Critical analysis of plume containment modeling in a thin heterogeneous unconfined aquifer: application to a bulk fuel storage terminal

Abstract

The reported hydrocarbon contamination and subsequent consultant work at a bulk fuel storage terminal has instigated the need to critically analyze modeling techniques in thin, heterogeneous, unconfined aquifers. This study provides an aquifer characterization such that a better understanding of the flow system can be input into an numerical model. A two dimensional plan view model was developed using MODFLOW. A steady state calibration was undertaken through the trial and error alteration of hydraulic conductivity given appropriate boundary conditions. Using the steady state model as initial conditions, a transient calibration was developed by a systematic trial and error alteration of recharge, specific yield, and boundary conditions. It was found that because the modeled area represented only a small fraction of the aquifer as a whole that the boundaries must be varied through time to simulate prolonged recharge and recession events. A critical analysis was performed to evaluate the potential problems and errors involved with employing simplified models and simplifying conditions given the complexities of system. In this work, the numerically derived head distribution solved within MODFLOW was used in conjunction with the U.S.G.S. MODPATH particle tracking program to simulate capture zones. It was found that the semi-analytical modeling effort performed by the consultants contracted by Chevron predicted a capture zone and drawdown at each well that were significantly less than there numerical counterpart. Similarly, the Javandel and Tsang method predicted a much smaller capture zone than the numerical model. It was shown that the effects of regional water level fluctuations through time on the size and shape of the produced capture zone must be taken into account in the pumping design. Also, the establishment of individual capture zones by each of the operators at the facility was going to prove detrimental to each operator's ability to contain the contamination within their site. Optimization techniques were shown somewhat valuable at the site because of the limiting constraints provided by the thin aquifer.