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Assessing nutrient management scenarios at the system level




Jobin, Olivia, author
Arabi, Mazdak, advisor
Hoag, Dana, committee member
Sharvelle, Sybil, committee member

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The exponential increase in urbanization and population has led to water quality degradation throughout the country. This can be linked to the increase in impervious surfaces from urban expansion, most wastewater treatment plants (WWTPs) not being equipped to handle higher nutrient inflows, and the exponential demand for food that has led to more intensive farming practices that erode and degrade the soil, further enhancing runoff. The overall goal of this study was to assess nutrient management scenarios at the system level. The objectives included: 1) determine a methodology that could be used to quantify nutrient load contributions from each sector at the watershed scale; 2): determining delivery ratios for each sector based on the ambient nutrient loads at the outlet of the watershed; 3): and assess the cost, equity, and water quality effects of conservation management practices, BMPs, wastewater treatment technologies, and water conservation practices. Assessing the effectiveness of agricultural management practices is often jeopardized by lack of comprehensive monitoring data and computational burden at larger scales. The Soil and Water Assessment Tool (SWAT) within the eRAMS platform was used to assess the benefits of different agricultural management practices at field and watershed scale for the South Platte River Basin (SPRB), a moderately large semi-arid watershed located in northeastern Colorado. The model was calibrated using measured field observations from a study site in the watershed where the target management practices were implemented and monitored for their effectiveness. The agricultural management practices studied included fertilizer application rate and timing, tillage practices (i.e. conventional, reduced, strip, and no-tillage), and center pivot versus surface irrigation for roughly 21,000 irrigated agricultural fields (740,000 acres) in the SPRB. Center pivot irrigation showed the highest potential for nutrient reduction while tillage practices had an intermediate effect. Due to interim warm water instream total nitrogen (TN) and total phosphorus (TP) levels being exceeded over the period of 2002-2015, nutrient management scenarios were assessed at the system level for the Cache la Poudre (CLP) watershed in Colorado. The CLP watershed consists of 13 WWTPs, as well as irrigated agricultural fields, forested land, rangeland and urban areas making it an ideal candidate for this analysis. The scenarios created involved a combination of different practices and technologies for each sector and their associated costs to determine cost effective solutions for the issue at hand. A Gini Index coefficient was also determined in order to determine how equitable each scenario was. Models were used to determine the nutrient load contributions over the 14 year time frame with and without the implementation of the different practices and technologies tested, and were validated based on previous research and monitoring data. It was found that TN reductions needed for regulations could be achieved through the adoption of carbon addition, WWTP effluent reuse, 10% adoption of strip tillage, and a 25% adoption of bio-retention basins for a total of roughly $6,000,000. Whereas the TP reduction needed for regulations for all hydrologic conditions could not be achieved with any combination of the practices looked into, however 2 out of the 3 reductions could be achieved from the adoption of Chem-P, WWTP effluent reuse, 10% adoption of strip tillage, and 25% adoption of bio-retention basins for roughly $11,000,000. Further research would be needed to determine a scenario that could achieve a 70% TP reduction and 40% TN reduction simultaneously at the outlet, which was needed at the system level to be in compliance with regulatory standards.


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