HGS Research Highlight - Solute transport processes in flow-event-driven stream–aquifer interaction

This post features a recent study by Xie et al., 2016, who used HydroGeoSphere to investigate the interaction between streams and groundwater controls on key features of stream hydrographs and chemographs.

Solute transport processes in flow-event-driven stream–aquifer interaction

Authors: Xie, Yueqing, Peter G. Cook, and Craig T. Simmons.

The conjunctive management of surface water and groundwater requires knowledge of the the exchange between these two water resources over large spatial and temporal scales. Our current knowledge about this regional-scale exchange has so far been limited to stable flow conditions due to assumptions of field methods. Many surface water gauging stations monitor electrical conductivity as well as stream stage. Because groundwater is usually more saline than stream water, these electrical conductivity records should contain information on surface water-groundwater exchange over large spatial and temporal scales, but the data does not appear to be widely used. This study attempted to improve the understanding of transient surface water –groundwater exchange processes at regional scales in order to make use of such data in the future.

To examine trends in solute concentrations within the stream requires a 3D stream-aquifer model that simulates both flow and transport within the stream channel and the aquifer. In this study, HydroGeoSphere was chosen to carry out numerical simulations. We simulated flow events through a highly simplified, synthetic 3D stream-aquifer system, and observed the resulting variation in stream water solute concentrations at downstream locations. The results show that the wave usually migrates faster than the average stream flow. As seen in the first figure, at 0.75 day, the wave peak and the concentration trough occur in approximately the same position. However, at 1.0 day, the concentration trough lags about 1.5 km behind the wave peak, and this lag increases with time. This study shows that, in most cases, the wave front will precede the solute front (arrival time lag), and the wave recession will precede the solute return (return time lag). Both arrival time lag and return time lag increase with increasing wave duration. However, arrival time lag decreases with increasing wave amplitude, whereas return time lag increases. This lag phenomenon is a result of combined effects of several processes including dispersion, wave propagation and bank

An example to show variation with distance of (a) stream depth, (b) the concentration of the stream water at different times.

An example to show variation with distance of (a) stream depth, (b) the concentration of the stream water at different times.

In addition, this study also discovered that the percentage of groundwater in the stream hydrograph increases with distance and this percentage can be very high, as seen in the second figure. This phenomenon has been observed repetitively and is referred to as the “old water paradox” in catchment hydrology, which requires slow-moving groundwater to discharge rapidly during flow events. In this study, the high contribution of groundwater is caused by in-stream and near-stream processes rather than increased flow of groundwater through the catchment. The combined effects of in-stream and near-stream processes can partially explain the “old water paradox”.

Variation in the proportion of groundwater in stream discharge with distance for various wave amplitudes.

Variation in the proportion of groundwater in stream discharge with distance for various wave amplitudes.

The complete manuscript is available from Journal of Hydrology.