TEMPORAL VARIATIONS IN ELECTRICAL CONDUCTIVITY IN A SECOND ORDER URBAN STREAM

Storm events can connect streams to hydrologic pathways which are inactive during baseflow conditions, or can alter the relative importance of specific pathways such as baseflow and runoff. These alterations can result in changes in stream discharge and water quality. To investigate the streamflow dynamics in the Bradley Creek watershed in Wilmington, NC we examined electrical conductivity (EC) behavior during baseflow and stormflow conditions. In addition to EC, rainfall amount, stream stage, and water temperature data were also collected. Most storm events resulted in decreases in EC in the stream, as well as in nearby retention ponds. This decrease in the electrical conductivity is typically attributed to the dilution of the stream water by low EC rainwater. However, following some storm events an increase in EC occurred within the stream. The increases in EC typically occurred during storm events which were preceded by extended periods of time during which there was not substantial rainfall. Pre-event EC values were close to those of shallow groundwater and suggest that stream flow was dominated by baseflow between storm events. Thus, the observed increases in EC during storm events were likely related to the ‘first flush’ phenomenon via surface runoff contributions. The increases were usually of short duration and were followed by a decrease in EC during the latter stages of storm events. The EC data were examined to evaluate the effect of factors such as rainfall amount and the time since previous rainfall on the EC increases. Additionally, the contribution of the ‘first flush’ to the total stream EC load was assessed by comparing storm and non-storm events.