Hydrogeology of a coal-seam gas exploration area, southeastern British Columbia, Canada: Part 1. Groundwater flow systems

Discovery of high contents of methane gas in coals of the Mist Mountain For mation in the Elk River valley, southeastern British Columbia, Canada, has led to increased exploration activity for coal-seam gas (CSG). CSG producti on requires groundwater abstraction to depressurize the coal beds and to fa cilitate methane flow to the production wells. Groundwater abstraction will have hydrodynamic effects on the now system, and an understanding of the g roundwater flow system is needed to evaluate these effects. The purpose of this paper is to describe the groundwater flow system in the area by means of a groundwater flow model and interpretation of hydrochemical and isotopi c analyses of groundwater and surface water. Groundwater flow for the Weary Creek exploration area is modeled in two ver tical sections. The model domains, based on classic upland-lowland conceptu al flow models, are approximately 10,000 m long and 4,000 m deep. Each cons ists of a fixed water-table boundary and no-flow boundaries along the trace s of major faults. Steady-state groundwater flow is calibrated to hydraulic -head, streamflow, and groundwater-recharge data. Simulated steady-state ve locity fields define regional and local flow components consistent with the conceptual model. The results are consistent with regional trends in delta H-2, delta O-18, t ritium, and TDS, which define two distinct groundwater groups (A and B) and a third of intermediate composition. An active, shallow, local now compone nt (group A) is recharged in beds cropping out along subdued ridges; this c omponent discharges as seeps along lower and mid-slope positions in the sou thern part of the study area. The waters are tritiated, relatively enriched in delta H-2 and delta O-18, and have low TDS. A deeper regional flow comp onent (group B), which originates at a higher altitude and which discharges to the Elk River valley bottom, is characterized by non-tritiated groundwa ter with relatively depleted delta H-2 and delta O-18, and higher TDS. Groundwater contributes less than 10% of the total direct flow to the Elk R iver, as indicated by flow measurements and by the absence of group A and g roup B characteristics in the river water. Thus it is hypothesized that gro undwater extraction during CSG production will have little impact on the ri ver. The groundwater flow model developed in this work is used in a compani on gaper to further test this hypothesis.