Effects of rain gardens on the quality of water in the Minneapolis-St. Paul metropolitan area of Minnesota, 2002-04

Rain gardens are a popular method of managing runoff while attempting to provide aesthetic and environmental benefits. Five rain-garden sites in the Minneapolis – Saint Paul metropolitan area of Minnesota were instrumented to evaluate the effects of this water-management system on surface and subsurface water quality. Most of these sites were in suburban locations and frequently in newer developments. Because of this they were affected by changing hydrology during the course of this study.

Less-than-normal precipitation during much of the study may have resulted in samples that may not be representative of normal conditions. However, the resulting data indicate that properly designed rain gardens enhance infiltration and can reduce concentrations of dissolved ions relative to background conditions.

The runoff events in one rain garden and several runoff events in the other rain gardens produced no sampled overflow during this study because the gardens captured all of the inflow, which subsequently infiltrated beneath the land surface, evaporated, or transpired through garden vegetation. Where measured, overflow had reduced concentrations of suspended solids and most nutrient species associated with particulate material, as compared to inflow. Many of these materials settle to the bottom of the rain garden, and some nutrients may be assimilated by the plant community.

Site design, including capacity relative to drainage area and soil permeability, is an important consideration in the efficiency of rain-garden operation. Vegetation type likely affects the infiltration capacity, nutrient uptake, and evapotranspiration of a rain garden and probably the resulting water quality. The long-term efficiency of rain gardens is difficult to determine from the results of this study because most are still evolving and maturing in relation to their hydrologic, biologic, and chemical setting. Many resource managers have questioned what long-term maintenance will be needed to keep rain gardens operating effectively. Additional or continued studies could address many of these concerns.