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Stray methane gas is the most common   elemental ratios and isotopic compositions   exposures and different environmental
            groundwater problem in areas of    can provide signatures of wastewater from   conditions, resulting in the identification
            Marcellus Shale development in     UOG production ( e.g., Akob et al., 2016;    of potentially toxic metals being mobi-
            Pennsylvania, followed by dissolved salts   Lauer et al., 2016;  McMahon et al., 2017) .  liz ed from black-shale drill-cuttings ( e.g.,
            from produced water ( Brantley et al.,   Residential water-supply wells in the   Stuckman et al., 2015) . Understanding
            2014) . Other contaminants linked to shale   vicinity of new oil and gas production   these processes will guide management of
            gas include metals, naturally occurring   wells are often sampled to provide pre-  these waste materials.
            radioactive materials ( NORM) , and   drilling information about water quality.
            organic compounds. Contaminants enter   The data are collected for liability reasons   SITE- BASED  PROJ ECTS
            surface water primarily through spills or   ( USEPA, 2016)  and are not well suited for   EX AMINING POTENTIAL
            leaks and infiltrate downward into shal-  interpreting sources of contamination   ENVIRONMENTAL IMPACTS OF
            low aquifers. No evidence supports aqui-  ( Table S2 [ see footnote 2] ) . Molofsky et al.   UOG D EVELOPMENT
            fer contamination by the upwelling of flu-  ( 2016)  assessed best practices for sam-  Access to UOG sites for environmental
            ids from production z ones ( e.g., Fisher and   pling, laboratory analysis methods, data   monitoring has been challenging for non-
            Warpinski, 2012;  Hammack et al., 2014;    management, and analysis protocols for   industry researchers ( Soeder, 2015) , but
            McMahon et al., 2017) .            residential water wells in areas of UOG   collaboration between academic, govern-
              Recent investigations have contributed   development.               ment, and industry researchers has been
            to a growing consensus that stray gas in                              improving. A multidisciplinary project
            aquifers results primarily from casing fail-  RESEARCH TO AD D RESS   begun in 2013 to examine potential envi-
            ures in older production wells, rather than   POTENTIAL IMPACTS OF LIQ UID    ronmental and human-health impacts of
            migration from z ones where hydraulic   AND  SOLID  W ASTES AND  SPILLS  UOG development, primarily in the
            fracturing was conducted in horiz ontal   Increases in UOG activities result in   Rocky Mountain region ( Table S3 [ see
            wells ( e.g., Brantley et al., 2014;  Lackey et   more environmental violations ( Kell,   footnote 2] ) , has produced more than 50
            al., 2017) . The challenges of understand-  2011)  and spills ( Lauer et al., 2016) . An   publications assessing air- and water-
            ing stray gas migration in the subsurface   11.3-million-liter spill of Bakken and   quality impacts;  wastewater treatment
            were illustrated by a test at the Borden   Three Forks produced water into a North   and re-use;  public health outcomes;  and
            groundwater research site in Ontario,   Dakota creek contained total dissolved   socio-political and economic factors asso-
            Canada. Methane was injected into the   solids ( TDS)  of 300 g per liter and high   ciated with UOG development. Potential
            well-characteriz ed, shallow sand aquifer,   concentrations of ammonium, barium,   water-resource risks have been assessed
            and migration was monitored spatially and   strontium, and radium ( Lauer et al., 2016;    near Marcellus Shale wells in Susquehanna
            temporally at high resolution ( Cahill et al.,   Coz z arelli et al., 2017) . Geochemical alter-  County, Pennsylvania, since 2015 ( Table
            2017) . The gas was transported in solution   ations in the stream persisted for at least   S4 [ see footnote 2] ) . Analyses of pro-
            by advection and diffusion and laterally in   six months after the spill, and fish kills   duced water and hydrocarbons from pro-
            the gas phase through interconnected lay-  were observed 7 km downstream of the   duction wells are providing signatures for
            ers of somewhat coarser sediments ( Fig. 1   spill site. Radium and strontium isotopic   potential contaminants like trace metals,
            # 7) . It persisted in the aquifer for more   signatures in downstream sediments   major ions, and hydrocarbons. Studies
            than a year, longer than expected.  resembled those from the spilled fluid   like these will provide insight into the
              Baseline water-quality data are needed   ( Coz z arelli et al., 2017) . Slow release of   natural spatial and temporal variation in
            to assess potential water-quality degrada-  spill-derived chemicals from sediment   water quality needed to detect impacts
            tion. Researchers from the USGS and   could provide a long-term contaminant   from UOG development.
            Northeast Midwest Institute investigated   source in aquatic ecosystems.  Field research projects involving part-
            decades of legacy water-quality data    Organic-rich shales were deposited in   nerships between DOE and industry are
            from the Susquehanna River Basin in   anoxic marine environments and contain   improving UOG-development technolo-
            Pennsylvania to determine if baseline con-  sulfide minerals, radionuclides, and   gies while reducing environmental and
            ditions prior to shale-gas development   reduced inorganic elements ( Chermak and   health impacts. The Hydraulic Fracturing
            could be determined ( Betanz o et al.,   Schreiber, 2014) . Hydraulic fracturing   Test Site ( HFTS)  in the Permian basin of
            2016) . Most of the existing water-quality   fluids often react with shale downhole,   Texas underwent environmental assess-
            monitoring sites were found in the lower   mobiliz ing inorganic compounds like bar-  ments before, during, and after develop-
            parts of the basin and established for   ium ( Renock et al., 2016)  or creating new   ment phases ( Fig. 1 # 1–6) . Air quality was
            nutrient and pesticide inputs to the   organic compounds that are found in the   monitored for methane, NO , and VOCs.
                                                                                                       x
            Chesapeake Bay. The data sets were not   produced fluids ( Kahrilas et al., 2016) .  Groundwater quality was monitored
            useful for assessing water-quality impacts   Horiz ontal drilling of a single shale well   within 4 km of production wells. Produced
            of shale-gas development in headwater   can generate several hundred tons of drill   water was analyz ed to evaluate potential
            streams. Impacts of UOG development on   cuttings, which may release harmful ele-  impacts to wellhead and casing integrity
            groundwater and surface-water quality   ments like arsenic, radium, and uranium   ( Table S5 [ see footnote 2] ) . New hydraulic
            can be difficult to distinguish from   ( Phan et al., 2015) . The leachability of   fracturing technologies were tested to
            impacts of septic systems and legacy coal   drill cuttings has been investigated in the   optimiz e hydrocarbon extraction effi-
            mining ( Messinger and Hughes, 2000) , but   laboratory under short-term and long-term   ciency. Preliminary findings indicate that


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