Direct Observations of Hydrologic Exchange Occurring With Less‐Mobile Porosity and the Development of Anoxic Microzones in Sandy Lakebed Sediments

Quantifying coupled mobile/less‐mobile porosity dynamics is critical to the prediction of biogeochemical storage, release, and transformation processes in the zone where groundwater and surface water exchange. The recent development of fine‐scale geoelectrical monitoring paired with pore‐water sampling in groundwater systems enables direct characterization of hydrologic exchange between more‐ and less‐mobile porosity during tracer tests. We adapt this technique to sandy interface sediments at a groundwater flow‐through kettle lake. Tracer experiments were conducted within controlled‐head permeameters over a range of specified downward flow conditions over several days. Although the bed was predominantly composed of highly permeable sands and gravels, cobble inclusions created less‐mobile flow zones at the centimeter scale. Less‐mobile porosity fractions, residence times, and rates of exchange were inferred from paired bulk and fluid electrical conductivity data, without the need for inverse model calibration. The conservative solute experiments were paired with 15NO3− and other reactive amendments, revealing anaerobic processes occurring at shallow sediment depths where pore‐water sampling indicated bulk‐oxic conditions. The average less‐mobile porosity residence times as evaluated with the geoelectrical method were on 1‐hr timescales, which appear to be biogeochemically important in the context of creating anoxic microzones within less‐mobile porosity of sandy interface sediments. Plain Language Summary Streams and lakes often exchange water with shallow aquifers, and this exchange can greatly alter water quality and impact greenhouse gas production. Water flows through the shallow sediments that separate surface and groundwater at varied rates due to sediment heterogeneity. However, existing fluid sampling methods typically only sense water moving through the faster pathways, and less‐mobile pore spaces are not visible. We present a method to directly measure varied flow rates and residence time within porosity that is less mobile compared to faster flowpaths. The method is applied to a natural lakebed where surface water and groundwaters exchange and indicates that less‐mobile porosity may be influential to the transport of nutrients and contaminants. Key Points Exchange with less‐mobile porosity can be directly quantified with the new interface‐scale geoelectrical methodology and analysis techniques Even high permeability sand and gravel lakebed sedime