Predicting Abiotic Reduction Rates Using Cryogenically Collected Soil Cores and Mediated Reduction Potential Measurements

Abiotic reduction of contaminants in iron based laboratory model systems has been studied extensively, but the resulting understanding is not sufficient to allow prediction of reaction rates under field conditions. This study demonstrates that rates of contaminant reduction by aquifer sediments can be predicted by combining several recent methodological innovations into an integrated protocol that is amenable to use on field samples. The protocol involves (i) cryogenic collection of core samples to preserve redox conditions, (ii) contaminant reduction rate measurements, (iii) oxidation-reduction potential measurements of sediments using electron shuttle-mediated potentiometry, and (iv) regression analysis of mass-normalized rate constants for contaminant reduction and reduction potential. The protocol was demonstrated using carbon tetrachloride as the model contaminant and 43 aquifer sediment samples from nine locations at four sites. The abiotic rate of degradation covers 5 orders of magnitude and a 450 mV potential range. The 95% confidence interval was used to represent the minimum reactivity that would be observed at each of the field sites under the measured reduction potentials. This provides a novel and effective protocol for screening abiotic reduction capabilities or establishing site specific rates.