APPROXIMATION OF BIODEGRADATION RATE CONSTANTS FOR MONOAROMATIC HYDROCARBONS (BTEX) IN-GROUND WATER

Two methods were used to approximate site-specific biodegradation rate s of monoaromatic hydrocarbons (benzene, toluene, ethylbenzene, and xy lenes [BTEX]) dissolved in ground water. Both use data from monitoring wells and the hydrologic properties of the aquifer to estimate a biod egradation rate constant that can be used in ground water solute fate and transport models. The first method uses a biologically recalcitran t tracer in the ground water associated with the hydrocarbon plume to normalize changes in concentration of BTEX under anaerobic conditions; attenuation of the tracer is attributed to dilution, sorption, and/or volatilization. Attenuation of BTEX in excess of the attenuation of t he tracer is attributed to biodegradation, although other processes ma y affect the observed rate. The second method assumes that the plume h as evolved to a dynamic steady-state equilibrium. A one-dimensional an alytical solution to the advection-dispersion equation is used to extr act the rate of attenuation that would be necessary to produce a stead y-state plume of the configuration found at the site. Attenuation is a ttributed largely to biodegradation because the analytical solution re moves the effects of sorption and dispersion and volatilization is ass umed to be minimal. Neither method fully accounts for the effects of c ontinuing dissolution of BTEX in the source area or nonlinear sorption . Therefore, the rates cannot be attributed fully to biodegradation, b ut still are useful for ground water contaminant fate and transport mo deling. The methods were applied to a data set from a JP-4 jet fuel sp ill at Hill Air Force Base, Utah. In estimates along two separate flow paths, natural attenuation rates for BTEX ranged from 0.006 to 0.038 day (-1), with most rates near 0.02 day(1). The rate for benzene range d from 0.025 to 0.038 day (1). The rates of attenuation of individual BTEX compounds as estimated by the two methods were in close agreement . For an individual compound, the rate estimated using the second meth od was at most 36 percent greater than, but usually within 20 percent of, the rate estimated using the first method, suggesting that intrins ic bioremediation was the dominant process that attenuated BTEX.