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In October 2011, samples (water and gas where available) were taken from 17 gas coalbed methane (CBM) and monitoring wells along the northwestern edge of the Powder River Basin in Montana and Wyoming. The University of Arizona group, whom I am closely working with, led this effort. Sampling constituted a transect from the edge of the basin where shallow groundwater interacts with the CBM system (~500 ft below the surface) into deep, more isolated CBM settings at depths exceeding 1500 ft. Along this transect lies an inferred biogeochemical gradient involving interaction of shallow, infiltrating groundwaters high in sulfate and thus favoring sulfate reduction and deep, sulfate-free methanogenic waters. Sampling was coordinated with two gas production companies, USGS, and the Montana Bureau of Mines and Geology.

Preliminary geochemical results for alkalinity and major anion concentrations imply that the hypothesized geochemical gradient was successfully captured both geographically and vertically in the sampling campaign.  The range of alkalinity concentrations (10.6-33.0 meq/L) implies that the samples bracket the inferred sulfate reduction-methanogenesis boundary. Likewise, the large range of carbon isotope ratios (δ¹³C) of dissolved inorganic carbon (-13.2 to 17.5 per mil) also encompass anticipated values for the progressive depletion of sulfate and generation of methane.  Many of the producing gas wells exhibit sulfate concentrations below instrumental detection (<0.5 mg/L).  However, we also documented producing CBM wells exhibiting detectable sulfate, implying that the onset of microbial sulfate production during mixing of shallow groundwater into the coal bed could preclude subsurface microbial methanogenesis and thus induce CBM production declines.  In addition to these clearly methanogenic conditions observed in production wells, we collected samples from monitoring wells too shallow and high in sulfate to be methanogenic. These provide a valuable point of comparison for understanding chemical or isotopic fingerprints of sulfate reduction vs. methanogenesis and interpreting microbiological characterization being conducted by colleagues at USGS and Montana State University. As with the results of the transect sampling, vertical sampling of three nested USGS monitoring wells also represents a gradient between high-sulfate (>1000 mg/L), nonmethanogenic waters interacting with coal in the shallow well, to apparently methane-rich and sulfate-free waters in the two deepest wells.

Additional analyses are pending. These include major and trace cation (metals) analysis, gas composition and isotope ratios, water mixing indicators (strontium and boron isotopes), and analysis of acetate, an important intermediate product in one methanogenesis pathway. 

An undergraduate student and my lab manager have been in change of analyzing the geochemistries of the waters collected.  The undergraduate will likely go into the field for the next (and last) round of sampling this spring.