MODELING THE FATE AND TRANSPORT OF VOLATILE ORGANIC COMPOUNDS AT A CLOSED UNLINED MUNICIPAL LANDFILL IN SHERIDAN, WYOMING

An unlined, uncapped municipal landfill was in operation in the city of Sheridan, Wyoming from 1950 to 1985. The landfill sits on the sedimentary rocks of the Eocene-age Wasatch Formation which consist of interbedded sandstone, claystone, mudstone, siltstone, and shale. Groundwater in this area flows northwest and the pre-landfill water table has risen thirteen feet, fully saturating a portion of the waste. A series of monitoring wells and piezometers have been installed to monitor the environmental impact of the landfill. Background wells show a wide range in TDS and pH. The leachate itself, monitored by piezometers, has a neutral pH and high TOC and chloride content. Downgradient wells indicate some impact from the migrating leachate on the groundwater. The purpose of this project is to determine the fate and transport of volatile organic compounds derived from the landfill waste and, in so doing, evaluate the potential for natural attenuation as a remediation approach. The organic compounds present in the leachate and downgradient wells are chlorinated aliphatic hydrocarbons and BTEX. Once the partitioning coefficients for these compounds have been either selected or experimentally determined, the modeling program PHREEQC will be employed to model the one-dimensional movement of the compounds in the groundwater and HYDRUS will be used to model gas phase transport. The groundwater model will include advective transport and dispersion as well as retardation due to sorption and biodegradation. Both spatial and temporal data from monitoring wells exist and will be used to calibrate the groundwater transport model. Soil gas samples will be collected to calibrate the gas phase transport model and aid in the mass balance calculation. The outcome of this model will dictate remediation means. If natural attenuation processes are at work, close monitoring may be all that is required. If the investigation results show that contamination may extend to domestic wells downgradient, more expensive remediation will be necessary. This project will not only help the leaders of the city of Sheridan decide on appropriate approaches for remediation, but could also be employed at other landfill sites in predicting contamination potential.