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Nitrate (NO3--N) removal studies in rain gardens show great variability in removal rates, and in some cases NO3--N was exported. A three-way factorial design (2x2x4) was devised for eight outdoor unvegetated rain gardens to evaluate the effects of hydraulic loading (two sizes and two flow rates), the presence/absence of a buried wood chip layer (2), and the presence/absence of a subsurface saturated zone (SZ) (2) on nitrate-nitrite (NO3--NO2-) removal. Captured stormwater runoff was used in this study. Results showed that the presence of a SZ reduced the NO3--NO2- mass by 75% compared to a 7% reduction without this zone. The presence of a SZ significantly decreased ammonia-N (NH3-N) mass reduction (p<0.00001).

The difference in total nitrogen (TN) mass reduction with the introduced SZ was not significant, largely due to NO3--NO2- mass reduction that was offset by NH3-N mass increases in rain gardens with the SZ. The buried wood chip layer showed no significant effect on N removal. No significant interactions were found between the factors. Overall, the results in this study highlight the importance of incorporating a subsurface SZ for NO3--NO2- load reduction and demonstrate that a buried wood chip layer as a carbon source did not make a significant contribution to N removal.