Quantifying Rates of Biodegradation in Contaminated Aquifers

Abstract

Rates of biodegradation can be measured in petroleum hydrocarbon-contaminated aquifers using a combination of field and laboratory methods. These methods are based on tracking concentration changes of substrates (both electron donors and acceptors) or final products of microbial metabolism over time. At one contaminated site in South Carolina, ground water is anoxic and sulfate reduction is the predominant terminal electron accepting process. Laboratory studies conducted with sediment cored from the site showed that 14C-toluene was mineralized to 14CO2 with a first-order degradation rate constant (ktol) of &3x2212;0.01 d−1 under sulfate reducing conditions. 14C-Benzene was degraded at low but measurable rates (kben = −0.003 d−1) under sulfate-reducing conditions whereas degradation under methanogenic conditions was negligible. These results illustrate the sensitivity of laboratory-measured biodegradation rates to terminal electron-accepting conditions, and show the necessity of carefully matching experimental conditions to in situ conditions. Concentration decreases of toluene along aquifer flowpaths, when the uncertainty of ground-water flow velocities was considered, indicated ktol values ranging from −0.0075 to −0.03 d−1. Because both laboratory and field methods have numerous sources of uncertainty, a combination of these methods is the most appropriate procedure for evaluating biodegradation rate constants in contaminated ground-water systems.