Seasonal and Spatial Variation of Metal Loads from Natural Flows in the Upper Tenmile Creek Watershed, Montana

Seasonal and spatial variation of metal loads can be significant in mining-impacted mountain watersheds in the western US due to a number of complex physical and biogeochemical factors. Anthropogenic influences, such as seasonal water diversion for municipal supplies, can increase this variability further. This study evaluates the seasonal and spatial variation of metal loads from estimated natural flows in a typical Rocky Mountain watershed impacted by historic hardrock mining and municipal water diversions: the Upper Tenmile Creek Watershed near Helena, Montana. Restoration of natural flows is being considered as part of broader watershed restoration measures, and an understanding of the variation in loads resulting from these flows is needed for restoration planning and design. Estimates of tributary and metal point and nonpoint source natural flows to the creek were used with representative total metals (cadmium, copper, lead, and zinc) concentration data for model input and to evaluate the variation of input loads. These loads were evaluated at key locations in the watershed for two seasons: spring snow-melt high flow in June and summer low flow in August. The Water Quality Analysis Simulation Program was used to model the resulting variation of total and dissolved metal loads with distance along the mainstem for the two seasons. Results show that total metal loads to the creek vary among input locations by up to >9,700-fold in June and up to >740-fold in August for copper. Several tributaries have the greatest loads during both seasons, although adits often exhibit the highest concentrations. For all locations, average input loads are up to 46 times higher in June for copper. Total and dissolved metal loads generally increase with distance along the mainstem and vary by up to >320-fold in August and up to >118-fold in June for zinc. Along the mainstem, average total loads are up to 68 times higher in June for lead. Many watershed and biogeochemical processes contribute to this variation, including variability in estimated natural flows, partitioning of metals between the dissolved and particulate phases, and attenuation in the hyporheic zone. Dissolved phases constitute a large proportion of the total metals and follow patterns very similar to those for total loads along the mainstem, especially for cadmium and zinc. Seasonal load differences are greatest for copper and lead because of greater sorption to solids and particulate loads during high flow associated with increased erosion and transport of solids.