Geochemistry of nutrients in upper Silver Bow Creek

Since the recent removal of contaminated sediment, upper Silver Bow Creek has shown a decrease in concentrations and loads of toxic metals. However, nutrient concentrations in the stream are extremely high, which leads to uncontrolled growth of algae and rooted macrophytes each summer. The Butte waste-water treatment plant (WWTP) discharges \u3e 10 kg/h NH4-N and \u3e 1 kg/h PO4-P. The WWTP outfall can be as much as 50% of the total flow in Silver Bow Creek during drought periods. Microbial oxidation of ammonium causes a 2-3 km long “dead zone” marked by a drop in dissolved oxygen concentrations to hypoxic levels during summer night-time periods. The dead zone is also marked by large diurnal oscillations in the concentrations of ammonium, nitrate, and nitrite, with higher NH4/NO3 ratios at night. The stable isotopic compositions of ammonium and nitrate show dramatic changes over short spatial and temporal scales. Notable trends include an increase in δ15N-NH4 and a decrease in δ18O-NO3 with distance downstream. Values of δ15N-NO3 vary in a more complex fashion. These changes in concentration, speciation, and isotopic composition of nutrients reveal much in terms of the biogeochemical processing of nutrients in the stream, but make it very difficult to collect a “representative sample” for monitoring or scientific purposes. Fortunately for the Clark Fork River, the Warm Springs treatment ponds capture and store the majority of the nutrient load passing down Silver Bow Creek. Unfortunately, the “over-ripe” nature of the treatment ponds causes seasonal release of dissolved arsenic from the pond sediment