LINKING GEOCHEMICAL AND HYDROLOGIC MODELS OF GROUND WATER IN TWO ROCKY MOUNTAIN CATCHMENTS: STRAIGHT CREEK, NM AND HANDCART GULCH, CO

An interdisciplinary approach utilizing geochemical and hydrological models has been initiated in two mineralized watersheds, Straight Creek, NM and Handcart Gulch, CO. Integration of geologic, hydrologic and geochemical components provides greater insight into processes controlling ground-water flow and composition. At Straight Creek, a mass balance approach of mineral weathering and solute evolution is combined with a water budget, and at Handcart Gulch the mass balance approach will be combined with a 3D finite element, coupled heat, mass, and fluid transport model. Detailed lithologic, mineralogic, structural, and hydrologic characterization of the study areas is required. Both catchments contain hydrothermally altered, pyritic lithologies that produce shallow acidic (pH 2.6-4.6) ground waters. In Straight Creek, 2.8 km ², ground-water data are obtained from eight alluvial wells (23 to 61 m. deep) and five bedrock wells (45 to 128 m. deep), and all waters are Ca-SO₄ type because of the ubiquitous occurrence of gypsum. For Handcart Gulch, 4.7 km², ground-water data are obtained from four deep boreholes (500 to 1100 m. deep) near and at the Continental Divide, along with nine shallower boreholes (3 to 50 m. deep) drilled adjacent to the stream. Deep ground waters at Straight Creek have circumneutral pH (6.5-7.8) and tend to be carbonate buffered. In contrast, the deepest ground waters sampled to date (100 and 200 m.) at Handcart Gulch are acidic (3.6-4.7). For Straight Creek, mass balance results in mmol/kg indicate the solute chemistry is dominated by dissolution of pyrite (6.9), gypsum (9.4), chlorite (1.1), kaolinite (0.6), albite (0.4), fluorite (0.25), sphalerite (0.14), and by precipitation of silica (4.6) and goethite (5.7). Preliminary solute fluxes in mmol/L/a for Straight Creek are SiO₂ (0.4-1.2), Ca (3-9), and SO₄ (100-350), using the age date for the most upgradient well as the upper bound for the age. These are among the fastest known rates of solute fluxes for a catchment because of the steep topography, strong pyrite mineralization, and ubiquitous occurrence of gypsum in the weathering zone.