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Scientific Investigations Report 2010–5086

Prepared in cooperation with the Naval Facilities Engineering Command Southeast

Contaminant Movement Around a Permeable Reactive Barrier at Solid Waste Management Unit 12, Naval Weapons Station Charleston, North Charleston, South Carolina, 2009

By Don A. Vroblesky, Matthew D. Petkewich, and Kevin J. Conlon

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ABSTRACT

The U.S. Geological Survey and the Naval Facilities Engineering Command Southeast investigated natural and engineered remediation of chlorinated volatile organic compound groundwater contamination at Solid Waste Management Unit 12 at the Naval Weapons Station Charleston, North Charleston, South Carolina, beginning in 2000. In early 2004, groundwater contaminants began moving around the southern end of a permeable reactive barrier (PRB) installed by a consultant in December 2002. The PRB is a 130-foot-long and 3-foot-wide barrier consisting of varying amounts of zero-valent iron with or without sand mixture. Contamination moving around the PRB probably has been transported at least 75 feet downgradient from the PRB at a rate of about 15 to 29 feet per year.

The diversion of contamination around the southern end of the PRB may be due to construction difficulties associated with the PRB installation or to reduced permeability in the PRB. An event that took place during installation of the PRB, which may have caused permeability loss, was the collapse and subsequent abandonment of a 110-foot-long trench originally designed to be the PRB on November 11, 2002, approximately 25 feet upgradient (west) from the final PRB. Guar gum with antimicrobial preservative in a polymer slurry had been used to stabilize the abandoned trench prior to collapse and was only partially recovered. Residual guar gum can cause permeability reduction in a PRB. It also is possible that permeability reduction took place within the PRB by slow degradation of the guar gum slurry or mineral precipitation. Despite the likely permeability reduction in and near the PRB immediately following installation, there is evidence that contaminants moved through the PRB and were degraded, consistent with the planned purpose of the PRB.

Volatile organic compound contamination in groundwater downgradient from the PRB is subject to attenuation by phytovolatilization, sorption, and biodegradation. Pulses of contamination increases have been observed in some monitoring wells downgradient from the PRB. The pulses may reflect downgradient transport of contaminant pulses; however, lateral shifting of the plume is a more likely explanation for the concentration changes at well 12MW-12S.

The ability to monitor the fate and behavior of the plume in the forest is severely limited because the present axis of maximum contamination in that area bypasses all but one of the existing monitoring wells (12MW-12S). Moreover, the 2009 data indicate that there are no optimally placed sentinel wells in the probable path of contaminant transport. Thus the monitoring network is no longer adequate to monitor the groundwater contamination downgradient from the PRB.

First posted June 7, 2010

For additional information contact:
Director
USGS South Carolina Water Science Center
720 Gracern Road, Suite 129
Columbia, SC 29210
http://sc.water.usgs.gov/

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Suggested citation:

Vroblesky, D.A., Petkewich, M.D., and Conlon, K.J., 2010, Contamination movement around a permeable reactive barrier at Solid Waste Management Unit 12, Naval Weapons Station Charleston, North Charleston, South Carolina, 2009: U.S. Geological Survey Scientific Investigations Report 2010–5086, 74 p.



Contents

Abstract

Introduction

Purpose and Scope

Description of the Permeable Reactive Barrier

Site History

Methods

Data Collection

Well Drilling

Conceptual Site Model

Movement of Contamination Around the Permeable Reactive Barrier

Potential Causes for Movement of Contamination Around the Southern End of the Permeable Reactive Barrier

Fate of Contamination Downgradient from the Permeable Reactive Barrier

Summary

References

Appendixes


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