Evaluating the impact of storage zones on backward contaminant transport: A comparative study of the classic advection-dispersion equation and storage zone models in riverine systems

Abstract

Accurate identification of unknown contaminant sources is a critical challenge in river management, where source characteristics must be inferred from limited observations. This study addresses this challenge by applying the inverse solution of the Transient Storage Model (TSM) using the Group Preserving Scheme (GPS) to evaluate the influence of storage zones in three test cases. In the first case, the direct and inverse TSM solutions were validated against tracer data from Uvas Creek and compared with results from the Advection-Dispersion Equation (ADE). The inverse TSM model outperformed ADE, accurately capturing delayed contaminant peaks and extended concentration tails due to storage effects, with statistical indicators (R² > 90 %) and significantly lower RMSE values. The second case simulated a hypothetical river with non-uniform hydraulic and geomorphological conditions, demonstrating TSM's robustness in handling complex scenarios, where ADE struggled to reconstruct concentration peaks, particularly at later times. In the third case, a framework using dimensionless Péclet and Damköhler numbers was developed to characterize river conditions and transport relationships. Results showed that increasing the Péclet number and decreasing the Damköhler number improved model accuracy, with TSM performing better under strong storage effects. For rivers with low Damköhler values, the ADE model effectively approximated contaminant distribution. However, when the exchange rate between storage zones and the river is high, the ADE model exhibited significant errors in identifying source locations. This study demonstrates the TSM's advantage for precise contaminant tracking in rivers with complex transport dynamics, providing valuable insights for enhanced river pollution management.