Water quality related to pharmaceutical and nitrogen compounds during infiltration of treated wastewater for Managed Aquifer Recharge

Abstract

As more regions in the world look to replenish depleted aquifers, treated wastewater (TWW) is increasingly infiltrated in Managed Aquifer Recharge (MAR) schemes. While MAR is a promising emerging technology, water quality issues—including potential contaminants present in the recharge water as well as the possibility of generation of pollutants along the infiltration flow path—should be considered. This dissertation documents and evaluates column experiments in which TWW was infiltrated through soil containing a considerable amount of organic matter (2.57% organic carbon). Three topics were investigated through column experiments: 1) cycling of nitrogen, 2) the fate of non-antibiotic pharmaceuticals and 3) the fate of antibiotic pharmaceuticals. In the study of nitrogen cycling, soil column experiments were operated with wetting and drying cycles. Ammonium, which was present only in trace concentrations in the TWW, increased in concentration with depth in the column and exceeded the EU Water Framework Directive limit of 0.5 mg/L (0.39 mg(N)/L) for up to a year, depending on the sampling depth. Pore water samples collected at the end of drying periods showed very high nitrate concentrations, indicating nitrification of some of the ammonium. Oxidation reduction potential often exceeded 200 mV during drying periods, showing conditions for nitrification, but dropped to below -100 mV during wetting periods, creating several possible pathways for ammonium production. Potential sources of ammonium are (1) dissolved organic nitrogen in the TWW, (2) nitrate in the TWW, and (3) organic nitrogen in the soil. δ15N in ammonium in pore water samples (mean 4.7 ‰) was slightly higher than δ15N the soil (2.4 ‰), indicating that the soil was likely the major source but also that nitrate (mean 17.2 ‰) may have been the source of some of the ammonium. Fractionation of 15N in nitrate as well as high (often >10 mg(C)/L during the first 50 days of infiltration) concentrations of acetate (a labile form of organic carbon) also indicate that dissimilatory nitrate reduction to ammonium may have formed some of the ammonium. In the study of pharmaceutical compounds, column experiments were conducted under three different conditions: continuous infiltration, wetting and drying cycles, and wetting and drying cycles with elevated concentrations of antibiotics in the inflow water, which may reduce microbially aided degradation of other compounds. A mass balance comparing pharmaceutical mass in the water phase over the 16-month duration of the experiments to mass sorbed to the soil was used to infer the mass of pharmaceuticals degraded. Results show sorption as the main attenuation mechanism for carbamazepine. About half of the mass of diclofenac was degraded with wetting and drying cycles, but no significant degradation was found for continuous infiltration, while 32% of infiltrated mass sorbed. Fenoprofen was degraded in the shallow and aerobic part of the soil, but degradation appeared to cease beyond 27 cm depth. Gemfibrozil attenuated through a combination of degradation and sorption, with slight increases in attenuation with depth from both mechanisms. Naproxen degraded progressively with depth, resulting in attenuation of more than 90% of the mass. In the column with elevated concentrations of antibiotics, the antibiotics attenuated to about 50% or less of inflow concentrations by 27 cm depth and within this zone, less degradation of the other compounds was observed. In this same experiment, the mass balance suggests that the antibiotic sulfamethoxazole was degraded during infiltration, while for sulfadimidine, sorption was an important attenuation mechanism.