THE BIOGEOCHEMICAL AND ENVIRONMENTAL IMPLICATIONS OF NATURAL GAS SEEPS INTO HEADWATER STREAMS

Recent advances in natural gas extraction technologies have made many gas deposits commercially viable. The increased resource extraction has created well founded concern for impacts to water resources. In this study, we examine the impact of a natural gas seep into an Ozark head water stream. The study site is located in central Arkansas, in the Middle Atoka Formation (Pennsylvanian), a thick sequence (7.6 Km) of sandstones and gray shales with minor coal seams. Local gas development is within a mile of the site on both sides of the stream. The gas wells are believed to be vertical however they have been stimulated and are at relatively shallow depth <1.5 Km TVD. The reservoirs in the Atoka have been reported as being under pressure and may have experienced natural leakage. Water samples were taken over a 4 Km stream reach. Gas has been reported bubbling into the stream since at least 2011. Dissolved gas during sample was 3.5ug/L. Gas levels were below detection by 500 meters downstream. No increase in trace metals was over the stream reach. Flow increased over the reach for 0.17 m3/second to 0.27 m3/second. Chloride concentrations show a small increase from 2.20 to 2.35 mg/L at the leak site. Estimations of methane evolution to the atmosphere for the dissolved phase are ~19 Kg/year, excluding direct gas to atmosphere input. While the impacts to the biogeochemistry of the stream were negligible; the import of methane to the atmosphere from such leaks may rival biogenic sources i.e. wetlands, and anthropogenic sources, i.e. landfills, natural gas infrastructure, and as such may represent the important impact of such natural leaks.