Evaluating best management practices for flood mitigation and stormwater contaminant removal in urban environments

Background/Question/Methods

Urban stormwater runoff is a major non point source of pollutants in urban water bodies. In addition, impervious areas generate large runoff volumes from large rainfall events, which could result in downstream flooding. As urban areas expand exponentially, best management practices that retain runoff locally, and can be retrofitted into existing urban infrastructure are gaining popularity. Such ‘low impact development’ (LID) practices need to be studied for their on-site volume and contaminant reduction, especially in sprawling metroplexes such as the Dallas Fort Worth area. A bioretention area or rain garden is such a practice that uses plant and soil characteristics to reduce the stormwater volume through retention and improve water quality through biochemical processes. A bioretention area was constructed to treat parking lot stormwater runoff in Dallas, TX. Native and adapted ornamental vegetation was selected to increase nutrient removal from the rain garden. Monitoring equipment to quantify the water balance were installed including monitoring wells, flumes and water level loggers. Inflow, outflow and storage volumes were monitored and water samples were collected on event basis and analyzed for nitrate-nitrogen (NO₃), Ortho Phosphorus (Ortho-P), Zn, Cu and sediments from November 2008 to present.

Results/Conclusions

Twenty five rainfall events greater than 7.5 mm, the amount that triggers runoff generation, were recorded. The volume reduction varied among events probably due to variation in rainfall intensity. On average the total runoff volume was reduced by 71%, a number higher than reported in the literature. The average infiltration rate was found to be 14.27 mm/hr (0.55 in/hr), higher than that of the underlying clay soil, which was likely indicative of the amendment of the soil in the bioretention cell with compost and expanded shale. NO₃, Ortho-P, Zn, and Cu load reduction was 83%, 65%, 83%, and 100%, respectively. NO₃ removal was higher than reported by others. Ortho-P reduction was lowest as consistent with the literature. There was a transient spike in Ortho-P in the outflow in the initial events that may have been due to the P rich compost used in the bioretention cell. LID practices offer tremendous potential for improving urban hydrology, reducing stormwater runoff and pollutants. In addition, they may serve as a tool for studying and separating the mechanics of pollutant removal via sediment binding, plant uptake and biogeochemical interactions such as denitrification.