The Somersworth Sanitary Landfill site is located in the central portion of Somersworth, approximately one mile to the southwest of the city proper. The 26-acre waste disposal area is situated entirely on land owned by the city of Somersworth. The landfill is adjacent to, and within approximately 400 feet of, Peter’s Marsh Brook, a secondary tributary of the Salmon Falls River, which serves as a water supply to Somersworth and Berwick, Maine.
The city has owned and managed the operation of the site since the 1930s. The landfill accepted municipal and industrial refuse for on-site disposal between the mid-1930s and 1981. Wastes deposited in the landfill included municipal trash, industrial wastes, and chemical wastes. Groundwater quality studies initiated at the site in 1980 indicated that the groundwater beneath the site was being contaminated by VOCs leaching from the landfill.
The site was placed on the National Priorities List in September 1983. During the course of the Remedial Investigation, it was found that organic contamination of groundwater and surface water existed in the area.
The Potentially Responsible Parties (PRPs) signed an Administrative Order to conduct the Feasibility Study (FS) under EPA and DES supervision. The FS was completed in December 1993. EPA signed the Record of Decision (ROD) in June 1994. The major components of the preferred alternative described in the ROD are: (1) the installation of a permeable, chemical treatment “wall”; (2) the extraction of bedrock groundwater with discharge up-gradient of the treatment wall; (3) the installation of a permeable cover while the chemical treatment wall is functioning; and (4) the evaluation of the need for an additional cover at the time of final closure.
A pilot-scale chemical treatment wall was constructed in 1996 utilizing a steel caisson to facilitate the removal of soils and placement of the wall materials. Constructability issues that became apparent as a result of the pilot study supported further evaluation of construction methods. A second pilot wall was constructed in November 1999 utilizing bio-slurry trenching technology. Construction methods and performance evaluations were considered and modifications made, as appropriate, to the full-scale installation. In September 2000, construction of the full-scale chemical treatment wall was completed.
The infiltration gallery, where extracted bedrock groundwater is discharge up-gradient of the treatment wall, was built concurrent with the permeable cover, which was completed during the spring and summer of 2001. Performance monitoring of the treatment wall and associated remedial systems will be considered in the design of a final cover, if a final cover is determined necessary.
As a result of discussions regarding existing, and potential future, reuse of the site, the administrative record was reviewed and consequently found landfill gas monitoring to be a relevant and appropriate regulatory requirement and a prudent measure to ensure the protection of human health.
Upon installation of perimeter landfill gas monitoring probes, methane levels were found to be in excess of the standard in multiple probes, indicating the potential for off-site migration of gas into nearby homes and businesses. Subsequently, DES required the installation of a landfill gas management system (GMS) to mitigate the off-site migration of landfill gases. The GMS was designed and constructed in 2003 and has since successfully captured landfill gases and vented them to the atmosphere via vertical pipes. Upon sampling and analysis of the vented gas, it was determined that engineering controls are not needed, as gas concentrations do not exceed ambient air standards.
The first “Five Year Review” report for the site was completed in September 2005 by EPA. The review confirms that clean up measures implemented at the site continue to protect human health and the environment. Two issues were identified during the review: (a) additional notification of affected property owners within the area of groundwater contamination is needed; and (b) continued monitoring is essential to ensure that groundwater cleanup is progressing and that long-term exposures will not exist in the future at the site.
EPA also finalized a Preliminary Close-Out Report (PCOR) in September 2005, which documents completion of all physical, remedial construction activities at the site. EPA and DES conducted a pre-final inspection in June 2004, and no outstanding construction items were identified. Therefore, no additional, substantial construction is anticipated at the site.
The Second Five Year Review for the site was completed on September 23, 2010. It concluded that the remedy is considered protective in the short-term, however in order to be protective in the long-term, several follow-up actions listed in the Five-Year Review needed to be taken including final closure of the landfill. As of August 2011, all of the additional data and documentation required by the Second Five year Review has been collected.
The additional groundwater data and historical information gathered has preliminarily confirmed that residents near the extraction well are not exposed to vapor intrusion from the contaminated groundwater.
Further evaluation (test pitting and surface soil samples) was performed on the eastern area of the site that was reclaimed by the city for recreational use. The objective was to confirm that the existing fill on top of the recreational areas meets the same protectiveness standard as the existing permeable landfill cover over the remainder of the landfill at the site. Preliminary results revealed a small area that needs additional cover material and arsenic as the only contaminant exceeding NH DES soil remediation standards.