ESTIMATING HYDRAULIC PROPERTIES OF COMPLEXLY FRACTURED AND METALLICALLY MINERALIZED BEDROCK IN AN ALPINE WATERSHED: HANDCART GULCH, COLORADO

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.