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Peter B McMahon

Peter is a Research Hydrologist with the USGS Colorado Water Science Center in Lakewood, CO.

Pete McMahon’s current research interests include understanding the effects of hydrocarbon production activities on groundwater quality and identifying natural and human processes that create regional- and national-scale patterns in the chemistry of groundwater. He uses geochemical, isotope, noble-gas, and groundwater-age tracers to understand the origin and fate of chemicals of concern in groundwater.
 

Education and Certifications

  • Ph.D., 1990, Geology, University of South Carolina at Columbia

  • M.A., 1984, Geology, University of Texas at Austin

  • B.S., 1981, Geology, University of Missouri at Columbia
     

Science and Products

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Two hands hold red oxidized soil

Oxidation/Reduction (Redox)

The redox state of groundwater—whether the groundwater is oxic (oxidized) or anoxic (reduced)—has profound implications for groundwater quality. Knowing the redox conditions of groundwater can help determine whether it contains elevated levels of many contaminants, including arsenic, nitrate, and even some manmade contaminants.
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Water Resources Mission Area, National Water Quality Program
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Oxidation/Reduction (Redox)

The redox state of groundwater—whether the groundwater is oxic (oxidized) or anoxic (reduced)—has profound implications for groundwater quality. Knowing the redox conditions of groundwater can help determine whether it contains elevated levels of many contaminants, including arsenic, nitrate, and even some manmade contaminants.
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Girl Drinking Water

Groundwater Quality in Principal Aquifers of the United States, 1991—2010

A new U.S. Geological Survey (USGS) report summarizes the major findings of national and regional assessments of groundwater quality in 40 Principal Aquifers in the United States. Knowledge of contaminant occurrence and the factors that control contaminant concentrations can ensure the availability and quality of this vital natural resource in the future.
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Ecosystems Mission Area, Contaminant Biology, Environmental Health Program, Toxic Substances Hydrology
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Groundwater Quality in Principal Aquifers of the United States, 1991—2010

A new U.S. Geological Survey (USGS) report summarizes the major findings of national and regional assessments of groundwater quality in 40 Principal Aquifers in the United States. Knowledge of contaminant occurrence and the factors that control contaminant concentrations can ensure the availability and quality of this vital natural resource in the future.
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USGS science for a changing world logo

Developing the next generation of USGS resource assessments

Resource assessments constitute a key part of the USGS mission, and represent a crucial contribution toward Department of the Interior (DOI) and broader Federal objectives. Current USGS energy and mineral assessment methods evaluate total technically recoverable resources (energy) or economically exploitable materials (minerals); the fiscal year 2010 budget for this work is $82M. To help stakehol
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John Wesley Powell Center for Analysis and Synthesis
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Developing the next generation of USGS resource assessments

Resource assessments constitute a key part of the USGS mission, and represent a crucial contribution toward Department of the Interior (DOI) and broader Federal objectives. Current USGS energy and mineral assessment methods evaluate total technically recoverable resources (energy) or economically exploitable materials (minerals); the fiscal year 2010 budget for this work is $82M. To help stakehol
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Photo of pivot irrigation in progress in the High Plains Aquifer region.

NAWQA High Plains Regional Groundwater Study

As part of the National Water Quality Assessment Program (NAWQA), the USGS has evaluated ground-water quality in the High Plains aquifer system. Beginning in 1999 and continuing for a period of 6 years, the High Plains Regional Groundwater Study intensively investigated the quality of groundwater resources within the study area. Water quality impairment coupled with water-level declines focus...
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Colorado Water Science Center
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NAWQA High Plains Regional Groundwater Study

As part of the National Water Quality Assessment Program (NAWQA), the USGS has evaluated ground-water quality in the High Plains aquifer system. Beginning in 1999 and continuing for a period of 6 years, the High Plains Regional Groundwater Study intensively investigated the quality of groundwater resources within the study area. Water quality impairment coupled with water-level declines focus...
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Cache la Poudre River

NAWQA South Platte River Basin Study

The South Platte River Basin study, conducted as part of the U.S. Geological Survey's National Water-Quality Assessment Program, combines information on water chemistry, physical characteristics, stream habitat, and aquatic life to provide science-based insights for current and emerging water issues in surface waters (streams, rivers, reservoirs) and groundwaters of the South Platte River Basin...
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Colorado Water Science Center
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NAWQA South Platte River Basin Study

The South Platte River Basin study, conducted as part of the U.S. Geological Survey's National Water-Quality Assessment Program, combines information on water chemistry, physical characteristics, stream habitat, and aquatic life to provide science-based insights for current and emerging water issues in surface waters (streams, rivers, reservoirs) and groundwaters of the South Platte River Basin...
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Oil well annular cement and casing damage data in mature oil fields undergoing hydraulic fracturing, South Belridge and Lost Hills Oil Fields, California

