Time Series of Electrical Conductivity Fluctuations give Insights into long‐term Solute Transport Dynamics of an Urban Stream

Artificial tracers are often used for quantitative estimates of solute transport properties in rivers. However, single-injection tracer tests give insights in transport characteristics limited to the ecohydrological conditions at the testing time. Series of time-consuming and laborious tracer tests would be required to properly capture seasonal changes. The present study uses intrinsic diurnal fluctuations of electrical conductivity caused by discharge of treated wastewater as a tracer to evaluate solute transport processes along a 4.7-km reach of the River Erpe, Germany. By reproducing the fluctuations recorded along the river using the solute transport model OTIS, this study investigated the long-term dynamics in solute transport properties. Individual 48-hour curves of electrical conductivity were used in the steady state configuration of the model to gain 48-hour-integrated estimates of selected transport parameters. Using a sliding window approach in 1-hour steps along the 2270-hour time series of electrical conductivity the temporal variability of solute transport between April and June 2016 was assessed. To test the identifiability of parameters using the proposed method, sensitivity analyses and a breakthrough curve analysis of selected 48-hour windows were implemented. With time advancing into the summer, a significant rising trend (Mann-Kendall test p-value < 0.05) of the cross sectional area of the channel was observed and attributed to the growth of macrophytes and a significant slightly decreasing trend for the storage rate was found. The presented method is of high value for river management, as promoting transient storage enhances biogeochemical cycling and benefits water quality.