Water Resources Research Act Program

Details for Project ID 2006MT86B

Temporal and spatial changes in the concentration and isotopic composition of nitrate in the upper Silver Bow Creek drainage, Montana.

Institute: Montana
Year Established: 2006 Start Date: 2006-03-01 End Date: 2007-12-31
Total Federal Funds: $29,200 Total Non-Federal Funds: $58,802

Principal Investigators: Chris Gammons, John LaFave

Project Summary: This proposal focuses on the distribution of nitrate and other nutrients in the Butte Summit Valley and upper Silver Bow Creek. Silver Bow Creek is a major headwater stream of the upper Clark Fork River, and the latter is known to have serious nutrient problems that further compromise the water quality of this mining-impacted watershed. Although few data are published on nutrient levels in Summit Valley, preliminary work by the Montana Bureau of Mines and Geology shows that nitrate concentrations in both groundwater and surface water in the Butte area are highly elevated. The State of Montana is currently reconstructing upper Silver Bow Creek in a very expensive reclamation project. However, very little is known regarding the concentration and speciation of nutrients entering the new Silver Bow Creek channel, nor their seasonal or diurnal variations. Maintaining nutrient loads below levels that lead to nuisance algae and eutrophication may be a key to the future aquatic health of Silver Bow Creek. The research plan involves: 1) biweekly to monthly sampling at three reference stations (two of which have permanent USGS gauging stations); 2) bimonthly synoptic sampling; 3) diurnal (24-h) sampling; and 4) stable isotope analysis. Nutrient types that will be quantified include dissolved nitrate, dissolved ammonia, dissolved phosphate, total N and total P. In addition, algal mass (chlorophyll a) coating the streambed will be monitored. Previous work on the upper Clark Fork River has documented substantial diurnal (24-h) fluctuations in dissolved nitrate concentration. However, it is not known to what extent total N and total P concentrations (the parameters on which local nutrient targets are based) may also vary diurnally. The synoptic sampling events will be designed to determine whether the majority of nutrients exiting the Summit Valley come from large point sources (such as the Butte-Silver Bow sewer discharge), or are mainly from non-point sources (e.g., agricultural runoff or septic tanks). To augment the conventional sampling and analytical methods, the stable isotopic composition of N and O in dissolved nitrate will be determined in a number of surface water samples. These will be compared to samples of possible nitrate sources (i.e., the municipal sewage discharge, agricultural runoff, chemical fertilizers, blasting residue from mining) to help determine the main source of nitrate contamination in the watershed. In addition, a set of diurnal samples will be collected to see if the isotopic composition of nitrate and co-existing dissolved N2 and O2 gas varies over a day-night cycle. Recent research by our group has shown that dissolved O2 in highly productive streams becomes isotopically lighter during the day (due to influx of biogenic O2) and isotopically heavier at night (due to consumption of light O2 by respiring organisms). We also have preliminary data that shows small but significant diurnal changes in the isotopic composition of dissolved N2 which may be linked to daily cycles in the rate of microbial de-nitrification reactions. Results generated in the project will provide the seed for continued research in this area. The proposed project will involve undergraduate and graduate (MS-level) students at Montana Tech, and will also have an outreach component that will involve K-12 students and their teachers. The latter activities will be coordinated through the Clark Fork Watershed Education Program, which is a new technical outreach program funded by the State of Montana and administered through Montana Tech.