Water quality characteristics of three rain gardens located within the Twin Cities Metropolitan Area, MN.

Abstract

Rain gardens are becoming an increasingly popular feature of Low-Impact Development. They promote decentralized stormwater management by treating stormwater runoff on-site. Rain gardens are effective because they store and infiltrate water allowing biogeochemical processes to occur, thereby removing pollutants from the runoff. Ensuring that rain gardens are functioning properly can be achieved by monitoring studies and visual assessments. This study focuses on the results from a water quality monitoring study and supplements the results with visual inspections of each site. Water quality benefits of rain gardens have been inconsistent and depend on various factors including size of rain garden, design specifications, location, vegetation, and contributing area. The U.S. Geological Survey monitored the water quality in three rain gardens located in the Twin Cities Metropolitan Area, Minnesota. The objective was to compare and contrast water quality characteristics from rain gardens with different designs and contributing area land uses. Representative water samples of the inflow, overflow, vadose zone, and groundwater were analyzed for nutrients typically found in urban stormwater runoff. Generally, lower median concentrations of nitrogen, phosphorus, and total suspended solids were found in the vadose zone and groundwater compared to the inflow, suggesting good performance in terms of water quality benefits. One site exhibited no difference in dissolved phosphorus concentrations; believed to be a result of the low concentrations entering the garden. Higher chloride concentrations were found in the soil and ground water compared to the inflow at two sites. Good evidence of nutrient removal in rain gardens resulting in relatively cleaner or unaffected recharge water reaching local ground water supplies was found from this study. Visual inspections of the three rain gardens provided additional evidence that these three sites are functioning well. There did not appear to be any major structural failures. The vegetation at each site also appeared to be healthy; however, the presence of wetland species and noxious weeds highlights the importance of maintenance.