Cities and the Environment (Jan 2011)
Water Quality Characteristics of Three Rain Gardens Located Within the Twin Cities Metropolitan Area, Minnesota
Abstract
A study was conducted by the United States Geological Survey (USGS) at three locations in the Twin Cities Metropolitan Area in Minnesota to assess the effect that bioretention areas, or rain gardens, have on water quality. The rain gardens are located at the University of Minnesota Landscape Arboretum (MLA), City of Hugo, and City of Woodbury. These sites were chosen because of their similar ages, differences in design, surrounding land use, precipitation patterns, and geology. This article reports the statistical analysis of six years of data obtained from these three sites. The data characterizes the water quality of the inflow, overflow, vadose zone, and groundwater of each rain garden. Nutrients analyzed included chloride, total suspended solids, ammonia, organic nitrogen, nitrate, and phosphorus. Lysimeters and wells had significantly lower nutrient concentrations compared to inflow for most nutrients. Increased nitrate occurred in the vadose zone at Woodbury and Hugo, suggesting some production of nitrate within the soil profile; however, groundwater beneath the rain gardens contained significantly lower concentrations of nitrate compared to the inflow, providing evidence of nitrate removal at deeper depths. Phosphorus concentrations were reduced in overflow and groundwater, with the exception of dissolved phosphorus at MLA. Rain garden and background wells often contained similar nutrient concentrations, suggesting that the rain gardens had little impact on the local ground water supplies. This unique six year study provides consistent evidence of the ability of these three rain gardens to reduce nutrient concentrations from urban stormwater.
