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A new USGS study, in cooperation with the Pennsylvania Geological Survey, identifies factors affecting the quality of groundwater used for domestic supply within the Marcellus Shale footprint in north-central and north-east Pennsylvania using a combination of spatial, statistical, and geochemical modeling.

Untreated groundwater, sampled during 2011–17 from 472 domestic wells in seven Pennsylvania counties (Potter, Clinton, Lycoming, Bradford, Sullivan, Wayne, and Pike), exhibited wide ranges in chemical quality.

  • Groundwater quality in the Delaware River watershed, in the eastern part of the study area where Marcellus gas has not been developed, was similar to that in the Susquehanna River watershed in the western part of the study area where natural gas production from the Marcellus Shale has been ongoing since 2008.
  • Natural variations in aquifer lithology combined with water-quality evolution along flow paths generally account for the observed water quality.
  • Sodium-enriched waters, which were mostly from shale and siltstone aquifers, had the greatest frequency of elevated pH (>8.5) and elevated concentrations of total dissolved solids (>250 milligrams per liter [mg/L]), bromide (>0.15 mg/L), methane (>7.0 mg/L), and lithium (>60 micrograms per liter).
  • Geochemical models indicate these characteristics could result from progressive mineral dissolution combined with cation exchange, plus mixing with locally important salinity sources, including Appalachian Basin brine and (or) road-deicing salt.
  • Most samples having elevated methane were from shale aquifers, which were present mainly in the Susquehanna River watershed and which also had the greatest density of gas wells compared to other lithologies.
  • Samples having elevated methane were also observed in the Delaware River watershed and other areas outside gas development.
  • Groundwater methane could be derived from microbial gas mixed with thermogenic gas, of deeper origin, that may have undergone degradation and (or) fractionation during migration. We propose the elevated methane results from “natural” sources of thermogenic and biogenic gas, and not from stray gas associated with development of natural gas.
Photograph of 2 people sampling a spring
Dennis Risser and Kyle Ohnstad sampling a spring in Salt Spring State Park, Pennsylvania (Charles A. Cravotta, III, USGS).

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