STABLE ISOTOPE ANALYSIS OF NITRATE PATTERNS IN BIORETENTION BASINS

Bioretention basins are implemented in urban areas to curtail stormwater runoff and pollution, but studies show inconsistent nitrate (NO₃^-) removal within basins. Nitrate concentrations sometimes increase in outflow samples compared to incoming runoff. It has been proposed that the presence of an internal water storage zone created by an underdrain establishes conditions required for microbial denitrification, the biotic process that removes nitrate by converting it to nitrogen gas (N₂). Because microbes preferentially consume lighter nitrate isotopes (¹⁴N and ¹⁶O), stable isotope analysis can provide evidence of denitrification. This study explores the impact of basin design and storm type on denitrification. Three basins in the Philadelphia area were selected for analysis: a large (~0.006 km²) wet suburban basin, a small (~0.0002 km²) suburban basin, and a medium-sized (~0.001 km²) urban basin. Unlike the suburban basins, the urban basin is equipped with a raised underdrain and internal water storage zone.

Time series and grab samples were collected from the inlet(s) and outlet of each basin during three storm events. Stormwater nitrate trends showed outlet concentrations lower than the inlet in the large suburban basin, similar outlet concentrations in the small suburban basin, and higher nitrate concentrations in the urban basin. The abundance of heavy ¹⁵N and ¹⁸O in select basin samples showed changing trends over time. Isotopic ratios indicated that microbial denitrification occurred in the urban basin with an internal water storage zone; alternatively, isotopic ratios for the suburban basins indicated mixing of atmospheric and soil nitrogen rather than denitrification. Differences in storm characteristics (including storm duration, antecedent dry periods, and storm intensity) do not appear to increase or decrease nitrate concentrations in any basin, suggesting that basin design is a more influential factor.