The role of water and mass balances in treatment assessment of a flooded natural wetland receiving wastewater effluent (Frank Lake, Alberta, Canada)

Abstract

Constructed, and to a lesser extent, natural wetlands are increasingly used to treat treated wastewater effluent from small municipalities and industries. Wetland treatment efficiency evaluation typically involves comparison of solute inflow and outflow concentrations, or estimation of the export of nutrients per unit wetland area. A more holistic approach, which evaluated water and solute mass balances, in addition to seasonal changes in water quality, yielded an improved understanding of where nutrient storage occurred, long-term wetland treatment capabilities, and downstream water quality impacts of a flooded natural wetland receiving wastewater. The lowest solute concentrations in the flooded wetland were observed soon after ephemeral creek flow in the spring, increasing concentrations over the summer attributed to evaporation, and the highest concentrations under winter ice. Chloride accumulation in shoreline sediments was attributed to transpiration pumping, and high salt and phosphorus accumulation in lake and shoreline sediments were a long-term concern at Frank Lake. Limited historic data suggested water quality impacts on the receiving watershed, the Little Bow River, were significant, with about 80, 230, and 20 times increased mass fluxes for chloride, total phosphorus, and total nitrogen, respectively. The flooded wetland, Frank Lake, is not a sustainable approach to wastewater effluent treatment in its current form.