RECONNAISSANCE OF WEATHERING RATES IN ALPINE CATCHMENTS OF CENTRAL COLORADO, USA

Headwater streams in central Colorado, USA, were sampled to provide a geochemical baseline and evaluate the effect of catchment lithology on stream-water chemistry. Annualized chemical weathering rates for each catchment were calculated using measured discharge and chemical analyses, and normalized to catchment area. Bedrock geology includes virtually all major rock types, but a focus of this study was to preferentially sample lithologically simple catchments. The primary anthropogenic disturbance is from hardrock mining. Catchments with historical mining had median annual weathering rates nearly four times greater than unmined catchments (20,000 vs. 5,400 kg/a-km²). Historical mining in some catchments has accelerated weathering rates by decreasing average grain size and exposing sulfide minerals to the weathering environment, creating acidic drainage. The median weathering rate for unmined catchments is lower than published rates, but the climate in this study area is drier and colder than the catchments described in the literature. In unmined areas, weathering rates were greatest in catchments dominated by Minturn Formation, a sedimentary unit that contains carbonate minerals and possibly gypsum. Igneous and metamorphic rock units had lower and variable chemical weathering rates. In an attempt to evaluate weathering mechanisms, Ge concentration in stream samples was determined by hydride generation-ICP-MS (detection limit 2 ng/L). Germanium concentrations of waters seem to be a better indicator of lithologic source, with the greatest median dissolved Ge in waters draining the Pikes Peak Granite (Ypp), a fluorine-rich granite. Greater Ge concentrations in these waters may be a result of F complexing. In addition, the role of mechanical weathering was evaluated by comparing chemical weathering rates for each catchment to the average catchment slope, which was determined from digital elevation models for each catchment. In general, weathering rates correlated positively with average catchment slope, with small but statistically significant correlation coefficients (r~0.4), in general agreement with other studies that have focused on single catchments and documented the role of mechanical weathering as a contributor to overall weathering rate.