Variation in Biogeochemical Processes Controlling Uranium and Chromium Induced by Soil Physical Complexity.

The fate and distribution of contaminants in soil and sedimentary environments is profoundly impacted by variations in subsurface permeability—not only will such variations alter the hydraulic flow-field, they may also induce redox zonation within pore domains dominated by advection or diffusion. For example, the production of labile organic carbon within micropore domains via fermentation may result in the local (µM to cm scale) depletion of terminal electron acceptors, thus resulting in steep chemical gradients (e.g. redox, pH, etc) over distances spanning millimeters.  Limited consumption or diffusion of soluble organic carbon within these small pore networks will result in the continued dominance of reducing conditions, whereas conditions may be appreciably less reducing within advective domains due to rapid electron acceptor replenishment (e.g. oxygen transport from infiltrating surface waters). Consequently, redox-sensitive contaminants such as uranium and chromium will exhibit drastic differences in chemical response across these domains; uranium and chromium may precipitate under reducing conditions yet remain soluble in more oxidizing advective regions.  To examine the effect of such steep chemical-permeability gradients on the fate of uranium and chromium, we have constructed novel flow cells to study uranium and/or chromium partitioning between different pore domains (i.e., to examine mass transfer between low and high zones of permeability consisting of clay and sand sized particles, respectively), which are equipped with a highly X-ray transparent polyester thin film, allowing elements of choice to be imaged with X-ray fluorescence during or after the completion of an experiment. Within zones of low permeability, reducing conditions are maintained with Shewanella sp. through an initial infusion of lactate, while conditions outside of clay domain remain relatively less reducing. Oxic influent solution containing uranium or chromium is passed through the advective (sand) domain within the cell, which undergoes advective transport and also diffuses into micropore domains (clay).  Our results illustrate that diffusion of uranium and chromium from oxic advective domains into reducing diffusive material results in rapid precipitation; uranium precipitates are chemically/physically protected until electron donor within the micropore domain is depleted, upon which time uranium may be oxidatively remobilized, while chromium forms mixed Fe(III)-Cr(III) solids which will not remobilize upon reversion to oxic conditions.