Application of isotope hydrograph separation to understand contributions of stormwater control measures to urban headwater streams



Insights into the effects of stormwater control measures (SCMs) on urban stream hydrology and in-stream processes are required to understand their effectiveness in mitigating the environmental problems associated with urbanization. Stable water isotopes were applied to understand processes occurring within SCMs and their effects on water sourcing in urban streams. We sampled ten events from June to November 2013 at four locations along a 360-m headwater stream reach in North Carolina and at four SCMs (two ponds, one wetland and one bioretention) that contribute to the reach. We used streamflow upstream of the SCM-affected reach and outflow from an intensively sampled retention pond as endmembers to quantify contributions of this pond's outflow to streamflow. Synchronous sampling revealed that SCM outflows have different isotopic signatures, likely a function of evaporation and mixing within the storage volume of each SCM. The SCMs also have distinctive isotopic signatures relative to the receiving stream. The isotopic signature of discharge from the intensively sampled pond reveals varying residence times (hours to weeks) within the structure. At sampled timepoints during ten events, this pond, which drains 25% of the watershed's impervious area, contributed an average of 10% (0–21%) of the streamflow on the rising limb and 12% (0–19%) of discharge at peak flow. During recession, this pond contributed an average of 32% (11–54%) of the stream's discharge, reflecting the SCM's design goals of temporarily storing and delaying run-off, mitigating the effects of impervious surface on peak flows. Based on this study, isotopes appear to be a robust tool for examining stormwater-stream dynamics. Copyright © 2015 John Wiley & Sons, Ltd.