Evaluating the Role of Rain Garden Soils in Nutrient Processing of Stormwater Runoff in Charlotte, NC
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Abstract
As urbanization increases, Best Management Practices (BMPs) are used to reduce pollutants otherwise released to streams. Rain gardens are a type of BMPs that are vegetated depressions with highly permeable soil to treat urban runoff. These structures are traditionally assessed using inflow-outflow studies with an emphasis on quantifying removal efficiency. This approach neglects controls on nutrient processing within the rain garden and their role as potential hotspots in urban systems. To bridge this gap, three rain gardens in Charlotte, NC were quantified for soil water nutrient concentrations (ammonium, nitrate, phosphate, DOC) during storms and seasonal potential denitrification rates. The rain gardens were located at Myers Park High School (MP), Park Road Park (PR), and Bruns Academy Elementary School (BR) and vary with vegetation, size, treatment area, and age (4,7, and 15 respectively). The results identified no significant difference in the runoff between site or season which indicated differences within soil water concentrations were caused by structural variances of the rain gardens. Results identified soil water ammonium concentrations were significantly different between sites (p=0.0201). Soil water ammonium concentrations were also found to be significantly lower in the summer compared to the winter when all sites were aggregated together (p=0.0201). Nitrate concentrations were significantly higher (p<0.0001) in soil water across sites, and significantly lower (p<0.0001) in winter soil water compared to summer. These trends were caused by the presence of an underdrain and high infiltration rates that limited favorable denitrification conditions from existing. The potential denitrification rates were 0.39, 0.12, and 0.65 μg/gDM/hr at MP, PR, and BR respectively. Overall, the nitrification process attributed to the decrease in soil water ammonium and the accumulation of soil water nitrate. Without prolonged anoxic conditions present in the rain garden nitrate was stored until subsequent storms exported nitrate to the receiving stream. The sites with the highest soil moisture percent after the storm also had higher denitrification rates. Bruns Academy had very high soil nitrate (2.75 mg/L) compared to the other sites (0.42 and 0.28 mg/L at MP and BR respectively) which may have been caused by a pollutant exposure prior to this study. Phosphate was variable between sites and was likely due to the difference in phosphorus in the original soil media. Between similarly constructed MP and PR, soil water phosphate was higher at MP (0.032 mg/L) than PR (0.007 mg/L). This suggests more adsorption occurred due to an increased ratio of the rain garden area to the treatment area as well as plant assimilation. DOC was different among sites and had an inverse relationship with potential denitrification rates. PR had the lowest denitrification rate and presented the largest soil water DOC concentrations with an average concentration of 14 mg/L. The age gradient of the rain gardens in this study proved rain gardens still have the ability to remove pollutants as the structures age (with the exception of nitrate). The addition of soil water concentrations and potential denitrification rates in this study showed internal processes of rain gardens should be explored further to understand the longevity of these structures, as well as nutrient export as aging occurs.