Assessment of in situ solvent extraction for remediation of coal tar sites: Process modeling

Abstract

Subsurface contamination at many former manufactured gas plants (MGP) resulted from on‐site disposal in wells, pits, and lagoons of coal tar, a by product of the gasification process. Coal tar is difficult to pump from the ground because of its density and viscosity. A technique employed with success in enhanced oil recovery, in situ solvent extraction, may be useful for remediation of coal tar‐contaminated sites. A water‐miscible, biodegradable solvent would be injected into the subsurface to promote dissolution of coal tar for subsequent removal via recovery wells. This paper presents the results of process modeling done in conjunction with a project to assess in a preliminary way the feasibility of in situ solvent extraction for remediation of coal tar sites. After evaluation of MGP site characteristics, possible injection‐recovery well schemes for the proposed process were developed. These comprised center injection and perimeter recovery patterns and patterns in which injection and recovery wells are placed on opposite sides of a contaminated area. A two‐dimensional model was then developed for simulating the proposed in situ solvent extraction process under simplified site conditions and applied to evaluate the performance of the various injection‐recovery schemes at a hypothetical MGP site. From model results it appears that an in situ solvent extraction process may be able to recover a significant mass of coal tar (more than 80%) from the subsurface within a time frame of 1 year or so, provided that site hydrogeologic conditions are conducive to process application. Pilot testing in the field is necessary to test the results of this preliminary assessment. There are also important implementation issues and some are discussed.