Technical Report No. 175
MODELING OF TRACE ORGANIC (DBCP) TRANSPORT IN PEARL HARBOR AQUIFER, O’AHU, HAWAII: Method of Characteristics, Phase II
Shlomo Orr and L. Stephen Lau
September 1988
ABSTRACT
A numerical model, based on the method of characteristics (MOC), is used to simulate the transport of DBCP through the basaltic aquifer in Mililani and vicinity and to predict the potential downstream contamination including Waipio Heights Wells H. The Phase H modeling is intended to improve upon the Phase I mixing-cell model. For the two-dimensional solute transport model and the large scale used, the aquifer is considered as homogeneous and isotropic, the solute transport as occurring in the upper irrigation-return lens, and the dispersion as Fickian. The lowest and the most abundant concentration detected in the deep core samples was assumed to be the input concentration at the water table, deep below the pineapple fields. This rather optimistic assumption would render a rather optimistic recovery of the aquifer. The results suggest that predicted DBCP levels will decline to below 20 ppt for the Mililani wells around the year 2000. Waipio Heights Wells R would experience DBCP increase up to 22 ppt around the year 2000, then decline slowly. Increased pumpage from Mililani Wells 11 would hasten aquifer recovery. The mutual relationship between the calibration of the solute transport and the groundwater flow models emphasizes the need for a more precise flow model that more precisely predicts dynamic changes in pumping stresses and recharge. The two models show a general similarity in the prediction of aquifer recovery, which is, however, slightly faster in the mixing-cell model. The mixing-cell model tends to overestimate dispersion, the lateral in particular. Both models show that leachate concentrations as low as 0.1 ppb below the top few feet of soil can contaminate a deep aquifer.