USING DUAL ISOTOPES TO IDENTIFY SOURCES OF NITRATE IN SUBURBAN PHILADELPHIA STORMWATER RETENTION BASINS

Understanding sources of urban runoff can help with better design of stormwater management projects (SMPs). Nitrate (NO₃^-) has a variety of sources in urban environments: deposition of atmospheric NO₃^- following combustion by vehicles/power generation, leaching of NO₃^- from organic material and debris, leaking from sewers, and runoff of applied fertilizer. The isotope ratios of ¹⁵N:¹⁴N and ¹⁸O:¹⁶O differ among the sources. For example, atmospherically deposited NO₃^- is marked by high δ¹⁸O because of enrichment during the reaction of NO₂ and ozone, while NO₃^- from sewage sources generally has high amounts of ¹⁵N.

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.