Identification of non-point sources of nutrient loading and proposed best management practices for Browns Gulch, Silver Bow County, MT.

摘要

Nutrients are essential to support stream ecosystems, however, if present in excess may lead to growth of algal blooms, excessive aquatic weeds, and alteration of natural aquatic ecosystems. Silver Bow Creek (SBC), the headwater stream of the Clark Fork River, is listed as impaired for nutrients (total nitrogen (TN) and total phosphorus (TP)), by the Montana Department of Environmental Quality. Browns Gulch is a major tributary to SBC, and drains agricultural and forested lands. To meet target nutrient TMDL concentrations in SBC, the tributary load inputs of TN and TP must be reduced by 93% in Browns Gulch. To identify the sources of nutrients, surface water samples were collected and analyzed for TN and TP at three flow stages from locations distributed along the stream. Browns Gulch water quality data exhibited that, in all the flow stages, TN and TP loads increase from up to down-stream. Data analysis suggests that runoff from agricultural lands (during spring and summer) is the main source of TN, and a supplemental source of TP. Irrigated and grazed areas correspond with a sharp increase in the stream nutrient load. Specific conductivity and alkalinity concentrations were highly correlated with TP concentrations at each flow stage. The Lowland Creek Volcanics are the predominant geologic formation in the Browns Gulch watershed and may be contributing consistent low levels of TP via groundwater. To reduce agricultural non-point source inputs, three best-management practices (BMPs) are recommended: vegetated filter strips, riparian exclusion fencing, and off-stream water sources. It is hypothesized that effective implementation of one of the three proposed BMPs on each agricultural property will significantly reduce tributary TN load input to below to TMDL load allocation. The TP load input will be reduced, however to quantify this reduction, an understanding of the fraction of phosphorus originating from agriculture is required.