Technical Report No. 174
TRACE ORGANIC (DBCP) TRANSPORT SIMULATION OF PEARL HARBOR AQUIFER, O’AHU, HAWAII: Multiple Mixing-Cell Model, Phase I
Shlomo Orr and L. Stephen Lau
Lumped parameter models were chosen to provide a preliminary appraisal of the fate of trace organics (DBCP) in a portion of the Pearl Harbor aquifer and to provide a management tool for policymaking. The models, which consider portions of the aquifer as mixing cells, extensively simplify the systems. Special attention focused on attenuation and travel time in the approximately 800 ft (244 m) thick, vadose zone. Pseudo first-order decay coefficients determined from pesticide residues in the topsoil were used to determine attenuation in the vadose zone. A separate mixing-cell model was developed for the topsoil to distinguish between the different mechanisms responsible for the attenuation of adsorbed pesticides, particularly leaching vs. volatilization. Thus, an alternative method that excludes the varying distribution coefficients is suggested to estimate the leaching fraction of strongly adsorbed pesticides. The results indicate that at least a decade will pass before the aquifer starts to recover and another five years before DBCP concentrations become undetectable. The concentration attenuation within the vadose zone is about three orders of magnitude greater than the concentration reduction resulting from dilution within the aquifer. The simulated concentrations are very sensitive to the decay coefficients determined for the unsaturated zone. We conclude that leachate concentrations as low as 0.1 ppb (ug/1) below the top few feet of soil can reach and contaminate deep goundwater, where (usually) the dilution is limited to less than 1:10. Due to the high persistence of many volatile organic carbons (dissolved in water) in very low concentrations, they can be considered as “conservative” tracers for modeling purposes.