INSIGHTS INTO GROUNDWATER FLOW IN AN ALPINE WATERSHED PROVIDED BY A COUPLED HEAT, MASS, AND FLUID TRANSPORT MODEL, HANDCART GULCH, COLORADO

Handcart Gulch in the Colorado Rocky Mountain Front Range provides a unique opportunity to address fundamental questions regarding groundwater flow in mountain watersheds. The study area contains four deep boreholes (500 to 1100 m) near and at the Continental Divide, along with nine shallower boreholes drilled along the trunk stream. Because the watershed hosts an unmined mineral deposit with associated naturally acidic (pH 2.6-4.6) and metal-rich waters, the role of groundwater in transporting these constituents in an alpine setting is being investigated. Field activities performed in 2003 and 2004 include the collection of basic geologic data, a stream tracer-dilution study, geophysical borehole logging, and pumping tests. Ground and surface water samples were collected and analyzed for dissolved noble gases, 3H, CFCs, major ions, and multiple isotopes. Rock and drill core samples were also collected to characterize their mineralogy and geochemistry.

A 3D finite element, coupled heat, mass, and fluid transport model of the watershed was constructed. The model simulates steady-state, base-flow conditions, and was calibrated with head, temperature, and age data. Preliminary results include the following. (1) The bedrock aquifer can be modeled as an equivalent porous medium, with modeled head, temperature, and age values matching measured values to a first order. Modeling heat flow places important constraints on recharge. Formal parameter estimation will be performed to investigate the uniqueness of the modeled flow field. (2) Bedrock groundwater is a substantial component of the hydrologic system, with about 30% of recharge leaving the site in the subsurface. (3) The stream may not be a good integrator of all groundwater. A considerable amount of groundwater flows down-drainage below the stream, underneath lower-permeability ferricrete deposits that impede the upward movement of groundwater to the stream. (4) Temperature profiles in the deep boreholes suggest that active groundwater circulation does not exceed a depth of about 150 m. (5) Groundwater age data indicate an effective porosity of 0.005-0.01 for the bedrock. Eventually, the reactive transport module of the model will be used to simulate the mobilization and transport of metals and ultimately to constrain weathering rates.