SIMULATION OF CARBON TETRACHLORIDE MIGRATION FROM THE 216-Z-9 TRENCH AT HANFORD

From July 1955 through June 1962, approximately 4,090,000 L of waste water, including 316,000 L of a mixture of dense, non-aqueous phase liquids (DNAPL), was discharged into the vadose zone of the Hanford Site through the 216-Z-9 Trench. Approximately 74% (by volume) of this DNAPL was carbon tetrachloride (CCl₄). Three-dimensional numerical simulations were conducted of the three-phase (aqueous, gas, NAPL) migration of the disposed liquids from the trench using the STOMP code. The model domain is 440-m (E-W) by 540-m (N-S) by 201-m (ground surface to top of basalt), and consists of 12 hydrostratigraphic units constructed from geologic interpretations of borehole data (including drillers’ logs, geologists’ logs, particle size data, calcium carbonate content, moisture content, and geophysical logs). EarthVision^® software was used to interpolate the hydrostratigraphic units between boreholes. Simulations were conducted from 1955 through present to evaluate the evolution and current status of the plume. In addition to the base case, 20 sensitivity cases were simulated to analyze the sensitivity of the model results to uncertainties in the total volume of DNAPL released, DNAPL fluid properties, source characteristics (area and infiltration rates), soil hydraulic properties (porosity, permeability, anisotropy, fluid entry pressures, pore geometry parameters), and maximum residual DNAPL saturation. Simulation results are compared with observed field data and are evaluated using spatial moment analyses and integrated fluxes of DNAPL, dissolved aqueous-phase CCl₄, and dissolved gas-phase CCl₄ crossing the model boundaries.