USING DUAL ISOTOPES TO IDENTIFY SOURCES OF NITRATE IN SUBURBAN PHILADELPHIA STORMWATER RETENTION BASINS
In two stormwater retention basins, one large (0.6 ha) and one small (0.02 ha) in suburban Philadelphia, we collected time series and grab samples of stormwater runoff entering via the basin inlets during six storm events (October 2019 – October 2020). We used the isotopic signatures of nitrogen and oxygen in NO3- to identify its sources. We determined that NO3- entering stormwater basins is a mix of NO3- from organic nitrogen and from atmospheric deposition, with organic N being the dominant source but atmospheric deposition being most prominent at the beginning of storm events. This trend was observed across a variety of storm types at the larger basin, but was less pronounced at the smaller basin. Data from an August 2020 storm sampled at both basins revealed a drop in the atmospheric fraction from 39% to 12% in the large basin inlet while this fraction made up 39% of NO3- in the small basin inlet at the start of the storm, and 34% at the end.