[1] Numerous studies have been devoted to the optimization of groundwater management strategies in coastal aquifers. However, little has been done to address the problem of initially contaminated aquifers, the subject of this paper. Corrective measures need to be optimally designed to improve water quality while minimizing changes in the existing pumping regime. To this end, we compare two optimization methods. The first one is linear and consists of maximizing pumping rates while constraining heads to prevent seawater inflow. The second one consists of minimizing the changes from current pumping rates, so as to preserve existing rights, while constraining concentrations, which leads to a nonlinear programming problem. In both cases, corrective measures include a reduction in pumping rates, inland artificial recharge, and a coastal hydraulic barrier. Not surprisingly, the nonlinear problem leads to a more efficient solution, both in terms of pumping rates and actual cleanup of the aquifer. Nevertheless, the linear formulation yields insights into the optimal allocation of pumping. More importantly, the linear formulation enables us to readily calculate the hydraulic efficiency (gain in pumping rate per unit increase in recharge rate) of corrective measures. The fact that efficiency is consistently greater than 1 proves that the hydraulic barrier not only increases resources but also protects the existing ones. Therefore we conclude that both optimization approaches are useful and should be used.

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SUMMARYPrediction of long-term settlement and control of gas pollution to the environment are two principle concerns during the management of municipal solid waste (MSW) landfills. The behavior of settlement and gas flow in MSW landfills is complicated due to the combined effect of mechanical deformation of the solid skeleton and continuous biodegradation of the waste. A one-dimensional settlement and gas flow model is presented in this paper, which is capable of predicting time evolution of settlement as well as temporal and spatial distribution of gas pressure within multi-layered landfills under a variety of operating scenarios. The analytical solution to the novel model is evaluated with numerical simulation and field measurements. The resulting efficiency and accuracy highlight the capability of the proposed model to reproduce the settlement behavior and gas flow in MSW landfills. The influences of operating conditions and waste properties on settlement and gas pressure are examined for typical MSW landfills.

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