Controls on major ion chemistry and metals in a suburban pond fed by municipal water and treated stormwater

Abstract

The quality of surface and ground water is threatened by stormwater runoff, which often contains heavy metals (such as copper, lead, and zinc), polychlorinated biphenyls (PCBs), fertilizers, and pesticides. In this study, we evaluated the effects of treated stormwater from a bioinfiltration swale on the dissolved metal load in a suburban park pond in Spokane, Washington. This pond, which has a porous bed, also receives a continuous flow of municipal water (sourced from local groundwater) to maintain a constant water level.Water samples were collected from the pond and the municipal water source every two weeks from January to August, 2016. Snow and road dust were collected in January, 2016. The results indicated that snow and dust from the roadway had elevated concentrations of major ions and heavy metals relative to their counterparts in local parks. However, the treated stormwater had a negligible impact on the dissolved load of the receiving pond for several reasons. First, heavy metal concentrations were significantly lower in the treated stormwater than the original runoff indicating successful removal by the bioinfiltration swales. Secondly, the dissolved major ion concentrations in the pond water tended to mimic the temporal fluctuations in the dissolved load of the municipal water. This can be attributed to the large volume of municipal water added to the pond (~86–97%) which dilutes the treated stormwater. Furthermore, the average residence of water in the pond is short (~39 days) such that the municipal water continuously flushes the pond. These findings establish a baseline for the dissolved load of this receiving pond and suggest that a decrease in municipal water input be carefully evaluated before implementation to prevent any adverse effects on the aquatic community. Moreover, we recommend that the heavy metal loading of the bioinfiltration swale media, and composition of treated stormwater, be monitored to ensure that metal mobilization from the swales does not occur especially during de-icing events.