Preliminary simulation of contaminant migration in ground water at the Lawrence Livermore National Laboratory

This report reviews a set of two-dimensional ground water flow and contaminant transport simulations in the upper aquifer materials beneath Lawrence Livermore National Laboratory (LLNL). The transport simulations focus on the migration, dilution, extraction, and potential degradation of existing aqueous volatile organic compounds (VOCs) in ground water, under both ambient flow and remedial pumping conditions. Simulations are used to show how, where, and at what rate vertically-averaged concentrations decrease to acceptable levels. Both the flow and transport calculations are made with a slightly modified version of the CFEST finite-element code. The hydrogeologic study area is defined to be the upper 200 feet of the saturated sediments within a 25 square mile area surrounding LLNL. Migration calculations focus on a single surrogate total-VOC (TVOC), equal to the sum of all VOC concentrations. Under natural flow conditions, and in the absence of intrinsic contaminant degradation, the results indicate that 800 y are required for the vertically-averaged TVOC concentrations to reduce to 5 parts per billion (ppb) or less in all parts of the study area. Although contaminant levels at several nearby agricultural wells may exceed 5 ppb during this time, levels at the municipal pumping wells in downtown Livermore never exceed 1 ppb. Under non-remedial conditions with an assumed linear TVOC degradation model corresponding to a 50 y half-life, simulation results indicate that all concentrations reduce to 5 ppb in 160 y, with very little impact on water quality at neighboring agricultural or municipal wells. Results of three additional simulations of TVOC migration under the influence of remedial pumping wells indicate that vertically-averaged TVOC concentrations uniformly reach 5 ppb (or below) within 50 to 75 y with little or no impact on water quality at nearby agricultural and municipal wells.