Case Studies > Haddam Demonstration Site > Rain Garden > Conclusions

Conclusions

The study rain gardens, designed for 2.54 cm of rainfall, accepted 99.1% of roof runoff (and associated pollutants) over two years. The study rain gardens retained ammonia-N, but did not consistently remove nutrients or metals. TP was leached from these rain gardens, especially immediately following construction. The creation of the saturated zone did reduce concentrations of TN, TP, and Zn, in underdrain outflow, and lowered soil redox potential. However, saturation increased ammonia-N concentrations slightly. In addition, Mn was released as a result of the saturated condition in the treatment garden. Overall metals retention was poor, and not in line with results of laboratory studies. Pb was exported more readily from the gardens than Cu and Zn. These rain garden soils were likely a source of Cu, Pb, Zn, TN, and TP.

Despite the poor overall retention of metals and TP, the mulch retained high percentages of some of these pollutants. Although plant uptake of metals was a negligible percentage of total inflow, winterberry (Ilex verticillata) shrubs bio-magnified Zn much more than the other shrubs. These results suggest the potential use of winterberry where zinc uptake may be desirable.

Surprisingly, the rain gardens did not decrease water temperature in the spring and summer. Perhaps the northern aspect of the roof and the shallow soil depth contributed to this unexpected finding.

The input to these gardens was roof runoff with low concentrations of pollutants. These water quality results may not apply to source water with higher contaminant concentrations. Methods of improving water quality renovation by rain gardens should be explored in future studies.
Overall, these rain gardens provided runoff control, but water quality renovation was not good. Installing a rain garden without an underdrain may not be appropriate in all situations. However, given the high overall retention of flow found for the 2.54 cm design method used in this study, a rain garden could be an effective BMP in reducing flow and pollutant loads if an underdrain were not connected to the stormwater system.

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