Transport of Lithium Tracer and E. coli in Agricultural Wastewater Treatment Wetlands

Agricultural waste must be managed effectively to protect surface and groundwater resources, as well as human health. Constructed wetlands can provide a low-cost environmentally acceptable method for the treatment of agricultural wastewater. An ionic tracer (Lithium chloride [LiCl]) and a biotracer (a naladixic acid-resistant strain of Escherichia coli) were injected into six pilot-scale constructed wetlands treating dairy wastewater: three surface-flow (SF) wetlands and three subsurface-flow (SSF) wetlands. Each wetland was 3.9-m long and 1.7-m wide. Residence time distribution functions were calculated for each wetland to investigate the hydraulic behaviour of each system during winter and summer conditions. During the summer study, the mean residence times for SF wetlands 2, 4, and 6 were 12, 16, and 14 days, respectively, while the mean residence time for SSF wetlands 1, 3, and 5 were 23, 18, and 22 days, respectively. The longitudinal dispersion coefficients were in the order of 10(-6) m(2).s(-1) for each wetland during the summer and winter. The mean residence time for SF wetlands 2, 4, and 6 during the winter study were 8, 10, and 10 days, respectively, while the mean residence time for SSF wetlands 1, 3, and 5 were 8, 9, and 10 days, respectively. E. coli effluent peaks often occurred prior to Li peaks, suggesting that bacteria may be motile within the wetland environment. This study suggests that dispersion is an important mass transport process in both SF and SSF wetlands. Long-term operation of SF and SSF treatment wetlands may cause reduced retention times and treatment efficiency due to organic matter accumulation and channelling. Cold winter temperatures may also increase the survival of bacteria within treatment wetland systems, decreasing the wetland's ability to reduce bacteria concentrations during the winter months.