Comparison Between Pump and Treat, Biorestoration, and Biorestoration/ Pump and Treat Combined: Lessons from Computer Modeling

Abstract

AbstractThis study compares pump and treat, bioremediation. and bioremediation/pump and treat for cleanup of a chemical plume using BIOPLUME II, a numerical solute transport code which simulates the transport of dissolved hydrocarbons under oxygen‐limited biodegradation. Nineteen remediation scenarios were evaluated under one set of hydrogeologic and chemical conditions using different injection and extraction well configurations and injected dissolved oxygen (D.O.) concentrations of 8 or 50 parts per million (ppm). The cleanup objective was to reduce the concentration of naphthalene in the plums by the greatest percentage without increasing the volume of contaminated ground water in the aquifer. Each scenario was ranked in terms of plume control and contaminant concentration reduction for a one‐year remediation period.Based on the performance criteria, scenarios that included pump and treat were superior in containing the migration of the plume and in restricting the plume from separating into smaller, localized plumes or “hot spots.” Bioremediation and bioremediation/pump‐and‐treat scenarios in which 50 ppm D.O. was injected throughout the plume (areal injection) or in the most concentrated portion of the plume (interior injection) achieved the greatest plume control and concentration reductions. Bioremediation cases with 8 ppm D.O. did not successfully control the plume or achieve major reductions in concentrations. The important conclusions drawn from the results were the following: (1) minimize the distance that the plume needs to be pulled toward extraction wells or that the D.O. needs to be circulated through to reach the contaminant zone; (2) inject water containing as high a level of D.O. as possible while meeting microbial and oxygen demand requirements: and (3) design the wellfield to produce convergent flow toward a central location and to minimize divergent flow and upgradient and intraplume stagnation areas.The study results suggest that bioremediation and bioremediation/ pump and treat should perform better than pump and treat for sand and gravel aquifers contaminated with biodegradable hydrocarbons. In particular, four remediation scenarios should be seriously considered in the design phase of biorestoration projects: areal injection/downgradient extraction, interior injection, interior extraction/exterior (i.e., outside of the plume) injection, and areal injection. Although the actual number of wells, injection and extraction rates, and D.O. concentrations will vary for each site, it is expected that areal injection: downgradient extraction and interior injection should perform well under high natural flow velocities, whereas interior extraction/exterior injection and areal injection should be more amenable to sites with lower natural flow velocities.