STORAGE DYNAMICS REVEALED BY WATER ISOTOPES PROVIDE INSIGHT INTO WATER QUALITY FUNCTION OF STORMWATER GREEN INFRASTRUCTURE

Increasingly popular, stormwater green infrastructure is touted as improving water quality, through filtration and retention that allows plant uptake and biogeochemical processing to occur. Many data sets reveal large inter-storm variability of water quality in green infrastructure effluent, suggesting that internal dynamics that control water transit time may play an important role in the water quality functioning of green infrastructure. We hypothesized that collecting data on water stable isotopes, in addition to solute chemistry, would provide insight into transit times and would help explain variability in water quality flowing out of three forms of green infrastructure. Water isotopes, chemistry, and fluxes were measured from bulk precipitation, inflow, outflow, and surface water storage for a green roof, bioretention cell, and wetland at a site in northeastern Ohio.

On the green roof, outflow isotopes were variable within storms, but flow-weighted averages were similar to bulk precipitation for each event, suggesting transit times of minutes to hours. First flush behavior for solutes was exhibited for some storms and some solutes, but much of the inter-event variability in solute export could be explained by precipitation amount and antecedent dry period of each storm. In the bioretention cell, inter-event storage and release of old water is sometimes observed in the outflow isotopes. Outflow nitrogen concentrations were generally lower when old water was discharged, suggesting that denitrification is occurring within the bioretention cell. However, antecedent dry period also appears to influence nitrogen concentrations, suggesting some discharge of new water even during moderately-sized storms. Isotopic hydrograph separation was possible for some storms in the wetland, and in these cases, solute concentrations in the outflow can be explained by mixing of new water with previously ponded water. Where solute concentrations can’t be explained by mixing, biogeochemical processing may be happening during the storm period.