Aerobic Degradation Potential Assessment from Oxygen and Carbon Dioxide Soil Gas Concentrations in Roadside Soil

Abstract

AbstractThe application of calcium magnesium acetate [CMA; Ca0.3Mg0.7‐(C2H3O2)2] to highways as a deicing agent induces aerobic acetate degradation by microorganisms in the capillary fringe, which may reduce O2 demand in the underlying groundwater. We assessed this aerobic degradation potential by computing the diffusive fluxes of O2 and CO2 from measured soil gas concentrations in the unsaturated zone of a mildly acidic roadside soil. The fluxes were relevant to the potential assessment because O2 is consumed and CO2 is generated by the aerobic degradation of organic substrates such as acetate. Soil gas O2 and CO2 were measured on a monthly basis in tubing clusters in the unsaturated zone over a 2‐yr period using portable meters. Quasi‐steady gaseous diffusion models were calibrated with the data, showing that O2 diffused into the fringe at a rate that was proportional to the exiting diffusive flux of CO2. The modeled fluxes were consistent with an independent estimate of acetate‐based microcosm reaction rates in the capillary fringe. Roadside soils have the potential to significantly degrade CMA under aerobic conditions before the deicing agent reaches the water table, thus reducing O2 demand in the groundwater.