Beaver activity improves pollutant reduction in a constructed stormwater wetland

Urban water bodies often receive pollution from stormwater runoff. Beaver activity can have varying effects on water quality, but the impact on urban systems is not well understood. This study evaluates pollutant reduction in a constructed stormwater wetland facility with and without beaver activity. The concentration of urban pollutants was measured in runoff at the inlet and outlet of the facility during storms: seven when beavers were absent and seven when beavers were present. Percent reductions of many pollutants, especially heavy metals and sediment, were significantly higher when beavers were present. None of the pollutants were significantly reduced in the facility when beavers were absent, while several were significantly reduced when beavers were present, including copper, mercury, lead, zinc, total phosphorus, and suspended solids. However, orthophosphate increased with beaver activity. There was evidence that organic compounds such as phthalates, herbicides, and preservatives may be reduced. The vast majority of applicable water quality standards were met at the facility outlet when beavers were both absent and present. Beaver dams in the facility appeared to allow for sediment settling and filtration of the water through the dam matrix of sticks, roots, and mud. Results for nutrients and sediment were similar to what has been found with beaver activity in other systems, and the results for heavy metals are encouraging as there are few studies on those pollutants. Coexistence devices like culvert protectors and pond levelers were constructed to allow beavers to remain in the facility without negative impacts to human maintenance efforts.