Dynamics of reactive mixing hot spots from hyporheic zones to fractured media

Reactive biogeochemical hot spots often form at the intersection of hydrological flow paths, where the confluence of complementary reactants promotes biogeochemical activity that can disproportionately influence macroscale production rates. Understanding where and when these hot spots form is thus critical to modelling biogeochemical processes. This presentation will discuss two mechanisms that contribute to the formation and transient dynamics of mixing-induced biogeochemical hot spots in the subsurface. The first mechanism is shear, which results from the differential velocity of neighboring streamlines (Bandopadhyay et al. 2017). Shear can be induced by medium heterogeneity (Le Borgne et al. 2015) or by nested streamlines patterns in coupled surface/subsurface systems such as hyporheic zones (Bandopadhyay et al. under review). In both cases, deformation of solute plumes by shear leads to a transient increase of concentration gradients and to the formation of dynamic mixing hot spots in zones of maximum compression. The second mechanism is mixing at fracture intersections, which allow fluids with contrasting redox chemistry to mix. Our recent field observations combining hydrogeological, geochemical and metagenomic analysis have demonstrated that the intersection of fractures triggers microbial hot spots in the subsurface (Bochet et al., under review). Simulations of this process suggest that favorable conditions for reactive hot spot development are likely seasonal, creating hot moments of microbial activity at different depths and different times of the year. These field observations and modelling results open new perspectives to understand and model mixing-induced biogeochemical hot spots as dynamic processes. References: Bochet et al. Fractures sustain dynamic microbial hotspots in the subsurface, under review, Nature Geoscience Bandopadhyay et al. Control of topography-driven shear flows on mixing processes from hyporheic zones to hillslopes, under review, Geophys. Res. Lett. Bandopadhyay et al. (2017). Enhanced reaction kinetics and reactive mixing scale dynamics in mixing fronts under shear flow for arbitrary Damkohler numbers. Adv. in Water Resour., 100, 78-95. Le Borgne et al. (2015) The lamellar description of mixing in porous media, J. Fluid Mech. vol. 770, pp. 458-498.