This data release contains oil well annular cement and casing damage data from parts of the South Belridge and Lost Hills Oil Fields in western Kern County, California. The study area coincides with where groundwater with less than 10,000 milligrams per liter total dissolved solids and outside an exempt aquifer are located overlying oil-producing zones in these fields. In the study areas, hydrauli
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California Water Science Center

Groundwater elevation data and models in and around select California oil fields

This data release contains groundwater elevation data and the associated model/code used to create groundwater elevation surfaces in and around select oil fields in California. The data and model provided here are intended to supplement interpretative manuscripts. Specific study areas will be added as Child Items found in this data release. Groundwater elevations are usually collected from individ
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California Water Science Center

Data for Distribution of Groundwater Age in Aquifers Used for Public Supply in the Continental United States, 2004 - 2017 (Version 1.1: June 2022)

This data release documents seven tables that contain environmental tracer data and lumped parameter model (LPM) results that are used for assessing the distribution of groundwater age in 21 Principal Aquifers of the continental United States. Groundwater samples were collected from 1,279 sites and analyzed for environmental tracers: tritium, tritiogenic helium-3, sulfur hexafluoride, carbon-14, a
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California Water Science Center

Geochemical and Geospatial Data for Per- and Polyfluoroalkyl Substances (PFAS) in Groundwater Used As a Source of Drinking Water in the Eastern United States

Concentrations of inorganic constituents, dissolved organic carbon (DOC), tritium, per- and polyfluoroalkyl substances (PFAS), volatile organic compounds (VOCs), and pharmaceuticals were measured in groundwater samples collected from 254 wells in 2019 and 2020. Concentrations of inorganic constituents, DOC, VOCs, and pharmaceuticals were measured at the U.S. Geological Survey (USGS) National Wate
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Water Resources Mission Area, Environmental Health Program

Quality-Control Data for Volatile Organic Compounds and Environmental Sulfur-Hexafluoride Data for Groundwater Samples from the Williston Basin, USA

In 2018, groundwater samples were collected from aquifers in the Williston Basin in parts of eastern Montana, western North Dakota, and northwestern South Dakota. This dataset includes quality-control data for volatile organic compounds that include data for source-solution blanks and field blanks. The dataset also includes data for sulfur hexafluoride in environmental samples of groundwater.
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Water Resources Mission Area

Data for Fluoride Occurrence in United States Groundwater

Data from 38,105 wells were used to characterize fluoride occurrence in untreated United States groundwater. The data were retrieved from the U.S. Geological Survey (USGS) National Water Information System (NWIS). Groundwater samples were collected from 1988 to 2017 in the conterminous United States. Data for groundwater included in this dataset are pH, water temperature, and concentrations of dis
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Colorado Water Science Center

Data Release for Hydrocarbons in Upland Groundwater, Marcellus Shale Region, Northeastern Pennsylvania and Southern New York, USA

Water samples from 50 domestic wells located 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. Methane (CH4) -isotope and pre-drill CH4 data indicate one prox
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Water Resources Mission Area, National Water Quality Program

Data for 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 microgram/L) in United States (U.S.) groundwater. Elevated manganese and dissolved organic carbon (DOC) concentrations were associated with shallow water tables and organic-carbon rich soils, suggesting soil-derived DOC supported manganese redu
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Water Resources Mission Area, National Water Quality Program

Well characteristics, water quality and age-date tracer data for wells along two regional flow paths in the Memphis aquifer, southwest Tennessee

In 2013, water-quality and age-date tracer samples were collected at wells located along two flow paths in the Memphis aquifer, southwest Tennessee, as part of the National Water Quality Assessment (NAWQA) Project of the National Water-Quality Program to better understand the vulnerability of the aquifer to contamination.. Each flow path began in the aquifer outcrop area (shallow well depths) and
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Lower Mississippi-Gulf Water Science Center

Data from 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 United States were sampled to identify which aquifers contained high methane concentrations (greater than 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.
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Water Resources Mission Area, National Water Quality Program

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

Groundwater samples were collected from domestic and public-supply wells in the Eagle Ford study area in 201516, in the Fayetteville study area in 2015, and in the Haynesville study area in 201415. One sample of produced water was collected from a gas well in the Haynesville  Shale in Rusk County, Texas in 2010, and 5 samples of produced water were collected from oil and condensate wells in the Ea
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Water Resources Mission Area, National Water Quality Program

Produced water chemistry for samples from four petroleum wells, southern San Joaquin Valley, California, 2014

The U.S. Geological Survey (USGS) in cooperation with the California State Water Resources Control Board collected produced water samples from four petroleum wells in the southern San Joaquin Valley on November 5, 2014. This digital dataset contains the site information, analyzing laboratories and methods, and water chemistry and quality control results for these samples. Water chemistry results i
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California Water Science Center
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Multiple-well monitoring site within the Poso Creek Oil Field, Kern County, California

