2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 18
Presentation Time: 8:00 AM-12:00 PM


KAHN, Katherine Gurley, Department of Geological Sciences, University of Colorado, P.O. Box 399, 2200 Colorado Avenue, Boulder, CO 80309, CAINE, Jonathan Saul, U.S. Geol Survey, P.O. Box 25046, MS 973, Denver, CO 80225 and GE, Shemin, Department of Geological Sciences, University of Colorado at Boulder, Boulder, CO 80309, katherine.l.gurley@colorado.edu

The Handcart Gulch (HG) watershed is located approximately 6 km SSE of Montezuma, Colorado in the Rocky Mountain Front Range. It has an area of roughly 5 km2 and elevations range from approximately 3,300 to 3,900 m. Topographic relief is extreme, up to 400 m in 1 km. This unmined watershed, comprised of complex Proterozoic metamorphic and igneous rocks, contains an average of approximately 10% fine pyrite locally. Iron oxide cemented alluvium, known as ferricrete, has precipitated in and surrounding the main stream channel as a result of the pyrite-rich rocks contacting the atmosphere.  

The USGS has been collecting data in HG since 2003. Four deep wells converted from mineral exploration boreholes (>475 m deep) and 9 shallow wells (<55 m deep) penetrate the crystalline, ferricrete, and surficial materials. Aquifer hydraulic tests were conducted and analyzed using MODFLOW in 10 of the 13 wells to estimate saturated hydraulic conductivities, including a 610 m deep well drilled at the continental divide. Infiltration tests were performed on various types of exposed surficial geologic units in the study area to estimate saturated conductivities. A stilling well was installed near the bottom of the study area equipped with a level logger to measure stream discharge.  

Preliminary modeling results suggest saturated hydraulic conductivities ranging from 3x10-8 near the surface to 1x10-11 m/s at 610 m deep in crystalline rock and 9x10-6 m/s at the surface to 7x10-6 at 7.6 m deep in ferricrete using storativities of 1x10-5 and less. Laboratory analysis of unfractured crystalline rock in HG performed by the USGS indicates intrinsic permeability values on the order of 1x10-17 m2, which indicates that fracture networks control the flow system. Saturated hydraulic conductivities derived from infiltration tests range from 1x10-1 m/s in the rock glacier to 6.2x10-5 m/s in the ferricrete. Estimates of discharge are on the order of 5x10-2 m3/s. Analysis of these data in conjunction with existing precipitation data and evapotranspiration estimates provide insights into hydrologic systems found in high alpine/sub alpine environments and help illuminate key processes responsible for the liberation and transport of trace metals and acid waters.