Wetlands are increasingly used to treat wastewater for small municipalities and industries. The evaluation of wetland treatment efficiency typically involves comparing solute inflow and outflow concentrations, or investigating the export of nutrients per unit wetland area. A more holistic approach, which included seasonal changes in water quality and solute mass balances, yielded improved understanding about where nutrients are being stored in a wetland, the long-term treatment capabilities, and downstream impacts on water quality.
The study area is Frank Lake (FL), a wetland complex in southern Alberta, Canada, which has received municipal and slaughterhouse wastewater since 1989. Average annual water, chloride and nutrient mass balances between 1993 to 2015 suggest that: i) the lake received roughly equivalent volumes of annual inflow from three sources: spring melt (via ephemeral creeks), precipitation, and wastewater; ii) about two-thirds of the lake’s water loss was via evaporation, and one-third was via discharge into the receiving watershed; iii) about one-fourth of the annual chloride mass flux into FL (estimated at 845 ± 18 tons/yr) was stored in the lake sediment and shoreline soils, while the remaining three-fourths was discharged into the receiving watershed; iv) more than 60% of the annual phosphorus mass flux was discharged into the receiving watershed (and the remainder stored in the lake sediment and shoreline soils); and v) about 6% of the nitrogen was discharged into the receiving watershed.
Frank Lake water quality varied seasonally, with the lowest chloride (tracer) concentrations observed soon after spring melt. Decreased ephemeral creek inflow combined with evaporation during the open-water season (from May to November) resulted in increasing chloride concentrations that annually exceeded surface water quality guidelines. The highest chloride concentrations (up to 1442 mg/L) were observed in pore water in the peripheral soils were attributed to transpiration pumping. Chloride concentrations in a single lake sediment core suggest diffusion has transferred up to 500 tons of chloride to the lake sediment since 1989.
Increasing salt and phosphorus accumulation in sediment and peripheral soils are a long-term concern at FL. An estimated 6,000 tons of chloride stored in the lake have caused soil electrical conductivity values to exceed agricultural criteria in some shoreline areas. This salt, and an estimated 220 tons of phosphorus accumulated in near shore and lake sediments since 1989, could be released into the receiving water body during an extreme runoff event. Although there was no evidence of halide precipitation, phosphorus and calcium minerals are present in the lake sediment.
Water quality impacts on the receiving watershed, the Little Bow River (LBR), were significant. Average annual discharge and mass flux estimations from FL were similar to increases observed in the LBR in historic data, when FL effluent roughly doubled the average annual flow, and increased mass fluxes by 80, 20, and 235 times for chloride, nitrogen, and phosphorus, respectively. Surface water quality standards were routinely exceeded in the LBR for chloride, electrical conductivity and phosphorus downstream (but not upstream) of the FL discharge.
Frank Lake is not