Design of multiple contaminant remediation: Sensitivity to rate‐limited mass transfer

Abstract

Optimal extraction well locations and pumping rates are compared, based on a limited number of deterministic simulations, for multiple contaminant plumes exhibiting rate‐limited, or nonequilibrium, mass transfer from a mobile to an immobile phase. This rate‐limited mass transfer results in what we define as rate‐limited, or nonequilibrium, transport. Two‐dimensional, nonequilibrium solute transport simulation and optimization are used to study the simultaneous remediation of carbon tetrachloride, 1,2‐dichloroethane and tetrahydrofuran that have been chromatographically separated during transport by groundwater. Minimum total pumping necessary for cleanup is compared for five different well geometries over remediation periods of 3 and 15 years. The sensitivity of these designs to first‐order mass transfer rates is examined for equilibrium and for three levels of nonequilibrium transport. The key results of this particular study are that (1) classic contaminant capture at downgradient wells is a poor design for contaminant cleanup; (2) for effective design of multiple plume remediation, the transport characteristics of each contaminant must be considered; (3) if solute transport is limited by mass transfer from an immobile to a mobile phase, there is a minimum remediation time which cannot be reduced by adjustment of pumping rates or changing well locations; (4) the optimal locations and pumping rates of extraction wells are less sensitive to nonequilibrium transport for long‐term remediation than for short‐term remediation; and (5) competition between pumping wells may significantly affect the feasibility and efficiency of specific multiple‐well cleanup designs. In addition, a method and a measurement are presented for evaluating the importance to remediation of rate‐limited mass transfer.