The Aerobic Critical Zone: deep transport and reactivity ofdissolved oxygen in fractured bedrock aquifers

National audienceThe persistence of dissolved oxygen in deep groundwater, sustains microbial life andbiogeochemical reactivity, with potential impacts on large scale nutrient cycling. Inaquifers, dissolved oxygen distribution is often heterogeneous and intermittent, but thedriving factors of this variation remain poorly identified. This study is based in twofractured-bedrock catchments in Brittany, one under natural flow-regime and the otherone under pumping regime. The study site is characterized by strong surfacegroundwaterconnectivity, rapid hydrologic response and deep microbial hot-spots.Multi-parameter borehole-logs, CFC data and groundwater level time series from theperiod 2009-2019 were analyzed over 34 piezometers located both in recharge anddischarge zones of both catchments. The variance of dissolved oxygen concentrationsdecreases with depth: shallow concentrations range between 2 - 8 mg/L while deepconcentrations range between 0.05 – 2 mg/L. Resulting from the competitive effects oftransport and reactivity, the observed distribution of dissolved oxygen concentrationswith depth was modelled with an analytical solution of a first-order transport-reactionmodel, which allowed constraining Damköhler numbers related to dissolved oxygen ingroundwater. Advection times and reaction rates were also estimated through Darcy'slaw calculations, CFC’s interpretations and batch tests, allowing an independentestimation of Damköhler number range related to dissolved oxygen in groundwater.The spatial distribution of dissolved oxygen within the natural flow-regime catchmentis transport-limited while being reaction-limited in the pumped site. Dissolved oxygendistribution is shaped by a different chemical reactivity linked to the weathering front.Moreover, preferential transport through fractures or enhanced hydraulic gradientsunder pumping conditions allow deep oxygen delivery and mixing between recent andaged groundwaters forming reactive hot-spots in subsurface