EVALUATING THE NATURAL ATTENUATION POTENTIAL FOR A CONTAMINANT PLUME COMPRISED OF CHLORINATED ETHENES AND BTEX AT A CLOSED UNLINED MUNICIPAL LANDFILL IN SHERIDAN, WYOMING

An unlined, uncapped municipal landfill near the city of Sheridan, Wyoming operated from 1950 to 1985. The landfill sits on primarily fine-grained sedimentary rocks of the Eocene-age Wasatch Formation. Groundwater in this area flows northwest and the pre-landfill water table has risen thirteen feet, fully saturating a portion of the waste. Piezometers installed in the saturated waste indicate localized dissolved chlorinated ethenes: PCE, TCE, and their degradation products cis-1,2-DCE and vinyl chloride. BTEX are also present in other portions of the saturated waste. Downgradient wells do not indicate measurable concentrations of these compounds. The Sheridan landfill site was therefore chosen as an ideal site to formulate, develop, and test a methodology for evaluating natural attenuation. Analysis of long-term temporal inorganic data and two recent rounds of organic sampling results were used to formulate a one-dimensional groundwater transport model using PHREEQC. Partitioning coefficients for solubility, sorption, and volatility were selected for each of the organic compounds present and used to modify the PHREEQC database. Biodegradation rates for the chlorinated compounds under anaerobic conditions were selected from the literature to evaluate fate and transport and predict the potential for natural attenuation. HYDRUS was used to determine the significance of gas-phase transport at the site, based on the volatility of the compounds under study. The outcome of these models is used to evaluate the long-term effects of the natural attenuation processes occurring at the site. These results can aid in decisions on proposed approaches for remediation, including natural attenuation, and can be employed at other landfill sites.