Combined Effects of Geological Heterogeneity and Discharge Events on Groundwater and Surface Water Mixing

Exchange between stream water (SW) and groundwater (GW) is an important mechanism controlling water quality in river-corridors. Different works have already recognized the complex interactions between hydrological and geological characteristics for SW-GW exchange fluxes (EF). However, it remains unclear how EF and subsequent SW-GW mixing are affected by different discharge events (e.g., peak discharge magnitude and event duration) that take place within different geological settings (e.g., highly permeable sand units juxtaposed to low permeable silt units). Here, to assess the combined effects of geological heterogeneity and discharge events on the EF patterns and subsequent SW-GW mixing in riparian aquifers, we combined a fully-coupled 3D numerical model with a mixing cell routine using 35 binary sedimentary geological settings (covering five different sand to silt ratios in the alluvial aquifer material) and eight different hydrological scenarios. Our results indicate that geological heterogeneity at the reach-scale has secondary effects on EF patterns and on the resulting net EF, mainly affecting the EF magnitudes. While EF magnitudes increased with increasing sand fractions (and hydraulic conductivity (K) values), including geological heterogeneity in the model generally enlarged SW infiltration, resulting in slightly higher net EF in comparison to the equivalent K homogenous cases. In general, SW-GW mixing under baseflow conditions decreased with increasing sand fractions. Furthermore, mixing was higher in the equivalent homogenous cases (e.g., similar K values) in comparison to the heterogeneous cases. On the other hand, the increase in SW-GW mixing due to discharge events was larger in sand units, as well as in the generated heterogeneous cases in comparison to their equivalent homogeneous cases. The results also indicated that more intense discharge events (higher peak discharge) promoted SW-GW mixing more than longer events presenting similar cumulative discharge values. Our work extends the knowledge on SW-GW mixing, critical for river restoration strategies and for downstream management of dam-regulated rivers, and sheds some light on potential future research direction in integrated SW-GW assessments and modelling.