Pedogenesis and Hydrologic Function in an Established Rain Garden.

As impervious landcover increases across the U.S., the natural hydrologic cycle is disrupted, resulting in increased stormwater runoff and contamination of receiving waters. To address these urban water problems, many cities use green infrastructure (GI), such as rain gardens, to infiltrate stormwater, thereby attenuating peak flows, and treating runoff. The ability of GI to remove trace elements and nutrients from runoff through physical and biological processes is well-documented. However, the flow reduction performance of rain gardens and the hydraulic function of the rain garden components over time are not well studied. To address this data gap, this study evaluated the hydraulic function of two linked, four-year-old rain gardens in Cincinnati, OH. From 2012 to 2015, we measured the unsaturated hydraulic conductivity of developing surface organic and A horizons, the drainage rate of the sandy subsoil in each rain garden, and in 2015, both infiltration and drainage on the surrounding turf, which served as an analogue for urbanized soil and land cover conditions. Over the course of the monitoring period, the development of an A horizon was observed in both gardens. In general, hydraulic conductivity within the gardens varied spatially and temporally. Surface and subsurface conductivities also varied between the upper and lower garden. These variations in conductivities were not statistically significant within or between the gardens, though favorable infiltration conditions within the gardens were four times higher than surrounding turf, highlighting rain garden effectiveness. These results suggest that over time, soil formation in rain gardens may occur, but may not have an appreciable nor deleterious effect on the hydraulic properties and overall function of the rain garden.