deposits (scale) during treatment that could adversely affect RO membranes. Speciation calculations showed that most boron in the irrigation example occurred as boric acid, which has relatively low removal efficiency by RO. Results of this preliminary study indicate that effective national or regional assessments of BGW resources should include geochemical characterizations that are guided in part by specific use and treatment requirements.

Groundwater↗

USGS2015

The quality of our Nation's waters: Water quality in principal aquifers of the United States, 1991-2010

About 130 million people in the United States rely on groundwater for drinking water, and the need for high-quality drinking-water supplies becomes more urgent as our population grows. Although groundwater is a safe, reliable source of drinking water for millions of people nationwide, high concentrations of some chemical constituents can pose potential human-health concerns. Some of these contaminants come from the rocks and sediments of the aquifers themselves, and others are chemicals that we use in agriculture, industry, and day-to-day life. When groundwater supplies are contaminated, millions of dollars can be required for treatment so that the supplies can be usable. Contaminants in groundwater can also affect the health of our streams and valuable coastal waters. By knowing where contaminants occur in groundwater, what factors control contaminant concentrations, and what kinds of changes in groundwater quality might be expected in the future, we can ensure the availability and quality of this vital natural resource in the future.

Circular↗

USGS2015

The water-energy nexus: An earth science perspective

Water availability and use are closely connected with energy development and use. Water cannot be delivered to homes, businesses, and industries without energy, and most forms of energy development require large amounts of water. The United States faces two significant and sometimes competing challenges: to provide sustainable supplies of freshwater for humans and ecosystems and to ensure adequate sources of energy for future generations. This report reviews the complex ways in which water and energy are interconnected and describes the earth science data collection and research that can help the Nation address these important challenges. The earth sciences have been a cornerstone in developing our current understanding of the water-energy nexus. A full understanding of the nexus, however, is limited by uncertainty in our knowledge of fundamental issues, such as the quantity of freshwater that is available, the amount of water that is used in energy development, the effects that emerging energy development technologies have on water quality and quantity, and the amount of energy required to treat and deliver freshwater. Enhanced data collection and research can improve our understanding of these important issues and thereby lay the groundwork for informed resource management. Relevant earth science issues analyzed and discussed herein include freshwater availability; water use; ecosystems health; assessment of saline water resources; assessment of fossil-fuel, uranium, and geothermal resources; subsurface injection of wastewater and carbon dioxide and related induced seismicity; climate change and its effect on water availability and energy production; byproducts and waste streams of energy development; emerging energy-development technologies; and energy for water treatment and delivery.

Circular↗