EVALUATING THE NATURAL ATTENUATION POTENTIAL FOR A LANDFILL LEACHATE PLUME COMPRISED OF BTEX AND CHLORINATED ETHENES

A closed, municipal landfill resides on the eastern edge of the corporate limits of the city of Sheridan, Wyoming. The landfill was unlined and uncapped and operated from 1950 to 1985. Since closure, the water table has risen and fully saturated up to thirteen vertical feet of the waste. Piezometers installed in the saturated waste show concentrations of BTEX and chlorinated ethenes that are slightly above the EPA MCL. Due to the low concentrations of contaminants present, this site was chosen as an ideal location to formulate and test a methodology for evaluating natural attenuation. The geochemical modeling program PHREEQC was employed to evaluate the natural attenuation processes: sorption, volatilization, dispersion, and biodegradation. The PHREEQC database was modified to include the organic compounds and partitioning coefficients. The transport command enabled a one-dimensional groundwater flow model that transports the leachate while simultaneously partitioning the compounds into the gas and solid phases and biodegrading according to the redox conditions present. The results show extremely low mobilities for each of the volatile organic compounds present in the leachate, even when conservative estimates for partitioning coefficients are used. This result shows that natural attenuation will lower organic concentrations to below the MCL without active remediation. Sorption and biodegradation are the dominant processes responsible for retarding these organic components in ground water, while dispersion and volatilization are secondary. This model could be used as a template for evaluating the natural attenuation potential at other small, municipal landfills across the Western U.S.