The impact of stormwater control structures on N fluxes in a coastal watershed.

Humans have significantly altered the landscape in urban areas resulting in increased volume and velocity of runoff following precipitation events. These changes in runoff have impaired streams and riparian areas that previously played a role in reducing watershed N flux through uptake and denitrification. The primary method used to control stormwater discharge is stormwater control measures (SCM) such as retention basins. Although the SCM were designed to mitigate hydrologic impacts associated with urban development, exactly how SCM compare to natural riparian areas in their ability remove N and the impact they have on watershed N fluxes is not well known. In this study we compared potential denitrification in SCM and riparian areas in Baltimore MD and estimated the impact of SCM on N flux in the Gwynns Falls watershed. Denitrification enzyme activity was higher in the SCM than in the riparian areas. Denitrification enzyme activity was highly correlated with soil moisture, soil organic matter, microbial biomass, and soil respiration in both SCM and natural riparian areas. However, under equivalent levels of soil moisture, soil organic matter, microbial biomass, and soil respiration-denitrification enzyme activity in SCM was higher than in riparian areas. SCM appear to function as hotspots of denitrification in the landscape and have higher potential denitrification compared to the natural riparian areas despite having similar amounts of microbial biomass. This higher potential denitrification is likely due to the engineered nature of the structures facilitating the interaction between soil and the nitrate-laden runoff. Within the entire Gwynns Falls watershed, there are 827 SCM, which collectively drain 21% of the watershed area. We estimated the impact of SCM on watershed N flux in several ways. First, we used removal efficiencies from the Chesapeake Bay program for each type of SCM along with their drainage areas to get an area-weighted N removal estimate of 3.3%. We also used the 50% difference in N concentrations observed between the SCM and the streams traversing natural riparian zones, along with the drainage area of all the SCM; this yielded an estimated 10% reduction in nitrogen flow in runoff due to SCM at the watershed scale. Based on watershed N retention estimates of 35% (wet years) to 85% (dry years), the SCM could be responsible for 4-9% of the N retained in the watershed.