STREAM METHANE MONITORING FOR EVALUATING GROUNDWATER IMPACTS ASSOCIATED WITH NATURAL GAS EXTRACTION - CONTINUED DEVELOPMENT AND APPLICATIONS

The baseflow of gaining streams can provide an integrated chemical signal for groundwater systems. It may be possible to evaluate impacts from natural gas drilling and hydraulic fracturing to regional aquifers by sampling such streams. The stream-reach methane (CH₄) mass balance method is one approach for establishing baseline groundwater quality prior to natural gas development and detecting subsequent contamination. This method has the advantage of using stream-integrated chemistry to indirectly monitor groundwater quality at the watershed scale, rather than relying on point values from monitoring wells. A tracer injection experiment in Nine-Mile Creek, a moderate-gradient stream in the Colorado Plateau Province of Utah, showed the downstream persistence of CH₄ for over 1.5 km downstream and yielded an “apparent” gas transfer velocity of 4.5 m/d. The “apparent” gas transfer velocity primarily describes CH₄ loss to the atmosphere, but may include microbial degradation. To evaluate the stream methane method in other climatic settings, a second experiment was conducted along West Bear Creek, a low-gradient stream in the Piedmont Province of North Carolina. The 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 applied through pilot-scale studies in the Marcellus shale-gas play of Pennsylvania by the USGS and Pennsylvania State University. Preliminary reconnaissance sampling has yielded stream methane concentrations of up 20 ppb (1300 nMol/L). Follow-up work along a gaining reach of Sugar Run using an un-calibrated stream-reach mass balance (with 1-D advective transport modeling) indicated inflowing groundwater with CH₄ concentrations of about 300 ppb (20,000 nMol/L); a gaining reach of Tunkhannock Creek showed a trend of ¹³C-CH₄ enrichment (from -55 to -39 permil), indicating a possible thermogenic source. Future work in both streams includes CH₄ isotopic fingerprinting to evaluate sources (biogenic or thermogenic) and tracer injections for quantifying gas transfer velocities.