Case Studies > Haddam Demonstration Site > Rain Garden > Methods > Modification

Modification to Increase Nitrate-N Retention

Retention of nirtrate-N in a laboratory pilot scale rain garden was reported to be 24% (Davis, et al. 2001). Retention of nitrate-N by the rain gardens at the Haddam site was 35% (Dietz and Clausen, 2005). Kim, et al. (2003) recommended the creation of a saturated zone at the bottom of the rain garden to promote denitrification and increase the poor retention of nitrate-N by rain gardens. To assess whether a saturated zone would reduce influent concentrations of nitrate-N, the paired watershed study design was used (Clausen and Spooner, 1993). The benefit of this method is that variance due to individual differences within rain gardens can be controlled. The two rain gardens were monitored for approximately one year. This period of time constituted a calibration period in which a regression relationship was developed between the two rain gardens for each of the constituents measured. The treatment was then applied. In this case, treatment consisted of raising the outlet of the underdrain pipe, creating a saturated condition in 0.5 m of the rain garden (0.25 m in the stone and 0.25 m in the soil mixture). Analysis of covariance (ANCOVA) was used to determine if significant differences due to treatment existed in the slopes and intercepts of the regressions for the calibration and treatment periods for all constituents measured. A significant decrease in nitrate-N concentrations would indicate that denitrification was most likely occurring. Although nitrate-N was the target pollutant, the effect on all pollutants measured was analyzed.

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