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Fate of Trace Organic Contaminants in Semi-Arid Areas
Civil and Environmental Engineering
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Semi-arid areas including the state of Nevada, has been exploring reclaimed water for direct or indirect potable reuse. Numerous research articles have discussed the occurrence of trace organic contaminants (TrOCs) in reclaimed water, surface water, soil, sediments, studied their photodegradation in water, and their transport and accumulation in plants when reclaimed water is used for irrigation. To safely reuse reclaimed water for irrigation in semi-arid areas, there is a need to determine the fate of TrOCs in various environmental media receiving reclaimed water. Therefore, the overarching goal of this dissertation was to investigate the fate of TrOCs in semi-arid areas to safely reuse the reclaimed water. To accomplish this goal, experiments were conducted to determine the fate of TrOCs in semi-arid areas. The specific objectives of this dissertation were to A) determine the fate of TrOCs in reclaimed water in an agricultural farm irrigated with reclaimed water for over 10 years, B) compare the fate of TrOCs in terminal lakes (TLs) influenced by reclaimed water to terminal lakes which are not influenced by reclaimed water, C) determine the photodegradation kinetics of TrOCs in terminal lakes and freshwater lake.To accomplish Objective A, the fate of 12 TrOCs in wastewater samples, 11 in plants and in soil pore water, and 10 in soil at an agricultural research farm were studied, which was irrigated with reclaimed wastewater for more than 10 years. Reclaimed wastewater contained 11 of 12 TrOCs, with concentrations ranging from 26 ± 9 ng/L (DEET) to 1539 ± 1899 ng/L (trimethoprim). Soils from 0 to 60 cm contained nine TrOCs from below the reporting limit (fluoxetine) to 329 ng/g (carbamazepine). TrOC concentrations decreased with increasing soil depth, except carbamazepine, which was more recalcitrant. Nine TrOCs were in the shoots and leaves of alfalfa grown in the reclaimed wastewater irrigated plots, at concentrations ranging from <1 ng/g (diphenhydramine and fluoxetine) to 49 ng/g dry weight (DW) (carbamazepine). Overall, despite some accumulation of TrOCs in the soil, alfalfa uptake was limited, which results in low exposure to foraging animals. To achieve Objective B, a preliminary assessment of the occurrence of ten TrOCs in three TLs receiving reclaimed wastewater and one TL which does not directly receive reclaimed wastewater were conducted. Concentrations of caffeine, carbamazepine, diphenhydramine, fluoxetine and meprobamate were significantly higher in TLs receiving reclaimed wastewater from a secondary treatment plant compared to those which received tertiary treated wastewater. Carbamazepine, fluoxetine, sulfamethoxazole, and trimethoprim were present at concentrations greater than is typical of other U.S. freshwater lakes, but other TrOC concentrations were at lower concentrations than in other freshwater lakes. Based on the outcomes of Objective B, carbamazepine and diphenhydramine were selected for further study because they were at higher and lower concentrations in TLs than U.S. freshwater lakes. In published literature, photodegradation of pharmaceuticals is considered as the main route of degradation of TrOCs in natural waters. To study photodegradation (Objective C), laboratory-scale solar simulated experiments were conducted on carbamazepine and diphenhydramine using surface water from terminal lakes and a freshwater lake. Photodegradation kinetics of CBZ and DPH in two terminal lakes (Walker Lake, NV and, Swan Lake, NV) were compared to the rates in one oligotrophic freshwater lake (Lake Tahoe). Dilutions of lake water samples containing 20 mg/L of CBZ and DPH were prepared and, exposed to simulated solar light for a maximum of 2 days. Photodegradation of CBZ and DPH followed a pseudo-zero order kinetics. Dissolved organic carbon up to 65 mgC/L and alkalinity of 5,198 mg CaCO3/L in terminal takes enhanced the rate of photodegradation rate of DPH and CBZ but photodegradation rates of DPH were higher than CBZ. Degradation rate constants ranged from 0.33 to 2.68 mg/L-hr for CBZ and DPH in terminal lakes compared to 0.19 to 0.88 mg/L-hr for CBZ and DPH in a freshwater lake. Overall, TrOCs were persistent at low concentrations in semi-arid areas. Water constituents in the presence of sunlight and soil have the potential to either degrade or sorb TrOCs, reducing the concentrations of aqueous and mobile TrOCs in the environment.