Methane occurs as a dissolved constituent in groundwater from confined aquifers in the Columbia River Basalt Group, Columbia basin, Washington. Isotopic compositions of methane in groundwater indicate that the methane is a mixture of biogenic (^dgr¹³C-CH₄ to -88^pmil and ^dgr²H-CH₄ to -265^pmil) and thermogenic (^dgr¹³C-CH₄ to -35^pmil, and ^dgr²H-CH₄ to -134^pmil) components. Chemical and isotopic data are consistent with entrainment of deep, coal-bed-generated methane in upwelling groundwater from below the Columbia River Basalt Group (>4 km) that mixes with near-surface groundwater. The areal distribution pattern of methane suggests that fault intersections are necessary for vert cal migration of deep methane through the basalt.

This study suggests that deep subbasalt coal-bed methane in the Columbia basin has infiltrated the shallow basalt groundwater system, and isotopic analyses of methane in groundwater from structurally favorable locations can be used to identify potential exploration targets. The wide areal distribution of methane in this large, relatively unexplored frontier province suggests economic gas reserves.