Stream Methane Monitoring for Evaluating Groundwater Impacts Associated with Shale Gas Development

The baseflow of gaining streams can provide an integrated chemical signal for groundwater systems. It may be possible to evaluate impacts from unconventional gas development by sampling such streams. The stream-reach methane (CH₄) mass balance method is one approach for establishing baseline groundwater quality prior to development and detecting subsequent contamination. This method has the advantage of using gaining streams to monitor groundwater quality at the watershed scale, rather than the point scale provided by monitoring wells. A tracer injection in Nine-Mile Creek, Utah, showed the downstream persistence of CH₄ for over 1.5 km downstream, with an “apparent” (atmospheric loss plus any microbial degradation) gas transfer velocity of 4.5 m/d. In West Bear Creek, North Carolina, injected CH₄ persisted for more than 2.5 km downstream. A noble gas (krypton) was also injected in order to separate atmospheric CH₄ loss from any microbial degradation. Comparison of the gas transfer velocities determined from CH₄ and Kr will indicate whether microbial degradation is a significant process (work is in progress). These methods are now being tested in pilot-scale studies in the Marcellus shale-gas play of Pennsylvania. Preliminary reconnaissance sampling has yielded stream methane concentrations of up to 70 ppb (background is generally < 2 ppb). Follow-up work along Sugar Run using an uncalibrated mass balance with transport modeling suggests that inflowing groundwater may have CH₄ concentrations of up to 3000 ppb; sampling along Tunkhannock Creek showed ¹³C-CH₄ enrichment (from -55 to -39 permil), indicating a possible thermogenic source. These results illustrate the utility of the method for locating areas of anomalously high stream CH₄, but more detailed assessment is needed. Future work in both streams includes additional ¹³C-CH₄ and noble gas fingerprinting to evaluate methane sources (biogenic or thermogenic) and tracer injections for quantifying groundwater inflow and gas transfer velocities.