Exploring controls on halogen and methane occurrence in groundwater of New York State.

The expansion of unconventional oil and gas development (UOGD) by means of horizontal drilling and high-volume hydraulic fracturing has been accompanied by concerns regarding the potential impacts to water resources and public health. The ban on UOGD in New York State (NYS) allows the natural processes that control groundwater chemistry and dissolved methane to be evaluated without complications that could arise in regions impacted by prior unconventional shale gas development. We evaluated the controls on the occurrence and spatial variability of methane, chloride, bromine, and iodine, covering much of NYS and encompassing a range of underlying bedrock geologies. Groundwater samples were collected from 108 domestic and public supply wells. Methane concentrations in groundwater ranged from <0.001–84.6 mg/L. The variables most consistently associated with methane occurrence in groundwater included: 1) bedrock geology penetrated by wells, 2) groundwater chemical composition, and 3) confinement characterization of the well. The geochemical parameters iodine, chloride, and sodium/calcium suggested that elevated concentrations of methane were likely associated with deep brines. Elevated methane concentrations were frequently accompanied by both high bromine and iodine concentrations in this study, indicating that the potential of halogens as tracers for dissolved methane needs to be further investigated. • Groundwater quality surveyed (including methane and halogens) in New York State. • Halogens used to understand the occurrence of methane in shallow groundwater. • Shallow aquifers influenced by the natural migration of formation waters. [ABSTRACT FROM AUTHOR]