A new method for tracing seepage from coalbed natural gas holding ponds, Powder River Basin, Wyoming

Coalbed natural gas (CBNG) production from coal seam aquifers requires large volumes of co-produced water to be pumped from the subsurface. The co-produced water ranges from high quality that meets state and federal drinking water standards to low quality due to increased salinity and/or sodicity. Water quality generally decreases moving from the southeastern portion towards the center of the basin. Most produced water is disposed into impoundments and other surface drainages where it may infiltrate into shallow groundwater. Groundwater degradation caused by infiltration of CBNG co-produced water holding impoundments into arid soluble salt rich soils is an issue of immense importance because groundwater is a major source for stock water, irrigation, and drinking water for many small communities in these areas. This study examines the potential of using stable carbon isotope signatures of dissolved inorganic carbon (delta 13CDIC) to track the fate of CBNG co-produced water after it is discharged into the impoundments. Other geochemical proxies like the major cations, major anions and trace elements were used in conjunction with field water quality measurements to understand the geochemical differences between CBNG co-produced waters and ambient waters in the study area. Samples were collected from the CBNG discharge outfalls, co-produced water holding impoundments, and monitoring wells from different parts of the Powder River Basin and analyzed for delta13CDIC. The CBNG co-produced waters from outfalls and impoundments have positive delta13C DIC values that fall within the range of +12‰ to +21‰, distinct from the ambient regional surface and groundwaters with delta 13CDIC values ranging from -10‰ to -15‰. The results from our study demonstrate that these contrasting delta 13CDIC signatures can be used to trace seepage out of CBNG co-produced water holding impoundments into shallow groundwaters.