Evaluation of Groundwater Resources in the Fraser River Watershed, Grand County, CO

Mountainous watersheds are characterized by complex geology and variable groundwater recharge.  Complicating factors in the Fraser River watershed, north-central Colorado, include groundwater withdrawals from private and municipal pumping wells and transbasin surface-water diversions.  The purpose of this study is to characterize the geology and hydrogeology of the Miocene Troublesome Formation, the primary aquifer of the watershed, and to develop a watershed-scale groundwater model to simulate long-term flow conditions.  Various pumping scenarios will be simulated to observe the effects of continued groundwater withdrawal on the sustainability of groundwater in the watershed.  Outcrop studies have revealed laterally discontinuous channel-fill deposits within a massive siltstone aquitard.  Pumping test and slug test data have produced upper and lower limits, respectively, on the hydraulic conductivity of these channel fills.  Hydraulic conductivity data from pumping tests are approximately 5.5x10^-5 m/s, whereas calculations from slug tests at two wells yield conductivities at 9.5x10^-6 m/s.  Laboratory tests of rock samples have quantified the hydraulic conductivity of siltstone, which is not represented by the screened intervals of pumping and monitoring wells, at approximately 3.4x10^-10 m/s.  Aquifer and aquitard units have hydraulic conductivities that fall within the reported ranges of sand and shale, respectively.  Soil-infiltration tests, conducted at various elevations and vegetation covers within the watershed, showed the near surface conductivities to be between 1.0x10^-5 and 1.4x10^-4 m/s.  Groundwater levels recorded throughout the watershed and obtained from the U.S. Geological Survey suggest that the general flow direction is from southeast to northwest, following the gradient of the Fraser River.  Preliminary modeling results show that composite groundwater table contours are similar to the topography of the watershed, and the Fraser River is a predominantly gaining water body from groundwater flow.