Mitigating Urban Wetland Impacts on Downstream Water Resources

Wetlands in older urban areas have often been integrated into stormwater management systems and exhibit severe degradation from receiving decades of untreated and uncontrolled urban runoff. Vegetative diversity is often poor and dominated by a few hardy species such as cattail (Typha latifolia) or reed canary grass (Phalaris arundinacea). These wetlands may also be affected by legacy impacts from agricultural uses that predate the urban development. Heavily impacted urban wetlands can become nutrient exporters, potentially degrading downstream water resources. Wetland 639W in Crystal, Minnesota, lies just upstream of Upper Twin Lake, which is an Impaired Water (EPA 303(d) list) for excess nutrients. The wetland receives stormwater from a 1,010 acre urban watershed. The concentration of total phosphorus at the wetland outlet is roughly double the concentration at the wetland inlet, indicating the wetland is exporting phosphorus. Evapotranspiration by the dense cattail stands in the wetland basin significantly draws down surficial groundwater levels during the summer growing months, speeding soil mineralization and leaving the soil surface friable. Sheet flow over the wetland during storm events releases dissolved phosphorus from the mineralized soil and detaches and mobilizes soil particles and plant detritus. In winter 2010-2011 the City of Crystal will modify the wetland outlet to limit outflow and restore a more natural wetland hydrology. This will reduce the periods of extended soil dryness and limit the direct discharge of phosphorus during smaller storm events. An upstream weir and overflow channel will provide a bypass for overflow and higher flows. The project is expected to reduce phosphorus export by an average 300 pounds per year.