James W. Mercer, PG, Ph.D. Executive Vice President GeoTrans, Inc.

Dr. Mercer is a hydrogeologist with GeoTrans, Inc. He received a B.S. from Florida State University and a M.S. and Ph.D. from the University of Illinois; all of his degrees are in geology. He spent 8 years with the U.S. Geological Survey in the Northeastern Research Group working on contaminant and heat transport issues, including multiphase flow. He co-founded GeoTrans in 1979, and in 1980 began working on DNAPL issues at Love Canal. In 1985, Dr. Mercer received the Wesley W. Horner Award of the American Society of Civil Engineers for the work preformed at Love Canal. Dr. Mercer continued to work on DNAPL issues and co-authored a book on DNAPL Site Evaluation in 1993. In 1994, he received the American Institutes of Hydrology’s Theis Award for contributions to groundwater hydrology. Dr. Mercer has served on the National Research Council’s Water Science and Technology Board and was a member of the U.S. EPA Science Advisory Board. He is currently on the Scientific Advisory Board of SERDP.

Presentation Description
Remediation Endpoints: Complex Sites Where MCLS Will Not Be Achieved In Reasonable Timeframes
Groundwater contamination involving dense nonaqueous liquids (DNAPLs) can be divided into two zones – a DNAPL or source zone and a down gradient dissolved plume. DNAPL zones can include (1) residual DNAPL, (2) pooled DNAPL, (3) sorbed contaminants and (4) dissolved contaminants diffused into fine-grained media. Unfortunately, sorbed contaminants and contaminants diffused into fine-grained media can effectively extend the source zone down gradient into the dissolved plume. Consequently, at some sites, even with mass removal or containment of the source zone, the dissolved plume could remain for an extended period of time. There is a need to manage both the DNAPL source zone and the dissolved or aqueous plume.

Much effort has been expended in recent years to develop and apply source zone remediation technologies. Although mass can be removed from the source zone, complete contaminant mass removal has generally not been possible. A 2003 USEPA Expert Panel Report concluded that, “As far as the Panel is aware, there is no documented, peer-reviewed case study of DNAPL source zone depletion beneath the water table where U.S. drinking water standards or MCLs have been achieved and sustained throughout the affected subsurface volume, regardless of the in-situ technology applied.” If MCLs can not be achieved at a site, what are acceptable remediation goals?

The remediation industry has more experience remediating light NAPLs or LNAPLs such that EPA has produced a decision-making framework for cleanup of LNAPL sites that provide potential remediation endpoints. Using this approach, the following set of conclusions is developed. For sites with simple, permeable media containing small, accessible DNAPL, source reduction can be very effective at reducing concentrations, especially if natural biodegradation is occurring down gradient of the source area. For sites having complex geology with large, multiple DNAPL sources, the expectation that source reduction will lead to significant concentration reduction is low. In between these endpoints are many sites where remedial performance and impacts of source reduction are difficult to estimate and uncertainties are high.

The focus in this discussion is the complex endpoint, where it is unlikely that MCLs will be met in any reasonable time frame. Case studies are presented where final remediation ranges from a technical impracticability waiver to risk-based site closure following source remediation where contamination remains in the subsurface. The case studies include both LNAPL and DNAPL sites.