Dual isotopes of nitrate reveal varying flow paths to stormwater retention basins

Abstract

Nitrate (NO3–) isotope ratios have been used to track sources of urban runoff, but their use as a tracer of flow paths has not been explored in detail. Urban flow paths are complex, and there is a gap in understanding how spatial and temporal variability impacts runoff chemistry. NO3– in urban systems can originate from atmospheric deposition, leaching from organic material, leaking from sewers, and fertilizer. Sources can be distinguished by their δ15N and δ18O signatures. We collected time series samples of stormwater runoff entering two stormwater retention basins, one large (0.32 km2 catchment) and one small (0.016 km2 catchment) in suburban Philadelphia. We used a Bayesian mixing model to determine that NO3– sources in the stormwater basin inlets were a mix of soil nitrogen (N) and atmospheric deposition across six storm events. Activation of impervious surfaces which collect atmospheric nitrogen was observed at the beginning of storm events and the portion of atmospheric versus soil N sources varied throughout the storms. However, storms with the same characteristics did not lead to the same isotopic patterns. Further evidence of the importance of flow paths was the contrast between the large and small basin. The large basin had more NO3– source variability between samples (ranging 6 % to 58 % atmospheric NO3–) compared to the small basin (16 % to 43 %). Thus, the NO3– isotopes suggest more distinct flow paths in the basin with the large catchment and more mixing of flow across land uses in the small basin’s catchment. Quantifying flow path variability from storm to storm and between different catchments can improve design and placement of urban stormwater control measures.