HYDROGEOLOGIC, ISOTOPIC, AND GEOCHEMICAL ANALYSIS OF GROUNDWATER-SURFACE WATER INTERACTIONS ALONG THE FRUITLAND OUTCROP: THE BACKDROP FOR COALBED METHANE DEVELOPMENT

There remains considerable uncertainty about how coal bed methane (CBM) production from Southwestern Colorado’s Fruitland Outcrop may affect the quantity and quality of nearby surface water, springs, wetlands, and groundwater systems along the northwestern margin of the San Juan Basin (SJB) near Durango, CO. Despite the fact that the SJB is one of the highest-producing and most extensively studied CBM basins in the world (Fasset 2000; Snyder and Fabryka-Martin 2007), controversy remains over the age of Fruitland Formation waters, the extent to which the basin undergoes active hydrologic throughflow, as well as whether CBM production near the Outcrop will impact existing surface-groundwater interactions. In order to more closely characterize the Fruitland Outcrop hydrology near the edge of the basin, geochemical and isotopic properties of surface water and groundwater sources were monitored from 2008-2009 in neighboring La Plata and Archuleta counties for use as baseline hydrologic information in an area soon to see increased CBM drilling. Water samples were collected from existing CBM wells, domestic wells, piezometers, springs, and the Florida and Piedra Rivers and tributaries. Oxygen-18 signatures for surface water and groundwater sources (domestic wells, piezometers, springs) suggested recharge by a mixture of rain and snow. Oxygen-18 concentrations did not vary significantly based on position to Outcrop (up-gradient vs. in-Outcrop vs. down-gradient) for surface water or groundwater. Tritium concentrations in all water bodies other than CBM wells approximated precipitation levels (mean=6.55 TU). Tritium concentration relative to Outcrop position was not significantly different for surface water, but was significantly different for groundwater sources when comparing in-Outcrop samples to down-gradient samples. In-Outcrop groundwater produced slightly “older” tritium signals than down-gradient groundwater. Results suggest tighter surface-groundwater connections at the edge of the basin and more circulation than expected, and may indicate a system that is already over-drafted by existing CBM production and heavy domestic well use. These results suggest that increased CBM production near the outcrop will likely affect the local hydrology.