A Breath in the Dark: Transport and Reactivity of Dissolved Oxygen in Fractured-Bedrock Aquifers
Résumé
The persistence of Dissolved Oxygen (DO) in deep groundwater, sustains microbial life and biogeochemical reactivity, with potential impacts on large scale nutrient cycling. In aquifers, DO distribution is often heterogeneous and intermittent, but the driving factors of this variation remain poorly constrained. This study is based in two comparable fractured-bedrock catchments in Brittany, a Natural-Flow Regime (NFR) and a Pumping Regime (PR) catchment.
Multi-parameter borehole-logs, groundwater residence-time tracers (CFC, SF6 and 36Cl), dissolved gases, cations and anions concentrations were analyzed over 34 piezometers located both in recharge and discharge zones of the catchments. Opposing gradients of DO and dissolved iron concentrations show that, at catchment-scale, DO reactivity is closely linked to Fe(II) oxidation. The depth-distribution of DO concentrations depends on groundwater flow regime. In the NFR catchment, DO is readily consumed in the first 70 m below surface. DO concentrations are shaped by a different chemical reactivity linked to the average depth of the weathering front. Conversely, in the PR catchment DO spreads over a wider range of depths with detectable DO concentrations as deep as 300 m.
Resulting from the competitive effects of transport and reactivity, the observed distribution of DO concentrations with depth was modelled with an analytical solution of a first-order reactive-transport model, which allowed constraining Damköhler numbers (Da) related to DO in groundwater. DO concentrations in the NFR catchment are transport-limited (Da>1): reaction rate is faster than advective transport and DO is consummed at shallow depths. On the other hand, stronger hydraulic gradients in the PR catchment enhance advection velocities and the DO distribution becomes reaction-limited.
The present study shows how the groundwater flow regime impacts the distribution of DO in the deep Critical Zone, as it affects the balance between transport and reactive processes. Enhanced hydraulic gradients under pumping conditions or preferential transport through fractures allow deep oxygen delivery with potential impacts on biogeochemical reactivity and microbial hot-spots formation in subsurface.