The Dominant Role of the Aquifer Matrix in Regulating Electron Acceptor Attenuation Rates

Electron acceptors, such as nitrate, in aquifers are attenuated via biologically mediated redox reactions in groundwater. Their reduction is coupled to the oxidation of electron donors. The main source of electron donors, particularly over long water-sediment contact times is the sediment/rock matrix. The physical and chemical properties of aquifer matrices, thus, govern the rate and extent of natural attenuation. In addition to catalyzing redox reactions, subsurface microorganisms, actively scavenge electrons from reduced mineral phases or, in the case of organic carbon, hydrolyze solid- to dissolved-organic-carbon (DOC). The release of aqueous electron donors is often the reaction-limiting step. Moreover, at the aquifer scale, both  the quantity and heterogeneous distribution of electron donors influence the overall extent of reduction. The extent of reaction hinges on flow paths that ensure long-enough contact time between aqueous electron acceptors and solid-phase electron donors, intrinsically linking hydraulic conductivity and sediment makeup with reactivity. Here, we show both analytically and numerically that hydrolysis of DOC from the matrix limits and conditions the rate of electron acceptor reduction. Our results show that this conditioning drives otherwise complex non-linear reactions to simple zero-order kinetics. In ongoing experiments with biostimulated aquifer sand, via a nitrate injection, preliminary results highlight that this limiting step is also relevant for matrices dominated by reduced iron minerals constitute the dominant electron donors. We expect to validate our theoretical analysis experimentally, and propose a framework to address the challenges in better representing both the reaction and transport properties of aquifers by accounting for sedimentological information.