STREAM MONITORING FOR EVALUATING THERMOGENIC METHANE MIGRATION ASSOCIATED WITH UNCONVENTIONAL GAS DEVELOPMENT

Shale gas development has enhanced energy production but remains controversial because of potential environmental impacts on water resources. In some cases, deep shale gas development has been linked with methane migration into shallow groundwater. Monitoring of methane (CH₄) within individual watersheds has generally been limited to domestic-supply wells, which may not be situated along predominant groundwater flow paths. A new technique has demonstrated that CH₄ concentrations can be measured in gaining streams to estimate groundwater impacts that may be caused by unconventional gas development. The technique utilizes measured stream CH₄ concentrations, groundwater inflow, and gas transfer coefficients in a 1-D gas stream-reach mass balance model to estimate groundwater CH₄ concentrations and fluxes. Gas tracer experiments in both Utah (Nine-Mile Creek) and North Carolina (West Bear Creek) demonstrated that injected CH₄ persisted downstream at the kilometer scale. It is assumed that stream methane eventually exsolves to the atmosphere as a greenhouse gas. But in the West Bear Creek experiment, injected krypton indicated that a significant fraction of the downstream decline was from microbial oxidation of CH₄. A pilot study in the Marcellus Formation shale-gas play in northern Pennsylvania used stream sampling in 15 watersheds as a reconnaissance tool for identifying areas with CH₄-laden groundwater discharge. Detailed geochemical characterization of one stream with high dissolved CH₄ (Sugar Run), including hydrocarbon concentrations/isotopes and noble gas concentrations, is consistent with a thermogenic Marcellus Formation gas source. Stream transport modeling indicates CH₄ is discharging along a 4-km reach of Sugar Run at about 1 kg d^-1. Due to the coalescing of flow paths in gaining streams, such monitoring provides an integrated signal of groundwater quality. The approach offers the first watershed-scale method for identifying locations where deep shale gas is migrating into shallow freshwater resources. While it has only been applied thus far in the Appalachian Basin (Eastern U.S.), it has global applicability and will be of interest to researchers and citizens in any country where unconventional shale-gas extraction is either ongoing or under consideration.