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 and vented landfill gases to the atmosphere.
The first "Five Year Review" report for the site was completed in September 2005 by EPA. The review confirmed that clean up measures implemented at the site continued to protect human health and the environment.
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, completed on September 23, 2010, concluded that the remedy is protective in the short-term, however, in order to be protective in the long-term, several follow-up actions needed to be taken including: (1) incorporate previous measures taken to a) control landfill gas, b) address potential future risk posed to recreational users, c) regulatory changes to cleanup standards, and d) land use restrictions into the remedy via a decision document; (2) collect additional overburden groundwater data to confirm that vapor intrusion exposure pathway to residents living near well B-12R is not complete; (3) conduct groundwater sampling for inorganics to confirm that concentrations are consistent with background levels; and (4) ensure that the depth of cover materials overlying waste in the City reclaimed area are adequate to prevent exposure risk to recreational users of the site.
EPA finalized a decision document (Explanation of Significant Differences) to incorporate the aforementioned remedy components into the Site Administrative Record in May 2013. Additional groundwater data and historical information gathered has confirmed that residents living near the extraction well are not being exposed to volatile organic vapors from the contaminated groundwater. Sampling also confirmed that concentrations of inorganics in groundwater are consistent with background levels. Lastly, further soil boring investigations performed on the eastern area of the site, formerly reclaimed by the City for recreational use, confirmed that two out of twenty seven sample locations had less than one foot of clean soil cover; consequently, to ensure public safety, additional clean soil was added to these areas during summer 2013.