RESUMO
Rising chloride concentrations in surface water due to applications of deicing practices is proving detrimental to aquatic systems. In this study, a new chloride module is developed for a version of the Soil and Water Assessment Tool specially designed for Canadian Shield catchments (SWAT-CS) to model long-term chloride dynamics in a headwater catchment in south-central Ontario, Canada. In this modified model (SWAT-CS-CL; extended SWAT-CS model for chloride), chloride sources, sinks, internal storages or pools, and movement between these components are depicted. Performance of SWAT-CS-CL is assessed using a two-stage evaluation process based on the generalized likelihood uncertainty analysis (GLUE) framework. SWAT-CS-CL was found to perform moderately well, with simulated monthly chloride in streams and lake outflow following overall chloride trends and capturing regular chloride dynamics. However, simulations fail to consistently reproduce some instances of large or low chloride fluxes. Limitations in simulating large chloride fluxes may be attributed to the inadequate ability for SWAT-CS-CL to closely simulate snowpack and snowmelt processes. Parameter transferability among sub-catchments does suggest that there is a potential to extend SWAT-CS-CL to other Canadian Shield catchments for chloride modelling. Further improvements are needed through more trials to other catchments in a same or different landscape, and by modifying the simulation structure, especially representation of snow hydrology and chloride inputs.
RESUMO
Land application of municipal wastewater to managed forests is an important treatment and water reuse technology used globally, but the hydrological processes of these systems are not well characterized for temperate areas with annual rainfall of 1200 mm or greater. This study evaluated the impact of municipal wastewater irrigation to the local water balance at a 3000-ha land application facility where secondary-treated wastewater is land applied to a mixed hardwood-pine forest over 900 ha. Stable isotopes of hydrogen (H) and oxygen (O), chloride concentrations, and specific conductance were used in combination with hydrometric measurements to estimate the wastewater composition in groundwater, surface water, and at the watershed outlet during dry and wet seasonal periods and during one large rainfall event. Wastewater and water bodies receiving irrigation were found to have significantly higher δH, δO, specific conductance, and chloride concentrations. Using these tracers, a two-component, three-end member geochemical mixing model estimated mean wastewater compositions in the surficial aquifer receiving irrigation from 47 to 73%. Surface water onsite was found to reflect the high wastewater composition in groundwater. Land-applied wastewater contributed an estimated 24% of total streamflow, with the highest wastewater compositions in surface water observed during major storm events and at low-flow conditions. Groundwater and surface water within the watershed were found to have proportionally higher wastewater compositions than expected based on the proportion of irrigation to rainfall received by these areas.
