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Evaluating Sources of Groundwater Quality Variability in Residential Water Wells for Pre-Drill Sampling

Abstract

Significant concern has been expressed regarding the potential impact of shale gas extraction on surrounding drinking water resources. However, determining whether changes in groundwater chemistry (e.g., methane, salts, etc.) are natural in origin or caused by drilling operations can be difficult, particularly when different sampling and analytical methodologies are employed. To better understand the sources of variability in water quality in residential water wells in NE Pennsylvania, an area of active Marcellus shale gas extraction, two field studies were completed at nine residential water wells to evaluate the significance of purge volume and the sample collection technique on water quality results. In the majority of wells, the variability of methane concentrations associated with different purge volumes (0.5 gallons up to 3 casing volumes) was less than ±30%. The comparison of sample collection techniques showed that open-system and closed-system sample collection methods yield similar results in the low range of methane concentrations (e.g., <10 mg/L). However, in the higher range of methane concentrations, open-system sample collection methods yield markedly lower dissolved hydrocarbon gas concentrations than closed-system methods. Findings from these field programs improve our understanding of the inherent variability in pre- and post-drill results and offer insight into methods for improving sample collection protocols and data interpretation.