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Two methods were used to approximate site-specific biodegradation rates of monoaromatic hydrocarbons (benzene, toluene, ethylbenzene, and xylenes [BTEX]) dissolved in ground water. Both use data from monitoring wells and hydrologic properties of the aquifer to estimate a biodegradation rate constant that can be sued in ground water solute fate and transport models. The first method uses a biologically recalcitrant 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 the tracer is attributed to biodegradation, although other processes may affect the observed rate. The second method assumes that the plume has evolved to a dynamic steady-state equilibrium. A one-dimensional analytical solution to the advection-dispersion equation is used to extract the rate of attenuation that would be necessary to produce a steady-state plume of the configuration found at the site. Attenuation is attributed largely to biodegradation because the analytical solution removes the effects of sorption and dispersion and volatilization is assumed to be minimal.