Pump and treat ground-water remediation system optimization

A ground-water management model using a nonlinear programming algorithm was developed to find the minimum cost design of the combined pumping and treatment components of a pump-and-treat remediation system and includes the fixed costs of system construction and installation as well as operation and maintenance. The fixed-cost terms of the objective function are incorporated into the nonlinear programming formulation using a penalty coefficient method. Results of applying the model to an aquifer with homogeneous hydraulic conductivity show that a combined well field and treatment process model that includes fixed costs has a significant impact on the design and cost of these systems, reducing the cost by using fewer, larger-flowrate wells. Previous pump-and-treat design formulations have resulted in systems with numerous, low-how-rate wells due to the use of simplified cost functions that do not exhibit economies of scale or fixed costs. Two example aquifers with heterogeneous conductivity fields were also investigated, and system costs similar to the homogeneous case were obtained. However, the introduction of aquifer heterogeneity did affect the remediation design, for example, well locations and pumping rates. Trade-offs between total remediation system cost and cleanup standard, remediation period, and typical design parameters of air-stripping towers are examined through sensitivity analysis. Generally, the optimal injection concentration is found to be approximately 70-80% of the cleanup standard. Designs with remediation periods around 5-6 yr have the minimum cost for the case study presented in this paper. Remediation periods beyond 6 yr are not economically justified, since the tower operating costs become dominant and offset the reduction of capital cost.