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Aigler, Brent V. ¹ ; Ge, Shemin ² ; McCaffree, Charles A. ³

¹ Environmental Studies & INSTAAR, CU-Boulder
² Geological Sciences & INSTAAR, CU-Boulder
³ University of Colorado at Boulder

Mountainous watersheds present unique challenges to groundwater modeling due to spatial variations in climate and topography. The objective of this study is to quantify the hydrogeology of the Fraser River watershed, located in north-central Colorado, to understand its groundwater-resource sustainability. Outcrop study, limited grain-size analyses, and previous watershed research suggest that the late Oligocene and Miocene Troublesome Formation is composed chiefly of siltstone and contains aquifers of permeable, well sorted, medium to coarse grained sand and gravel interpreted as channel-fill deposits. A steady state groundwater-flow model of the Fraser River watershed was developed and calibrated to 24 observation wells and to baseflow of the Fraser River. The modeled hydraulic head distribution is similar to topography, and modeling results confirm quantitatively that surface recharge is the major source of groundwater in the watershed and varies as a function of elevation and precipitation. Groundwater discharges from the watershed through evapotranspiration and baseflow to rivers. Based on the steady state groundwater-flow model, future pumping scenarios in the Winter Park West well field are investigated with transient modeling to examine the groundwater resources of the Troublesome Formation aquifers. Drawdown from pumping simulations stabilized within months to years for various groundwater-withdrawal scenarios, suggesting a sustainable use of groundwater in the well field.