THE EFFECTIVENESS OF RAIN GARDENS AT MITIGATING NITROGEN RUNOFF TO A PIEDMONT LAKE, GREENVILLE, SOUTH CAROLINA

Urbanization alters the flow and chemistry of surface water systems. The addition of impervious cover increases runoff and degrades water quality. Rain gardens have become a popular Low Impact Development (LID) strategy to help mitigate the impacts of urban runoff by reducing runoff, capturing heavy metals and hydrocarbons, and diminishing nutrient and sediment loads to receiving surface water systems. The success and effectiveness of a rain garden is dictated by its design (i.e., size, fill media, vegetation type and density, and underlying geology) and the region’s storm characteristics (i.e., rainfall patterns, frequency, and intensity). This study looked at the effectiveness of four rain gardens on Furman University’s campus in Greenville, South Carolina at minimizing nitrogen loading to Furman Lake. The rain gardens were installed in 2008 as part on an ongoing lake restoration project aimed at improving the lake’s degrading water quality. Inflow and outflow water samples were collected at each garden for six major storm events from June – July 2012. Roof, parking lot, and lawn runoff and precipitation samples were also collected for each storm event from sites within the lake’s watershed. Dry atmospheric deposition was identified as the primary source of nitrogen loading to the lake, with loading rates seemingly governed by the duration of dry periods between rain events. Overall in the gardens, nitrate levels were effectively reduced from inflow median concentrations of 3.517 mg/L to outflow median concentrations of 0.0534 mg/L for the six storm events, although the efficiency varied among gardens. The two rain gardens with the deeper, wider infiltration zones and denser vegetation were markedly more effective at reducing nitrate loading to the lake compared to the other two gardens which were only marginally effective. The results highlight the importance of proper and appropriate garden design and size to ensure successful water quality improvement.