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Watershed Scale Impacts of Stormwater Green Infrastructure on Hydrology, Nitrogen Fluxes, and Combined Sewer Overflows in the Baltimore, MD and Washington, DC area
Pennino, M., R. McDonald, AND P. Jaffe. Watershed Scale Impacts of Stormwater Green Infrastructure on Hydrology, Nitrogen Fluxes, and Combined Sewer Overflows in the Baltimore, MD and Washington, DC area. American Geophysical Union Fall 2016 Meeting, San Francisco, CA, December 12 - 16, 2016.
Stormwater green infrastructure (SGI), such as rain gardens and detention ponds, has become a common method municipalities implement to mitigate flooding, water quality, and combined sewer overflow problems in urban areas. However, there have been few studies documenting how SGI impacts hydrology and nutrient exports at the watershed and regional scale. We found a significant reduction in hydrologic flashiness and nutrient export in watershed with higher stormwater green infrastructure density. This indicates that as cities implement more stormwater green infrastructure, there is likely to be even more reductions in hydrologic flashiness and nutrient exports. Reducing hydrologic flashiness will help prevent local flooding and stream erosion and reducing nutrient export will help reduce the potential for downstream eutrophication in estuaries and coasts. These results have implications for improving watershed management and ecosystem and human health through improving stream water quality, reducing urban flooding, and combined sewer overflows.
Despite the increasing use of urban stormwater green infrastructure (SGI), including detention ponds and rain gardens, few studies have quantified the cumulative effects of multiple SGI projects on hydrology and water quality at the watershed scale. To assess the effects of SGI, Baltimore County, MD, Montgomery County, MD, and Washington, DC, were selected based on the availability of data on SGI, water quality, and stream flow. The watershed scale impact of SGI was evaluated by assessing how increased spatial density of SGI correlates with stream hydrology and nitrogen exports over space and time. The most common SGI types were detention ponds (58%), followed by marshes (12%), sand filters (9%), wet ponds (7%), infiltration trenches (4%), and rain gardens (2%). When controlling for watersheds size and percent impervious surface cover, watersheds with greater amounts of SGI (>10% SGI) have 44% lower peak runoff, 26% less frequent runoff events, and 26% less variable runoff than watersheds with lower SGI. Watersheds with more SGI also show 44% less NO3− and 48% less total nitrogen exports compared to watersheds with minimal SGI. There was no significant reduction in combined sewer overflows in watersheds with greater SGI. Based on specific SGI types, infiltration trenches (R2 = 0.35) showed the strongest correlation with hydrologic metrics, likely due to their ability to attenuate flow, while bioretention (R2 = 0.19) and wet ponds (R2 = 0.12) showed stronger relationships with nitrogen compared to other SGI types, possibly due to greater denitrification in these sites. When comparing individual watersheds over time, increases in SGI corresponded to non-significant reductions in hydrologic flashiness and combined sewer overflows compared to watersheds with no change in SGI. This study shows that while implementation of SGI is ongoing, some regions are beginning to have enough SGI to see significant impacts on hydrology and water quality at the watershed scale.