Nitrogen Retention in Bioretention Mesocosms with Outlet Controls

Retention of nitrogen (N) in bioretention systems is often less effective than observed for other pollutants. To examine factors that improve N retention, bioretention mesocosms 240L in size were used to retain dissolved nitrogen by bioretention systems, using different media amendments with varying hydraulic responses. Notwithstanding high hydraulic loads, cumulative retention of total nitrogen (TN) and nitrogen oxides (NO_x) was as high as 48%. In contrast to vegetated systems, TN retention by barren media was only 3%. TN retention in all vegetated systems substantially exceeded projected plant N uptake, suggesting that denitrification also contributes to TN removal in bioretention systems. The 63% NO_x retention observed in the outlet controlled treatment was significantly better than the 45% retention observed in the corresponding unrestricted treatment. During high concentration stormwater runs, TN retention ranged from 27% to 64%, and NO_x retention ranged from 19% to78%, with the unrestricted treatment being the least effective. During the low concentration stormwater runs, TN retention ranged from 2% to 50%, while NO_x retention ranged from 28% to76%. Again, the unrestricted treatment was the least effective. NO_x retention in the non-restricted treatment was significantly lower than the corresponding controlled treatment in every case, and in two of three cases for TN. These results document the importance of a saturated zone and extended retention time on improving N retention, even under high hydraulic loading rates.