Targeted sampling reveals localized brine signals without evidence of regional water quality impacts from...

Unconventional oil and gas development (UOGD) in the Marcellus Shale of southwestern Pennsylvania has raised persistent concerns regarding groundwater quality. Previous work identified potential regional associations between UOGD activity and elevated salinity in heavily drilled counties based on samples collected between 2008 and 2018. Here, we collected 97 water samples from private wells, springs, and streams in 2024 to evaluate whether such associations persist and to assess whether impacts, if present, are regional or highly localized. Sampling targeted previously identified chloride hotspots, decommissioned wastewater impoundments, and recent spills, with comparisons to control locations. Major ions, trace metals, dissolved gases, isotopes, and organic compounds were analyzed and compared with drinking water standards, historical datasets, and proximity to UOGD features while accounting for geologic, hydrogeologic, and topographic controls. Most samples met drinking-water standards, and statistical comparisons revealed no robust regional differences between treatment and control samples after Benjamini-Hochberg correction. Fixed-effects models showed no evidence of widespread groundwater degradation attributable to UOGD. Among spill sites sampled, only one groundwater sample exhibited elevated salinity and brine-associated constituents. Geochemical, isotopic, and topographic evidence suggests this anomaly reflects localized influence from produced water or natural Appalachian Basin brine migrating along deeper flow paths rather than surface transport. Our results indicate that contamination patterns are best explained by highly localized rather than region-wide problems, consistent with rare and localized detectable impacts within the targeted sampling framework used here and highlighting challenges in defining appropriate controls given hydrogeologic heterogeneity. This work emphasizes combining regional screening with targeted, hydrogeology-informed monitoring to assess UOGD-related groundwater contamination.