Effect of subsurface fluid mixing on microbial growth and bioclogging in fractured media: field observations and modelling

International audienceSubsurface environments host a large diversity of microorganisms, constituting a significant part of the Earth's biosphere and exerting a first order control on biogeochemical cycles. Subsurface bacteria can also cause significant disturbances in subsurface operations by altering hydraulic properties through bioclogging. However, due to challenging observation conditions, the dynamics of subsurface microbial activity are poorly understood. Here we investigate the effect of fluid mixing on subsurface microbial growth. From field observations combining hydrological, geochemical, and metagenomics characterization, we document two mixing processes inducing microbial growth: i) naturally occurring mixing generating biotic iron-oxidation in a fractured aquifer (Bochet et al. Nature Geoscience accepted), ii) man-induced mixing leading to bioclogging by iron and manganese precipitates in a geothermal dipole (Burté et al. Environmental Science and Technology 2019).From the analysis of field data and the development of a mechanistic model, we demonstrate that mixing-induced microbial activity can occur naturally at the intersection of fractures where fluids with contrasting redox chemistry mix. Fractures provide pathways for intermittent oxygen delivery in the deep subsurface, sustaining dynamic microbial hot spots when intersecting with fractures bearing iron-rich groundwater. In geothermal dipoles, fluid mixing is triggered by pumping, which leads to the convergence of waters of contrasting chemistries in the pumping borehole. We develop a reactive transport model to simulate the kinetics of the associated precipitation reaction, which generates bioclogging in injection boreholes. These findings suggest that fluid mixing, either naturally occurring or driven by anthropogenic activities, is a major driver of microbial activity in the subsurface, substantially influencing Earth's biogeochemical cycles and subsurface water and energy resources.Biobliography:Bochet et al. Intermittent delivery of oxygen through fractures sustains deep microbial hotspots in the subsurface, accepted in Nature GeoscienceBurté et al. Kinetic study on clogging of a geothermal pumping well triggered by mixing-induced biogeochemical reactions. Environmental Science & Technology 2019 53 (10