A.55 Penobscot River (Dunnett’s Cove), Maine

A.55.1 Contacts

Regulatory Contacts:

Kathy Howatt
Maine Department of Environmental Protection
207-446-2642

A.55.2 Summary

Environment	River
Scale	Full
Primary source(s):	Sewer discharge from a manufactured gas plant.
COCs	Coal tar NAPL
Final Remedy	Cap designed to trap NAPL and dredging

A.55.3 Site Description

From 1851 to 1963, Bangor Gas Works operated a manufactured gas plant. Wastewater from the plant, containing coal and oil tar, was discharged into a sewer that in turn discharged to the Penobscot River.

CSM summary: Much of the tarry sediment hardened at the bottom of the river but a portion of the tar impacted sediment remains unhardened in what has been termed the active zone. Tar is present in sediments over approximately 11 acres of Dunnett’s Cove. Gas released from the contaminated sediment facilitates NAPL migration to the surface.

Based on observation, 3 conditions were necessary to facilitate NAPL migration:

• The sediment contains liquid tar
• The sediment produces gas bubbles at a rate to increase the buoyancy of tar and facilitate upward migration
• The gas is in contact with the tarry sediment.

A.55.4 Remedial Approach

Final selected remedy: Cap designed to trap NAPL

Bench scale tests showed that a sand capA covering over material (contaminated sediment) used to isolate the contaminants from the surrounding environment. allowed gas to migrate upward carrying NAPL. A second bench scale test showed that sand amended with organophilic clayClay minerals whose surfaces have been ion exchanged with a chemical to make them oil-sorbent. Bentonite and hectorite (plate-like clays) and attapulgite and sepiolite (rod-shaped clays) are treated with oil-wetting agents during manufacturing. Quaternary fatty-acid amine is applied to the clay. Amine may be applied to dry clay during grinding or it can be applied to clay dispersed in water. also exhibited NAPL breakthrough. The selected remedy, a low permeability1) Characteristic of a material or membrane that allows liquids or gases to pass through it; 2) The rate of flow of a liquid or gas through a porous material. AquaBlok cap with underlying gas venting layer, addresses the problem with gas accumulation and NAPL migration.

Lessons learned: Facilitated transport of NAPL at manufactured gas plant (MGP) sites must be taken into consideration. A sand cap by itself or with an amendment may not effectively isolate NAPL contamination at MGP sites.

Additionally, source area characterization is important. Sediment in the designated active zone posed the greatest exposure risk.

Why the remedy was selected: The cap only needed to be replaced in those areas that still posed an unacceptable exposure risk.

A.55.5 References

Case Study, Sediment Remediation, Bangor Landing, Bangor, ME, Maine Department of Environmental Protection, April 2010, http://www.newmoa.org/cleanup/cwm/sediments10/HowattMECaseStudy1.pdf.

Publication Date: August 2014