IntroductionThe Poso Creek Oil Field is one of the many fields selected for regional groundwater mapping and monitoring by the California State Water Resources Control Board as part of the Oil and Gas Regional Monitoring Program (RMP; California State Water Resources Control Board, 2015, 2022b; U.S. Geological Survey, 2022a). The U.S. Geological Survey (USGS), in cooperation with the California St
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Rhett R. Everett, Peter B. McMahon, Michael J. Stephens, Janice M. Gillespie, Mackenzie M. Shepherd, Nicole C. Fenton
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Water Resources Mission Area, California Water Science Center

Groundwater quality near the Montebello Oil Field, Los Angeles County, California

Groundwater quality and potential sources and migration pathways of chemical constituents associated with hydrocarbon-bearing formations were assessed by the U.S. Geological Survey for the California State Water Resources Control Board Oil and Gas Regional Monitoring Program (RMP). Groundwater samples were collected as part of the RMP from 21 preexisting wells used for public supply, monitoring, o
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Jennifer S. Stanton, Michael Land, Matthew K. Landon, David H. Shimabukuro, Peter B. McMahon, Tracy A. Davis, Andrew G. Hunt, Theron A. Sowers
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Water Resources Mission Area, Geology, Geophysics, and Geochemistry Science Center, California Water Science Center

Quality of groundwater used for public supply in the continental United States: A comprehensive assessment

The presence of contaminants in a source water can constrain its suitability for drinking. The quality of groundwater used for public supply was assessed in 25 principal aquifers (PAs) that account for 84% of groundwater pumped for public supply in the U.S. (89.6 million people on a proportional basis). Each PA was sampled across its lateral extent using an equal-area grid, typically with 60 wells

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Kenneth Belitz, Miranda S. Fram, Bruce D. Lindsey, Paul Stackelberg, Laura M. Bexfield, Tyler D. Johnson, Bryant Jurgens, James A. Kingsbury, Peter B. McMahon, Neil M. Dubrovsky
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Water Resources Mission Area, California Water Science Center, Colorado Water Science Center, New Mexico Water Science Center

Over a third of groundwater in USA public-supply aquifers is Anthropocene-age and susceptible to surface contamination

The distribution of groundwater age is useful for evaluating the susceptibility and sustainability of groundwater resources. Here, we compute the aquifer-scale cumulative distribution function to characterize the age distribution for 21 Principal Aquifers that account for ~80% of public-supply pumping in the United States. The aquifer-scale cumulative distribution function for each Principal Aquif
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Bryant Jurgens, Kirsten Faulkner, Peter B. McMahon, Andrew Hunt, Gerolamo C. Casile, Megan B. Young, Kenneth Belitz
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Water Resources Mission Area, California Water Science Center, Colorado Water Science Center

Occurrence of water and thermogenic gas from oil-bearing formations in groundwater near the Orcutt Oil Field, California, USA

Study regionSanta Barbara County, California, USA.Study focusTo analyze a wide array of newly collected chemical, isotopic, dissolved gas, and age dating tracers in conjunction with historical data from groundwater and oil wells to determine if water and/or thermogenic gas from oil-bearing formations have mixed with groundwater in the Orcutt Oil Field and surrounding area.New hydrological insights
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Robert Anders, Matthew K. Landon, Peter B. McMahon, Justin T. Kulongoski, Andrew Hunt, Tracy Davis
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Water Resources Mission Area, California Water Science Center

Perfluoroalkyl and polyfluoroalkyl substances in groundwater used as a source of drinking water in the eastern United States

In 2019, 254 samples were collected from five aquifer systems to evaluate per- and polyfluoroalkyl substance (PFAS) occurrence in groundwater used as a source of drinking water in the eastern United States. The samples were analyzed for 24 PFAS, major ions, nutrients, trace elements, dissolved organic carbon (DOC), volatile organic compounds (VOCs), pharmaceuticals, and tritium. Fourteen of the 24

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Peter B. McMahon, Andrea K. Tokranov, Laura M. Bexfield, Bruce D. Lindsey, Tyler D. Johnson, Melissa Lombard, Elise Watson
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Water Resources Mission Area, Environmental Health Program, California Water Science Center, Colorado Water Science Center, New England Water Science Center, New Mexico Water Science Center, Pennsylvania Water Science Center

Predicting regional fluoride concentrations at public and domestic supply depths in basin-fill aquifers of the western United States using a random forest model

A random forest regression (RFR) model was applied to over 12,000 wells with measured fluoride (F) concentrations in untreated groundwater to predict F concentrations at depths used for domestic and public supply in basin-fill aquifers of the western United States. The model relied on twenty-two regional-scale environmental and surficial predictor variables selected to represent factors known to c
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Celia Z Rosecrans, Kenneth Belitz, Katherine Marie Ransom, Paul E. Stackelberg, Peter B. McMahon
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Water Resources Mission Area, California Water Science Center, Colorado Water Science Center, New England Water Science Center, New York Water Science Center

