Fate and Transport Modeling of Selected Chlorinated Organic Compounds at Operable Unit 1, U.S. Naval Air Station, Jacksonville, Florida

Abstract

The U.S. Naval Air Station occupies 3,800 acres adjacent to the St. Johns River in Jacksonville, Florida. The Station was placed on the U.S. Environmental Protection Agency's National Priorities List in December 1989 and is participating in the U.S. Department of Defense Installation Restoration Program, which serves to identify and remediate environmental contamination. One contaminated site, the old landfill, was designated as Operable Unit 1 (OU1) in 1989. The major source of ground-water contamination was from the disposal of waste oil and solvents into open pits, which began in the 1940s. Several remedial measures were implemented at this site to prevent the spread of contamination. Recovery trenches were installed in 1995 to collect free product. In 1998, some of the contamination was consolidated to the center of the old landfill and covered by an impermeable cap. Currently, Operable Unit 1 is being reevaluated as part of a 5-year review process to determine if the remedial actions were effective. Solute transport modeling indicated that the concentration of contaminants would have reached its maximum extent by the 1970s, after which the concentration levels would have generally declined because the pits would have ceased releasing high levels of contaminants. In the southern part of the site, monitoring well MW-19, which had some of the highest levels of contamination, showed decreases for measured and simulated concentrations of trichloroethene (TCE) and dichloroethene (DCE) from 1992 to present. Two upgradient disposal pits were simulated to have ceased releasing high levels of contamination in 1979, which consequently caused a drop in simulated concentrations. Monitoring well MW-100 had the highest levels of contamination of any well directly adjacent to a creek. Solute transport modeling substantially overestimated the concentrations of TCE, DCE, and vinyl chloride (VC) in this well. The reason for this overestimation is not clear, however, it indicates that the model will be conservative when used to predict concentration levels and the time required for the contamination to move through the system. Monitoring well MW-97 had the highest levels of contamination in the central part of the site. The levels decreased for both the measured and simulated values of TCE, DCE, and VC from 1999 to present. Simulating the source area as ceasing to release high levels of contamination in 1979 caused the drop in concentration, which began in the 1990s at this well. Monitoring well MW-89 had the highest levels of contamination in the northern part of the site. In order to match the low levels of contamination in wells MW-12 and MW-93, the pit was simulated as ceasing to release contamination in 1970; however, the installation of a trench in 1995 could have caused the source area to release additional contamination from 1995 to 1998. The effect of the additional dissolution was a spike in contamination at MW-89, beginning in about 1996 and continuing until the present time. Results from the last several sampling events indicate that the TCE and DCE levels could be decreasing, but VC shows no apparent trend. Several more years of sampling are needed to determine if these trends are continuing. Based on the solute transport modeling predictions, TCE, DCE, and VC will have migrated to the vicinity of creeks that drain ground water from the aquifer by 2010, and only relatively low levels will remain in the aquifer by 2015. Because the creeks represent the point where the contaminated ground water comes into contact with the environment, future contamination levels are a concern. The concentration of chlorinated solvents in the creek water has always been relatively low. Because the model shows that concentrations of TCE, DCE, and VC are declining in the aquifer, contamination levels in the creeks also are anticipated to decline.