Rain gardens have been recommended as a best management practice to treat stormwater runoff. However, no published field performance data existed on pollutant removal capabilities. Replicated rain gardens were constructed in Haddam, CT, to capture shingled-roof runoff. The gardens were sized to store the first 2.54 cm (1 inch) of runoff. Influent, overflow and percolate flow were measured using tipping buckets and sampled passively. Precipitation was also measured and sampled for quality. All weekly composite water samples were analyzed for total phosphorus (TP), total Kjeldahl nitrogen (TKN), ammonia-nitrogen (NH 3 -N), and nitrite+nitrate-nitrogen (NO 3 -N). Monthly composite samples were analyzed for copper (Cu), lead (Pb) and zinc (Zn). Redox potential was measured using platinum electrodes. Poor treatment of NO 3 -N, TKN, organic-N, and TP in roof runoff was observed. Many Cu, Pb and Zn samples were below detection limit, so statistical analysis was not performed on these pollutants. The only pollutants significantly lower in the effluent than in the influent were NH 3 -N in both gardens and total-N in one garden. The design used for these rain gardens worked well for overall flow retention, but had little impact pollutant concentrations in percolate. These results suggest that if an underdrain is not connected to the stormwater system, high flow and pollutant retention could be achieved with the 2.54 cm design method.

This study compared lag time characteristics of low impact residential development with traditional residential development. Also compared were runoff volume, peak discharge, hydrograph kurtosis, runoff coefficient, and runoff threshold. Low impact development (LID) had a significantly greater centroid lag‐to‐peak, centroid lag, lag‐to‐peak, and peak lag‐to‐peak times than traditional development. Traditional development had a significantly greater depth of discharge and runoff coefficient than LID. The peak discharge in runoff from the traditional development was 1,100% greater than from the LID. The runoff threshold of the LID (6.0 mm) was 100% greater than the traditional development (3.0 mm). The hydrograph shape for the LID watershed had a negative value of kurtosis indicating a leptokurtic distribution, while traditional development had a positive value of kurtosis indicating a platykurtic distribution. The lag times of the LID were significantly greater than the traditional watershed for small (<25.4 mm) but not large (≥25.4 mm) storms; short duration (<4 h) but not long duration (≥4 h) storms; and low antecedent moisture condition (AMC; <25.4 mm) storms but not high AMC (≥25.4 mm) storms. This study indicates that LID resulted in lowered peak discharge depth, runoff coefficient, and discharge volume and increased lag times and runoff threshold compared with traditional residential development.

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