Gasoline, kerosene, diesel, and heating oil have composition ratios of ethylbenzene to xylenes (EXR) of approximately 0.17±0.05.  Upon a release, aerobic bacteria rapidly use available oxygen and drive the release environment anaerobic.  Ethylbenzene and xylenes have nearly identical boiling points, vapor pressures, water solubilities, and carbon-water sorption coefficients.  Therefore, the major fate and transport mechanisms of evaporation, water washing, groundwater velocity retardation, and mechanical dispersion treat ethylbenzene and xylenes alike.  Anaerobic biodegradation will remove xylenes faster than ethylbenzene and the EXR in groundwater will increase with time.

Hydrocarbon biodegradation and hydrocarbon ratios can be reasonably simulated using first-order exponential approximations.  Given the difference in ethylbenzene and xylenes biodegradation rates and similar fate and transport properties will not significantly effect the EXR differently over time, simulating the EXR using a first-order exponential approximation will eliminate the need to know initial ethylbenzene and xylenes concentrations and allow for modeling groundwater data to predict RDRs using a minimum (0.17-0.05=0.12) and maximum (0.17+0.05=0.22) initial EXR representing a new release.

Ethylbenzene and xylenes data that exhibit anaerobic, first-order exponential biodegradation are used to estimate RDRs, or at least indicate the release could not have occurred after the estimated RDR, that were in reasonable agreement with known release dates.  The field examples suggest that regression analysis of post release data can be used to provide reasonable estimates of RDRs, without the need for early time data.  EXR data are modeled and projected forward in time to support a new from old release determination.  EXR data not showing an increase are associated with free product conditions.