WATER ISOTOPES PROVIDE INSIGHTS INTO THE HYDROLOGIC FUNCTIONING OF STORMWATER GREEN INFRASTRUCTURE

Increasingly popular, green infrastructure for stormwater management is touted as decreasing runoff volumes and improving water quality. These water quality improvements are attributed to filtration by flow through the green infrastructure's substrate and retention of water that allows biogeochemical processing to occur. Often, however, green infrastructure is treated as a "black box" and processes are inferred from inflow and outflow concentration data. Stable isotopes of the water molecule have the potential to shine a light into the black box, because they are an ideal tracer of hydrological processes occurring within the system. Here we explore insights to be gained by examining water isotopes for an extensive green roof and a bioretention cell for which simultaneous flow and water quality data are available. The green roof has a shallow substrate (~8 cm) with an underdrain, while the bioretention cell has a thick substrate (~1 m), with an underdrain and potential exfiltration into the surrounding soil. Water isotopes were measured from bulk precipitation, grab samples of inflow to the bioretention cell, and outflow from the underdrains of the green roof and bioretention cell.

In small, low intensity events, both the green roof and bioretention cell effectively smooth out any short term fluctuations in precipitation isotopes. Under more intense rainfall, the green roof outflow exhibits short-term fluctuations, which mirror the precipitation isotope time series, while the bioretention cell outflow is similar to the bulk precipitation. These differences probably reflect saturation and rapid transit of water through the green roof substrate during intense storms, whereas the thicker bioretention substrate may not develop dominant preferential flowpaths that effectively bypass treatment within the cell. Neither the green roof nor the bioretention cell show evidence of significant evaporation of water that reaches the outflow, but inter-event storage and release of old water from the bioretention cell is observed across a series of storms. Connecting such isotopic insights to the observed water chemistry is an on-going challenge.