The Origin of Dissolved Gas in Shallow Aquifers Overlying the San Juan Basin's Ignacio Blanco Gas Field Near Bayfield, La Plata County, Colorado

Abstract Revised rules governing coalbed methane well spacing in the San Juan Basin now include new regulations requiring environmental monitoring of shallow groundwater. This paper provides a case study illustrating how a variety of data, collected as part of a long-term environmental baseline sampling and analysis program, can be used to interpret the origin of dissolved methane in groundwater. Results indicate that carbon isotopic analysis of dissolved methane alone is not always sufficient to characterize the origin of shallow gases in the San Juan Basin; deuterium analysis of methane and carbon isotopic analysis of CO2 may sometimes be necessary. When placed in the context of both local physical and chemical hydrology and local and regional geology, this information documents the origin, transport, accumulation, and secondary alteration of biogenic methane.