Saturation To Improve Pollutan
(Michael E. Dietz* and John C. Clausen)
Rain gardens have been recommended as a best management practice to
treat stormwater runoff. Replicate rain gardens were constructed in
Haddam, CT, to treat roof runoff. The objective of this study was to
assess whether the creation of a saturated zone in a rain garden
improved retention of pollutants. The gardens were sized to store 2.54
cm (1 in) of runoff. Results show high retention of flow; only 0.8%
overflowed. Overall, concentrations of nitrite+ nitrate-N, ammonia-N,
and total-N (TN) in roof runoff were reduced significantly by the rain
gardens. Total-P concentrations were significantly increased by both
rain gardens. ANCOVA results show significant reductions in TN (18%)
due to saturation. Redox potential also decreased in the saturated
garden. Rain garden mulch was found to be a sink for metals, nitrogen,
and phosphorus, but rain garden soils were a source for these
pollutants. The design used for these rain gardens was effective for
flow retention, but did not reduce concentrations of all pollutants
even when modified. These findings suggest that high flow and pollutant
retention could be achieved with the 2.54 cm design method, but the use
of an underdrain could reduce overall pollutant retention.
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