Hydrogeological analysis of a mountainous groundwater system: Fraser River Watershed, Grand County, Colorado

The purpose of this study is to quantify the hydrogeology of the Fraser River watershed, a mountainous drainage system in north-central Colorado. Groundwater withdrawal is concentrated in the channel-fill deposits of the late Oligocene and Miocene Troublesome Formation, deposited in the Fraser basin. Limited grain-size analyses suggest that the aquifers are composed chiefly of unconsolidated medium- to coarse-grained sand and pebble gravel. Aquifer hydraulic conductivity values calculated from multiwell pumping tests are approximately 1x10-4 m/s, one order of magnitude greater than the values from single-well slug tests. Numerical models developed to simulate pumping tests yield sand-and-gravel hydraulic conductivity values ranging from 1x10-5 m/s to 1x10-3 m/s. Laboratory measurements of samples of indurated siltstone of the Troublesome Formation produce conductivity values ranging from 2x10-10 m/s to 6x10 -10 m/s. The large variation range in hydraulic conductivity reflects the influence of varying lithology and the scale of tested volumes.;A steady state groundwater-flow model of the Fraser River watershed is developed and calibrated using 24 observation wells in the Fraser River valley and estimated baseflow of the Fraser River. Modeling results suggest that surface recharge is the major source of groundwater in the watershed and varies as a function of elevation. Groundwater exits the watershed through evapotranspiration and discharge to rivers. The hydraulic head distribution from the steady state model follows topography and is used as the initial condition for modeling groundwater-withdrawal scenarios. Drawdown from daily pumping from the thick sands and gravels of the Troublesome Formation stabilizes within one year for hydraulic conductivity of 1x10-5 m/s and 1x10-3 m/s. When the duration of pumping is doubled, the drawdown stabilizes within two to four years. The radius of the cone of depression varies in the range of hundreds of meters. Pumping wells withdraw approximately two and 15 percent of the groundwater flow through the well field for hydraulic conductivity of 1x10-3 m/s and 1x10-5 m/s, respectively. This study suggests that the groundwater system can sustain current and future withdrawals.