A Groundwater Vulnerability Analysis of a Mountainous Watershed, Crested Butte, Colorado

The DRASTIC (Aller et. al. 1987) ground-water contaminant vulnerability model has commonly been applied in hydrogeologically and structurally uncomplicated basins with unconfined aquifer conditions; however, its use in more complex mountainous hydrogeologic environments with spatially variable aquifer confinement has been limited.  This study involved implementing a DRASTIC model for the East River watershed, a structurally complex mountainous catchment near Crested Butte, Colorado.  The DRASTIC methodology uses the vertical relationship between the hydrogeologic parameters of Depth to groundwater, net Recharge, Aquifer media, Soil media, Topography, Impact of the vadose zone, and hydraulic Conductivity of the aquifer to assign ratings of ground-water vulnerability to locations.  A GIS raster-based methodology was used to overlay the DRASTIC model components. Depth to groundwater data for unconfined aquifers was collected from State water well permit records; water depth data for confined aquifers was generated based on a subsurface hydrogeologic model constructed from geologic mapping.  Net recharge was calculated using  precipitation, soil type, evapotranspiration, and land use data.  Aquifer media were characterized from the geologic mapping and unit descriptions. Values for aquifer hydraulic conductivity were extracted from available water well permit records in combination with data from published hydrologic maps and literature.  Soil media and impact of the vadose zone media data were collected from published soil and geologic maps.  A 10-meter resolution digital elevation model (DEM) was used to calculate topographic slope. Following the DRASTIC methodology, vulnerability ratings were calculated and datasets and maps were generated to represent the vulnerability of groundwater to pollution within the East River watershed for use by the US Forest Service in their land-use management decision making.  This study demonstrates a methodology for implementing DRASTIC in complex mountainous hydrogeologic environments.