In Situ Microcosm Biodegradation Rates of Alkylbenzenes Under Iron-Reducing Conditions

In petroleum hydrocarbon plumes iron-oxyhydroxides on the aquifer solids can be important electron acceptors for natural biodegradation. Due to the difficulty of identifying specific redox zones, in situ rate estimates under iron-reducing conditions are rare. To measure degradation rates, we used an in situ microcosm to isolate an aquifer volume in a predominantly iron-reducing zone of a crude-oil plume near Bemidji, MN. The microcosm was a stainless-steel cylinder open at the bottom with an internal volume of 4.76 L, and estimated pore space of 1.43 L. Microcosm pore water was displaced by injecting water from an adjacent well spiked with bromide and a series of alkylbenzenes. Concentrations in samples from the microcosm initially oscillated but then decreased monotonically after 13 days. Using the eight samples collected from days 13 to 1055, first-order and Monod degradation rates were fit to each compound. For comparison, rates were also computed from concentrations measured in wells along a flow-path spanning the microcosm location. Well sample concentrations were corrected for dilution (constrained by modeling) and converted to time using a flow velocity of 6 cm/day obtained from a tracer test. For a given compound, the fitted plume and microcosm degradation rates were nearly identical. Among all compounds, rates varied by a factor of 10, with toluene the highest and ethylbenzene the lowest. Fits of the Monod degradation expression generally gave low values for half-saturation constants ranging from 0.01 to 0.5 mg/L. In the microcosm, benzene and ethylbenzene had zero-order rate profiles but stalled at threshold concentrations of ~0.1 mg/L. Toluene and o-xylene microcosm rates were slightly lower than the plume scale rates, probably because these compounds were not present in the microcosm sediments for 10 years before the start of the experiment due to complete degradation in the methanogenic zone of the plume.