Nutrient Sources and Transport in the Central Region of the Big Sioux River, South Dakota
Agricultural and municipal wastes account for most of the nutrients entering natural waters (Mitchell 1972). Agricultural land run-off is currently the largest and most complex pollution contributing factor in South Dakota. Municipalities are significant contributing factors in local areas, but the total pollution load they contribute is much less than that from agricultural land run-off (Matthew 1968). The Big Sioux River flows through the intensively farmed eastern edge of South Dakota. The drainage area of the Big Sioux River is 24,880 km and has the greatest population density of any major watershed in South Dakota (Matthew 1968). According to Applegate (1971) there were an estimated 858,000 cattle in 1970 in the counties in which the Big Sioux River is located with the potential to produce 6,229,000 kg P and 283,140,000 kg N per year. Twenty-five cities and towns use the Big Sioux River and its tributaries as receiving waters for their sewage effluent (Matthew 1968). Skille (1971) stated that the Big Sioux River carried high nutrient loads and contributed large amounts of nutrients to the Lake Poinsett water system. This nutrient load has increased the eutrophication of the lake-river system of eastern South Dakota and has caused concern among neighboring states over the quality of the water in the Big Sioux River as it leaves South Dakota. The first objective of this study was to determine the amount of phosphate, organic nitrogen and organic carbon transported by the Big Sioux River. The second objective was to determine the amount of these nutrients contributed by three agricultural drainage areas and the city of Brookings, South Dakota.