QUANTIFYING STORMWATER RUNOFF CONTAMINANT MITIGATION BY BIORETENTION CELLS

This research study has three objectives, quantify the presence and concentration of selected contaminants in urban stormwater runoff from several urban locations within Lincoln, Nebraska, quantify the effectiveness of two, 10-year old bioretention cells, for reducing concentrations of contaminants in effluent and quantify the effectiveness of six rain gardens. The primary research sites were two bioretention cells using automated samplers collecting grab samples of stormwater runoff influent and effluent for at least six precipitation events during a two-year period. Additional research sites included samplers for two rainfall, six rain garden, two commercial rooftop, two commercial parking lot, and six residential rooftop sites. Stormwater runoff sample collection occurred from April 21, 2013 to May 22, 2014. Comparing the average influent against the average effluent, both the large and small bioretention cells had pollutant reductions (+) respectively for metolachlor 70%/21%, propazine 21%/45%, TKN 28%/9%, zinc 49%/69%, total fecal coliform 99.6%/88%, total E.coli 97%/66%, TSS 93%/71%, and oil/grease 52%/17% , while cells were a source(-) for nitrate -431%/-91%, and TDS -79%/-41%. The cells differed in pollutant removal/source for acetochlor, atrazine , DEA, total phosphorus and conductivity as the large cell had pollutants calculated at; 75%, 75%, 52% -255% and -45% respectively, while the small cell measured opposite of what the large cell at; -11%, -14%, -14% 5% and 25% respectively. Concentrations of pollutants in the rain garden runoff were 51% and 66% less than residential rooftop runoff for zinc and nitrate respectively. Concentrations of pollutants TKN and total phosphorus measured in the rain gardens runoff were 15% and 129% respectively greater than residential rooftop runoff. Similarities seen between the rainfall and bioretention cells in seasonal changes and statistically similar pollutant levels during the same storm events, indicate that rainfall contributed a majority of pollutant concentrations to both cells. Pollutants measured at all of the sites showed a decreasing trend from spring to late summer, with the exception of zinc.