The role of midsummer urban aquifer recharge in stormflow generation using isotopic and chemical hydrograph separation techniques

Results dispute the traditional view that groundwater contribution to urban streams during storm events is too small to significantly influence stream flow rates. The study used a two-component hydrograph separation technique involving oxygen-18 and electrical conductivity to quantify event and pre-event components of stream flow associated with a midsummer precipitation event in a highly urbanized, formerly glaciated catchment. Isotopic separation results showed that only about 15% of the rainfall amount contributed to overland flow, with groundwater accounting for 22% of the total stormflow. Elevated electrical conductivity values, possibly associated with the flushing of urban contaminants during the storm, suggested a slightly higher (30%) groundwater contribution to total stormflow. It was also demonstrated that nearly 6% of total rainfall contributed to aquifer recharge representing significant summer recharge in the urban catchment. The findings are contrary to the prevailing assumption that stormflow in highly urban catchments is largely overland flow. (C) 2010 Elsevier B.V. All rights reserved.