U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Use of Column Studies and a ReactiveTransport Model to Measure Biodegradation Rates of Hydrocarbons

Abstract

A laboratory method for determining biodegradation rates of volatile hydrocarbons in unsaturated porous media (subsurface sedimentary material) was developed. Glass columns were filled with samples of porous media from the unsaturated zone at a gasoline-contaminated site in Galloway Township, N.J. Vapor-phase hydrocarbons (benzene, toluene, or p-xylene) were added to the porous media at the bottom of the column and were transported vertically along a concentration gradient. Aerobic biodegradation of hydrocarbons is indicated by the production of carbon dioxide (CO₂). The vapor-phase hydrocarbon and CO₂ concentrations were monitored at several depths through sampling ports. Fluxes of hydrocarbons and CO₂ leaving the column also were measured frequently. A mathematical model was used to calculate CO₂-production rates as a function of depth. These distributions and stoichiometric relations describing hydrocarbon biodegradation were used to calculate depth-specific hydrocarbon-degradation rates. Degradation rates generally increased with depth, and appeared to be coupled to soil-moisture content, which also increased with depth. Degradation rates also increased with the number of methyl groups on the aromatic ring, and rates were greater in unprocessed sediment than in sieved sand from which fine particles had been removed. This method could be used to determine optimum biodegradation conditions in the design of bioremediation strategies.