Fate and transport modeling of selected chlorinated organic compounds at Operable Unit 3, U.S. Naval Air Station, Jacksonville, Florida

Abstract

Ground water contaminated by the chlorinated organic compounds trichloroethene (TCE), cis-dichloroethene (DCE), and vinyl chloride (VC) has been found in the surficial aquifer beneath the Naval Aviation Depot at the U.S. Naval Air Station, Jacksonville, Florida. The affected area is designated Operable Unit 3 (OU3) and covers 134 acres adjacent to the St. Johns River. Site-specific ground-water flow modeling was conducted at OU3 using MODFLOW, and solute-transport modeling was conducted using MT3DMS. Simulations using a low dispersivity value, which resulted in the highest concentration discharging to the St. Johns River, gave the following results. At 60 years traveltime, the highest concentration of TCE associated with the Area C plume had discharged to St. Johns River at a level that exceeded 1x103 micrograms per liter (ug/L). At 100 years traveltime, the highest concentration of TCE associated with the Area D plume had discharged to the river at a level exceeding 3x103 ug/L. At 200 years traveltime, the Area B plume had not begun discharging to the river. Simulations using a first-order decay rate half-life of 13.5 years (the slowest documented) at Area G caused the TCE to degrade before reaching the St. Johns River. If the ratio of the concentrations of TCE to cis-DCE and VC remained relatively constant, these breakdown products would not reach the river. However, the actual breakdown rates of cis-DCE and VC are unknown. Simulations were repeated using average dispersivity values with the following results. At 60 years traveltime, the highest concentration of TCE associated with the Area C plume had discharged to St. Johns River at a level exceeding 4x102 ug/L. At 100 years traveltime, the highest concentration of TCE associated with the Area D plume had discharged to the river at a level exceeding 1x103 ug/L. At 200 years traveltime, the Area B plume had not begun discharging to the river. "Pump and treat" was simulated as a remedial alternative. The concentration of TCE at Area B trended rapidly downward; however, one isolated pocket of TCE remained because of the low-permeability sediments present at this area. The concentration of TCE at Area C trended rapidly downward and was below 1 ug/L in about 16 years. The concentration of TCE at Area D also trended rapidly downward and was below 1 mg/L in about 18 years.