Document Type

Thesis - University Access Only

Award Date

2012

Degree Name

Master of Science (MS)

Department / School

Civil and Environmental Engineering

First Advisor

Delvin E. DeBoer

Abstract

The Big Sioux River, draining a major watershed in eastern South Dakota, has shown increased nitrate levels throughout the years along the reach of the river. Increased nitrate levels may be due to increasing inputs from wastewater treatment plants, agricultural runoff and ground water. This thesis investigates relationships between nitrate discharged to the river from various sources and nitrate concentrations measured at several locations along the river. The primary objective was to examine the hypothesis that the wastewater discharges from Watertown, Brookings, and Sioux Falls contribute a majority of the nitrogen load to the Big Sioux River throughout the year. Data for this study were obtained from several sources, reorganized to enable the evaluations conducted in this thesis, and subjected to several tests to examine trends and correlations. Nitrate concentration data at various points along the river obtained from South Dakota Department of Environment and Natural Resources (SD DENR) along with river flow data from United States Geological Survey (USGS) were used to determine the nitrate mass loadings in the river and to observe seasonal variations of nitrate concentrations along the river. Similarly, nitrate concentrations and flow data of effluent from wastewater treatment plants were used to examine nitrate concentration variations and nitrate mass loadings of wastewater treatment plant effluents. Nitrate trends through the years were also examined in the river and the wastewater treatment plant effluents using the Kendall Family of Trend Tests developed by the USGS. Spatial and temporal analysis of nitrate concentration in the Big Sioux River using the Microsoft Excel plots of nitrate concentrations with the stations along the river showed high concentrations of nitrate in the river during winter (December - February). The nitrate concentrations spiked after each wastewater treatment plant (WWTP) discharged their effluent in the river. The results of the Mann Kendall trend tests exhibited statistically significant (p value< 0.1) increasing nitrate trend in Watertown WWTP effluent and decreasing nitrate trend in Brookings WWTP effluent, whereas the increasing trend in Sioux Falls WWTP was not statistically significant. Seasonal Kendall trend tests at 12 SD DENR water quality monitoring (WQM) stations showed increasing trends in 11 stations and a decreasing trend in one station (460655, statistically significant). Out of eleven increasing nitrate trend stations only two stations 46BS08 and 460702 were statistically significant. Regional Kendall trend test along the river exhibited a statistically significant increasing trend in fall and winter months (October - March) of the year. The nitrate loss study was performed using the mass balance in three different sections of the river. The average nitrate load input downstream of Watertown was 903 Ibid, 896 Ibid downstream of Brookings, and 13,790 Ibid downstream of Sioux Falls. The nitrate loss study near Watertown and Brookings showed maximum loss (76% of input nitrate load) during the month of August when there were maximum temperature and low flow condition in the river. Denitrification rates were estimated using the empirical equations using the temperature and DOC data. Denitrification rates were as low as 0.148 g NI m2.d in cool months (October - March) and high as 3.556 g NI m2.d during warm months (June - August). Annual average nitrate loadings were 426 lbld from the Watertown WWTP, 527 lbld from the Brookings WWTP, and 3,206 lbld from the Sioux Falls WWTP. An evaluation of ground water nitrate loading using the nitrate loadings from the river and the wastewater treatment plants estimated ground water contributions to be about 25 - 42% of the differential nitrate loading (difference between downstream and upstream nitrate loading) in the river between Watertown and Sioux Falls. During winter, WWTPs were the major sources of nitrate discharging to the Big Sioux River. The increasing Watertown WWTP nitrate concentration and increasing nitrate concentration in the SD DENR WQM station following the discharge point indicated Watertown WWTP effluent nitrate concentrations had an increasing nitrate concentration effect downstream of the river. Nitrate load input downstream of Sioux Falls was substantially greater than Watertown and Brookings which reflected the combining effect of the Morrell' s Meat processing plant effluent and Sioux Falls WWTP effluent discharged into the river.

Library of Congress Subject Headings

Nitrates -- Environmental aspects -- Big Sioux River (S.D. and Iowa)
Waste disposal in rivers, lakes, etc

Publisher

South Dakota State University

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Rights Statement

In Copyright