Partitioning and Aerobic Biodegradation of Hydrocarbon Vapors in Soil from a Weathered Petroleum Spill Site

We dose aerobic soil microcosms from the unsaturated zone of a weathered petroleum spill site with a blend of alkane and aromatic hydrocarbon vapors, subject to abiotic controls. The controls quantify volatile, dissolved, and sorbed partitions, with the latter including deposition to the air water interface. The live microcosms document recalcitrance of one alkane, zero order biodegradation of the remaining 4 alkanes, and first order biodegradation of the 5 aromatics in the dose. We use coupled models of substrate transport in the three partitions to explain the calibrated kinetics of constituent substrate utilization, under the hypothesis of submerged and interfacial biomass fractions. The latter are assumed to occupy the air water interface. The soluble aromatics sustain submerged biomass growth through first order liquid diffusion through the soil moisture, while the insoluble alkane vapors partition directly to the interfacial microorganisms, establishing zero order kinetics at a maximum specific substrate utilization rate. Heterotrophic plate counts confirm the biomass model, and permit us to express the partitioned model parameters as asymptotic Monod kinetics for each biodegradable constituent.