Noble gas signatures constrain oil-field water as the carrier phase of hydrocarbons occurring in shallow aquifers in the San Joaquin Basin, USA

Noble gases record fluid interactions in multiphase subsurface environments through fractionation processes during fluid equilibration. Water in the presence of hydrocarbons at the subsurface acquires a distinct elemental signature due to the difference in solubility between these two fluids. We find the atmospheric noble gas signature in produced water is partially preserved after hydrocarbons pr
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Ruta Karolyte, Peter H. Barry, Andrew Hunt, Justin T. Kulongoski, R. L. Tyne, Tracy Davis, Michael Wright, Peter B. McMahon, C. J. Ballentine
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Water Resources Mission Area, California Water Science Center

Relative risk of groundwater-quality degradation near California (USA) oil fields estimated from 3H, 14C, and 4He

Relative risks of groundwater-quality degradation near selected California oil fields are estimated by examining spatial and temporal patterns in chemical and isotopic data in the context of groundwater-age categories defined by tritium and carbon-14. In the Coastal basins, western San Joaquin Valley (SJV), and eastern SJV; 82, 76, and 0% of samples are premodern (pre-1953 recharge), respectively;
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Peter B. McMahon, Matthew K. Landon, Tracy Davis, Michael Wright, Celia Z. Rosecrans, Robert Anders, Michael Land, Justin T. Kulongoski, Andrew Hunt
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Water Resources Mission Area, California Water Science Center, Colorado Water Science Center, Geology, Minerals, Energy, and Geophysics Science Center

Geochemistry and age of groundwater in the Williston Basin, USA: Assessing potential effects of shale-oil production on groundwater quality

Thirty water wells were sampled in 2018 to understand the geochemistry and age of groundwater in the Williston Basin and assess potential effects of shale-oil production from the Three Forks-Bakken petroleum system (TBPS) on groundwater quality. Two geochemical groups are identified using hierarchical cluster analysis. Group 1 represents the younger (median 4He = 21.49 × 10−8 cm3 STP/g), less chem
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Peter B. McMahon, Joel M. Galloway, Andrew Hunt, Kenneth Belitz, Bryant Jurgens, Tyler D. Johnson
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Water Resources Mission Area, California Water Science Center, Colorado Water Science Center

Origin and geochemistry of formation waters from the lower Eagle Ford Group, Gulf Coast Basin, south central Texas

The lower Eagle Ford Group (LEFG) is one of the most productive continuous hydrocarbon plays in the United States but few associated produced waters data and minimal interpretation have been published. This effort focuses on results from compositional and isotopic data from 39 produced water samples collected from horizontal wells producing from the LEFG in south central Texas. The depth of the LE
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Mark A Engle, Colin A. Doolan, Janet K. Pitman, Matthew S. Varonka, Jessica Chenault, William H. Orem, Peter B. McMahon, Aaron M. Jubb
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Water Resources Mission Area, Energy Resources Program, Central Energy Resources Science Center, Colorado Water Science Center, Geology, Energy & Minerals Science Center, Brine Research Instrumentation and Experimental (BRInE) Laboratory

Fluoride occurrence in United States groundwater

Data from 38,105 wells were used to characterize fluoride (F) occurrence in untreated United States (U.S.) groundwater. For domestic wells (n = 11,032), water from which is generally not purposely fluoridated or monitored for quality, 10.9% of the samples have F concentrations >0.7 mg/L (U.S. Public Health Service recommended optimal F concentration in drinking water for preventing tooth decay) (8
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Peter B. McMahon, Craig J. Brown, Tyler D. Johnson, Kenneth Belitz, Bruce D. Lindsey
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Water Resources Mission Area, California Water Science Center, Colorado Water Science Center, New England Water Science Center, Pennsylvania Water Science Center
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Bakken Shale unconventional oil and gas production has not caused widespread hydrocarbon contamination to date in groundwater used for water supply

A new USGS study reports that shale-oil and -gas production from a major production area in Montana, North Dakota, and South Dakota has not caused...

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A comprehensive assessment of fluoride in groundwater

Fluoride occurs commonly in groundwater used for drinking, but typically at concentrations less than the drinking-water standard for human health...

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Marcellus Shale natural-gas production not currently causing widespread hydrocarbon contamination of domestic wells that tap shallow upland groundwater

A new USGS study reports that shale-gas production in northern Pennsylvania has not currently caused widespread hydrocarbon contamination in the...

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Unconventional Oil and Gas Production Not Currently Affecting Drinking Water Quality

Decades or longer may be needed to fully assess the effects of unconventional oil and gas production on the quality of groundwater used for drinking...

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