CONTROLS ON MAJOR ION CHEMISTRY AND ZINC IN AN URBAN POND FED BY MUNICIPAL AND TREATED STORM WATER

Stormwater contamination is a growing concern as urban development continues and impermeable surfaces increase. As stormwater moves across roadways, parking lots, and other impermeable surfaces, it collects heavy metals, polychlorinated byphenyls (PCBs), fertilizers, pesticides, and deicing salts. A best management practice (BMP) for treating runoff is the use of a bioinfiltration swales. These swales reduce contaminants through natural processes such as infiltration, plant uptake, and cation exchange. Bioinfiltration swales were constructed in Spokane, Washington, in 2010, as a way to minimize the amount of untreated stormwater that has been historically released to the Spokane River during storm events. A series of bioinfiltration swales were built in curb extensions along a residential, arterial road. Street runoff filters through the swales and is piped to a pond in a nearby park. Because the base of the pond is highly porous, municipal water is continually added to maintain a constant water level.

This study investigated temporal changes in the major, minor, and trace element concentrations in the pond. Water samples were collected from the pond and the municipal water source every two weeks from January to August, 2016. Road dust, snow, and untreated stormwater were also sampled. Results indicate that the dissolved load in the pond mimicked that of municipal water, except during the spring and summer months when silica and calcium concentrations were lower in the pond. We attributed these differences to diatom productivity and calcium carbonate precipitation, respectively. We found elevated concentrations of zinc and other heavy metals in stormwater runoff and surface soils from the swales. Despite this, the pond water chemistry was not affected because the amount of runoff is a small fraction of the municipal water added to the pond. These results may provide insight to city planners regarding storm and pond water management in urban environments.