The water-table aquifer in Vega Alta, Puerto Rico, has been contaminated with volatile organic compounds. A three-dimensional ground-waterflow and solute-transport model was developed and calibrated to evaluate the effects of remedial alternatives designed to reduce the magnitude and extent of a trichloroethylene plume in the water-table aquifer. The development of the model was based on the computer code HST3D, developed by the U.S. Geological Survey. Heads measured from February 1983 to April 1992 were used to calibrate the ground-water-flow component of the model. Trichioroethylene concentrations measured in ground-water samples in January 1990 and March 1992 were used to calibrate the solute-transport component of the model, which consisted in the calibration of the longitudinal and transverse dispersivities, the distribution coefficient, and the solute influx at the source of trichloroethylene. Model input values assigned to specific storage, dispersivity, net recharge rates, effective porosity, riverbed conductivity, horizontal and vertical hydraulic conductivities, initial heads and trichloroethylene concentrations, and the locations of specified-head, river-leakage, and no-flow boundaries are described in this report. The root mean square error of simulated water-table heads from the ground-water-flow component of the calibrated model was 0.81 foot. The root mean square error of the simulated trichloroethylene concentrations, from the solute-transport component of the calibrated model, was 29 micrograms per liter of trichloroethylene.

The four remedial alternatives simulated in this report were the revised Record of Decision, and alternatives A, B, and C. Remedial alternatives were simulated to study the movement of the trichloroethylene plume in the aquifer from March 1992 to March 2022, while wells located within the extent of the trichloroethylene plume pumped water at specified rates and depths at the prevailing trichloroethylene concentrations. The trichloroethylene mass in the Vega Alta water-table aquifer was estimated to be 12,984 and 12,814 pounds in January 1990 and March 1992, respectively. The solute influx to the aquifer was estimated to be 22 pounds per year under long-term net recharge rates. The trichloroethylene mass remaining in the aquifer was estimated to be 5,720 pounds in March 2022 after simulating the revised Record of Decision remedial alternative. The trichloroethylene mass remaining in the aquifer was estimated to be 5,194, 4,786, and 3,689 pounds in March 2022 after simulating remedial alternatives A, B, and C, respectively.