Establishing baseline water quality for household wells within the Marcellus Shale gas region, Susquehanna County, Pennsylvania, U.S.A.

Abstract

Flowback fluids associated with hydraulic fracturing shale gas extraction are a potential source of contamination for shallow aquifers. In the Marcellus Shale region of northeastern Pennsylvania, certified water tests have been used to establish baseline water chemistry of private drinking water wells. This study investigates whether a single, certified multiparameter water test is sufficient for establishing baseline water chemistry from which possible future contamination by flowback waters could be reliably recognized. We analyzed the water chemistry (major and minor inorganic elements and stable isotopic composition) of multiple samples collected from lake, spring, and well water from 35 houses around Fiddle Lake, Susquehanna County, PA that were collected over approximately a two-year period. Statistical models estimated variance of results within and between households and tested for significant differences between means of our repeated measurements and prior certified water tests.Overall, groundwater chemistry varies more spatially due to heterogeneity of minerals within the bedrock aquifer and due to varying inputs of road salt runoff from paved roads than it does temporally at a single location. For wells located within road salt-runoff zones, Na+ and Cl− concentrations, although elevated, are generally consistent through repeated measurements. High acid neutralizing capacity (ANC) and base cation concentrations in well water sourced from mineral weathering reactions, and a uniform stable isotopic composition for well water, suggests long flowpaths for groundwater that dampen seasonal variability of most elements. Exceptions occur for two wells within road salt runoff zones that show the greatest range of concentrations for Na+ and Cl−, suggesting that these wells have a faster pathway to surficial recharge. Additionally, sampling protocols can induce variability for Fe, Mn, and Pb, making other elements identified in flowback fluids (Ba, Br, Ca, Cl, Mg, Na, Sr) more dependable indicators of contamination. Although there is general concordance between our repeated measurements and the certified test results, characterizing baseline chemistry is strengthened when results from multiple households are combined to establish regional upper baseline limits that will have a low probability of being exceeded by future samples unless conditions have changed.