A.43 Laconia, NH

A.43.1 Summary

Environment:	Freshwater
Scale:
Contaminants of Concern:	PAHs average of 25 ppm and max of 20,210 ppm, VOCs, TPH levels as high as 88,000 ppm
Final Remedy:	Installation of a sealed Waterloo sheet pile barrier in certain area for dry excavation. Gravity dewatered: water then filtered and disposed. Disposal of most sediment at a commercial thermal desorption facility, 20 miles from site. The rest was sent to a hazardous waste landfill 150 miles from site.

A.43.2 Site Description

The target areas for the Messer Street Manufactured Gas Plant in Laconia, NH, consisted of two one-half-acre areas in Winnipesaukee River and one three-quarter-acre area in Lake Opechee. PAHs released from coal tar discharges from a former manufacturing gas plant located adjacent to the river was the main contaminant in the target area. Results of sediment samples collected from the river indicate TPH concentration as high as 88,000 ppm in sediment from 0-2 ft and 87,000 ppm in sediment 4-6 ft. The upper 2 ft of sediment was the target of the sediment removal project, about 13,000 yd³ of contaminated sediment was estimated to be removed.

The one-quarter mile long Winnipesaukee River connects Lake Opechee (upriver) to Lake Winnisquam (down river). The average flow velocities in the main channel of the river are very high because of the difference in water elevation between Lake Opechee and Lake Winnisquam.

The high concentration of PAHs and TPH in the target area, coupled with the high flow velocities of the river in the area makes active sediment removal (dredging and excavation) to be the preferred remedial alternative. Mechanical dredging with a Cable Arm bucket was the designated primary dredging method to remove about 40% of the contaminated sediment especially from the deep section (up to 20 ft of water) of the site. The remaining sediment was to be removed by excavation when the water levels in the lakes were lowered by up to 5 ft.

The mechanical dredging started with 2.5-y³ Cable Arm environmental clamshell bucket, which was replaced with a similarly designed 4.0-cy bucket both of which failed to produce the desired result because they could not effectively penetrate the entire 2 ft of mostly sandy sediment. When these two bucket sizes failed, a custom built hydraulically operated 1.5-cy enclosed bucket attached to an excavator was used to complete the dredging. A crane was used to operate the Cable Arm environmental clamshell buckets and an excavator was used for the hydraulically operated enclosed bucket. A modular barge was used to convey the crane and excavator for in-river dredging. A second modular barge was used to convey the roll-offs containing dredged material from dredge sites to upland sediment handling locations. Land-based operation using dry excavation was used to remove sediment from the area that could be accessed from the land. Combined volume of sediment dredge by all the various equipment was between 12,000 to 13,000 yd³. Following sediment removal, one foot of backfill material was placed over area where sediment was removed. About 8,250 yd³of gravel and native stones was used for the backfill. Native stones were used in area of high river velocities. The same equipment used to remove sediment from each area was also use to place the back fill material in the area. The dredged areas were backfilled with gravel and native stones.

Pre-dredge and post dredge bathymetryThe measurement of or the information from water depth at various places in a body of water. measurements were used to determine that the targeted sediment volume has been completely removed. Post-dredge sampling was not conducted to determine the level of contaminants after the dredging. The removal contractor attributes their success in this project to the following factors: 1) availability and use of different dredge type for the in-river phase of the project and 2) working long hours and days to meet project schedules.

A.43.3 Remedial Objectives

Remove majority of contaminated sediment in 3 acres span.

Dredge depth: 2-5 ft

Contaminated sediment thickness: up to 2 ft.

Combined target removal area: 3 acres.

A.43.4 Remedial Approach

Dry and wet mechanical dredging were used at this site.

Wet and Dry dredging. Dry by installing a sealed Waterloo sheet pile barrier.

Mechanical cable arm clamshell, conventional clamshell, and hydraulic bucket used—depended on sediment type.

Silt curtain installed around the perimeter of the dredge areas. Unanticipated current surges required an extra ballast be added to hold the silt curtains in place. Sheet piling was installed along the river side of localized target areas to eliminate river flow.

A.43.5 Monitoring

Performance: In phase 1, clam shell buckets had trouble collecting sandy sediment. Three passes were required, once in Phase 1 and twice in Phase 2. Post dredging and pre dredging concentrations were significantly different. Cleanup was considered successful. All areas backfilled with mostly gravel material.

A.43.6 References

Maxymillian. Messer Street Former Manufactured Gas Plant Remediation. http://www.maxymillian.com/portfolio/messer_street_gas_plant_remediation.html

Agency for Toxic Substances & Disease Registry as updated Apr 2010. Public Health Assessment Messer Street Manufactured Gas Plant Laconia, Belknap County, New Hampshire. May 2000. http://www.atsdr.cdc.gov/hac/pha/pha.asp?docid=1187&pg=0

Major Contaminated Sediment Sites Database; Sept. 2004 as updated 2008. http://www.smwg.org/MCSS_Database/MCSS_Database_Docs.html

Publication Date: August 2014