MEASURING RATES OF BIODEGRADATION IN A CONTAMINATED AQUIFER USING FIELD AND LABORATORY METHODS

Rates of biodegradation were measured in a petroleum hydrocarbon-conta minated aquifer using a combination of field and laboratory methods. T hese methods are based on tracking concentration changes of substrates (both electron donors and accepters) or final products of microbial m etabolism over time. Ground water at the study site (Hanahan, South Ca rolina) is anoxic, and sulfate reduction is the predominant terminal e lectron accepting process. Laboratory studies conducted with sediment cored from the site showed that C-14-toluene was mineralized to (CO2)- C-14 with a first-order degradation rate constant (k(tol)) of -0.01 d( -1) under sulfate-reducing conditions, Under nitrate-amended, Fe(III)- amended, or nonamended (methanogenic) conditions, toluene was not sign ificantly mineralized, C-14-Benzene was degraded at low but measurable rates (k(ben) = -0.003 d(-1)) under sulfate-reducing conditions where as degradation under methanogenic conditions was negligible, These res ults illustrate the extreme sensitivity of laboratory-measured biodegr adation rates to terminal electron-accepting conditions, and show the necessity of carefully matching experimental conditions to in situ con ditions. Concentration decreases of toluene along aquifer flowpaths, w hen the uncertainty of ground-water Bow velocities was considered, ind icated k(tol) values ranging from -0.0075 to -0.03 d(-1). Concentratio n decreases of sulfate and concentration increases of dissolved inorga nic carbon (DIC), when normalized for assumed stoichiometric oxidation of toluene coupled to sulfate reduction, yielded a k(SO4) range of -0 .005 to -0.02 d(-1), and a k(DIC) value range of +0.00075 to +0.003 d( -1). Because both laboratory and field methods have numerous sources o f uncertainty, a combination of these methods is the most appropriate procedure for evaluating biodegradation rate constants in contaminated ground-water systems.