Hyporheic Exchange Flow Under A Unit, Medial Gravel Bar: A High-Resolution Field Study

The impacts of Hyporheic Exchange Flows (HEF) on phenomena such as stream water quality, greenhouse gas emissions, river temperature, and nutrient cycling are broadly recognized. Much research has investigated the biogeochemical and physical processes induced by HEF; however, there is a scarcity of field studies scrutinizing HEF provoked by Medial (within the stream) Gravel Bars (MGBs), in spite of the significant role that they play in stream water quality and self-depuration capacity. Furthermore, the few existing field studies were conducted on rather large bars or fluvial islands with a limited number of observation wells. The typical incongruity between the observation domain and spatial resolution of HEF studies, together with the well-described complexity of sedimentary architecture in MGBs, hamper our capacity for appropriately describing and elucidating hyporheic processes. Hence, the heterogeneity of river bed sediments, alongside the high temporal and spatial variability of hyporheic processes, underscore the necessity of using approaches with a higher resolution, and first focusing them on simpler systems, such as unit MGBs. Consequently, we aim to instrument a small, unit MGB in a natural stream with a 3-D high-resolution grid of mini-piezometers nests (less than 1 m grid spacing in x-y and less than 0.2 m spacing in depth z). In order to capture the physical variables involved in hyporheic flow,e.g., flow velocities, residence time, flow rates, as well as the biogeochemical variables such as dissolved oxygen, NO3, NH4, DOC, and POC, each nest will contain three or four PVC wells screened at different depths, where we will measure piezometric levels, as well as a continuously screened well measuring the level of the alluvial water table. We will estimate hydraulic conductivity by performing constant-head injection tests at each mini piezometer. We will perform extensive tracer injection experiments using both a conservative saline tracer (measured as electrical conductivity at a frequency of 10 hz) and a non-conservative thermal tracer (measured with iButtons at a frequency of 1 hz). This will be done for at least two different steady flows and water quality variables, e.g., DO, NO3, NH4, DOC, and POC will be screaned with less than one hour time intervals. Our results will provide a 3-D picture of hydraulic conductivity and its spatial variation due to sediment heterogeneity. Depicting MGB-induced hyporheic processes, e.g., biogeochemical processing, nutrient cycling, and water filtration rates. The resulting high-resolution dataset will allow us to assess the performance of typical field techniques in capturing the actual spatial variability of HEF.