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Finding land and water management practices to reduce selenium and nitrate concentrations in an agricultural river valley applying a regional-scale stream-aquifer model




Shultz, Christopher David, author
Gates, Timothy K., advisor
Bailey, Ryan T., committee member
Hoag, Dana K., committee member

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The long-term practice of irrigated agriculture within the Lower Arkansas River Valley (LARV) in southeast Colorado has contributed to a number of land and water management concerns, including elevated concentrations of dissolved selenium (Se) and nitrate (NO3) in the stream-aquifer system. The goal of this study was to develop and calibrate a stream-aquifer flow and reactive transport model to simulate conditions within a representative region of the LARV, then to apply the model to evaluate the potential effectiveness of alternative land and water best management practices (BMPs) to improve conditions. Using a MODFLOW-SFR model to simulate groundwater and stream flow, linked to an RT3D-OTIS model to simulate reactive transport of solutes, enabled comprehensive regional-scale modeling of the coupled stream-aquifer system. Through an extensive calibration and testing process, including manual and automated calibration using PEST, parameter values were estimated and runs were conducted to describe spatiotemporal distributions of groundwater levels and concentrations, mass and return flow rates to streams, and stream concentrations for baseline conditions. Similar runs were conducted for individual and combined BMPs to analyze their effectiveness in reducing groundwater and stream water pollution from Se and NO3, assuming their broad implementation over the study regions. The considered BMPs include two land BMPs, namely reducing applied fertilizer application (RF), and enhancing riparian buffer zones (ERB); and three water BMPs, reducing applied irrigation (RI), lease-fallowing irrigated land (LF), and canal sealing to reduce seepage (CS). Results reveal substantial spatial and temporal variability in Se and NO3 concentrations over the region. Moreover, they show that by implementing such BMPs, Se and NO3 groundwater concentrations could be lowered by as much as 23% and 40%, respectively, and stream concentrations of Se and NO3 could be lowered by as much as 57% and 33%, respectively. The most effective stand-alone land BMP was ERB, and the most effective stand-alone water BMP was CS. By coupling groundwater and stream flow modeling, this study has provided a number of insights not perceived in precursor modeling studies in the study region which examined only groundwater concentrations and mass loading. Some of these findings include: (1) BMPs which alter water management alone are likely to result in an increase in NO3 concentration in the streams (this is because the chemical reduction of groundwater return flows through the riparian zone is so effective under baseline conditions that practices which lower rates of return flow, without also substantially lowering concentrations, diminish the dilution effect on stream flow), (2) lower mass loading of Se and NO3 to streams due to a BMP does not necessarily imply a lowering of stream concentration since there are interactive effects of concurrent reductions in return flow rates, and (3) though there are prospects for substantial lowering of total Se concentrations in streams in the LARV, it is unlikely that the current Colorado chronic standard of 4.6 µg L-1 for total Se could ever be achieved practically. Furthermore, the linked models presented in this thesis could be applied to other irrigated stream-aquifer systems to simulate reactive transport of Se and NO3.


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