Hyporheic exchange in heterogeneous streambeds under losing and gaining flow conditions

Recent studies have shown that stream-groundwater interactions (gaining or losing flow conditions) have a major impact on hyporheic exchange fluxes. However, the physical complexity of natural streams has limited our ability to study these types of interactions systematically in the field, and to evaluate their importance to biogeochemical processes and nutrient cycling. In this work we were able to quantify the effect of losing and gaining fluxes on hyporheic exchange in heterogeneous streambeds by combining experiments in a laboratory flume and modeling. Tracer experiments were conducted for measuring hyporheic exchange with using a dye and NaCl under various combinations of overlying water velocities and losing or gaining fluxes. The goal of the modeling was to extent and to supplement the hydraulic conditions covered by the flume experiments with a wide range of scenarios. In the model, the surface water domain was represented by a CFD model which generates the head distribution. Subsurface flow and transport is simulated in 3D using MIN3P. The flume experiments revealed that hyporheic exchange fluxes under losing and gaining flow conditions were similar, and became smaller in a comparable manner when the losing or gaining flux increases. Heterogeneity had little effect on hyporheic exchange under gaining flow conditions due to the compaction of the hyporheic zone. However, local hydraulic conductivity led to preferential flow paths of stream water under neutral conditions (strong horizontal flow component) and losing conditions (strong vertical flow component). Preferential flow paths were also evident under gaining conditions but affected mainly the upwelling groundwater. The model was set up to resemble the conditions in the flume, and reproduced the flume data very well. The propagation of dye fronts as observed in the flume is currently implemented into the model. The combination of experimental and modeling results enable us to systematically study the coupling between flow conditions and flow patterns in a representative heterogeneous streambed. We are currently extending these findings to evaluate the implications of flow patterns in heterogeneous streambed for biogeochemical processes.