The Dover Municipal Landfill accepted municipal and industrial refuse for on-site disposal from approximately 1961 to 1979. The site occupies approximately 55 acres of land four miles to the southwest of the city of Dover in a generally rural setting. The site is approximately 2,100 feet south of the Calderwood municipal water supply well, 600 feet west of the Cocheco River, and 1,400 feet northeast of the Bellamy Reservoir. The Bellamy Reservoir, along with water from multiple municipal production wells located in the area, supplies drinking water to Portsmouth, Newington, Newcastle, Greenland, and parts of Rye, Madbury and Durham, New Hampshire.
The site was added to the National Priorities List in September 1983 after sampling of residential well water in the vicinity of the site revealed contamination had migrated approximately 200 feet to the east of the landfill. The city of Dover promptly installed a water main to service all affected and potentially affected homes in the area of the site. Studies undertaken by others at the Dover Municipal Landfill indicated that the Bellamy Reservoir and the Calderwood municipal supply well, as well as private residential wells located in the vicinity of the site, were potentially threatened by groundwater contamination emanating from the landfill.
The Remedial Investigation was completed in March 1989. In 1988, the Potentially Responsible Parties (PRPs) signed an Administrative Order with EPA and DES to conduct the Feasibility Study, which was completed in early 1991
EPA issued the Record of Decision (ROD) formalizing the preferred remedy in September 1991. The source control component of the preferred remedy included capping the landfill, installing a leachate collection trench and/or groundwater extraction wells, and treating leachate on-site with discharge to the Cocheco River or pretreating leachate on-site with discharge to the Dover publicly owned treatment works (POTW). The management of migration portion of the remedy included monitored natural attenuation of the eastern plume and active extraction with treatment of groundwater in the southern plume.
In 1993 the PRPs proposed further study of the southern plume. They believed the contaminants to be naturally attenuating such that the plume would not impact the Bellamy Reservoir. The agencies agreed to allow the PRPs to conduct additional studies of the southern plume. The report findings were inconclusive and recent groundwater quality data suggest that a plume of groundwater containing elevated concentrations of contaminants of concern is migrating south toward the Bellamy Reservoir.
Construction of the preferred remedial action, described in the 1991 ROD, was scheduled to begin in June 1997. However, the agencies agreed to postpone implementation of the remedy to allow the PRPs to explore an alternative remedy at the site. Specifically, the PRPs constructed a pilot-scale treatment zone demonstration (TZD) to evaluate the site-specific viability of augmenting naturally occurring biodegradation of contaminated groundwater by injecting sodium benzoate and oxygen.
The TZD operations began in December 1997 and continued through November 2001. In November 2001, the PRPs proposed to abandon enhanced bioremediation as the preferred remedy and introduced an alternate remedy that included a permeable vertical barrier along the down-gradient landfill toe to facilitate the injection of air and stripping of contaminants (alternate remedy). The agencies agreed to proceed with the alternate remedy by amending the 1991 ROD.
In September 2004, EPA signed an Amended Record of Decision (AROD) that changed the preferred source control remedy from impermeable cap with extraction and treatment of groundwater to an air sparge trench that would remove, destroy, or immobilize contaminants in-situ. The amended consent decree/statement of work included the 1991 remedy (cap, groundwater pump and treat) as the contingent remedy, should the sparge trench fail. The remedy for the extended plumes remained unchanged from the 1991 ROD; that is, an active pump and treat system in the Southern Plume and monitored natural attenuation of the Eastern Plume.
In 2008, the PRPs finalize and implement the remedial design of groundwater extraction system in the Southern Plume and implemented a full-scale air sparge/soil vapor extraction remedy in the northwest portion of the landfill.
The PRPs prepared a source-control focused feasibility study (SC-FFS) in 2007 as the result of new information that had been collected from the work carried out for the Source-Control Pre-Design Investigations required by the Amended ROD. The SC-FFS concluded that modifying the source control remedy from an air sparging trench to an extraction well system with offsite treatment at the Dover POTW would be preferable for numerous reasons detailed in the report. Modifying the remedy to an extraction system has several significant advantages which include utilization of a proven technology, eliminating uncertainties associated with implementation of the sparge trench, and avoiding potential adverse interaction between the Southern Plume extraction and Northwest Landfill remedies and the sparge trench remedy. The extraction and conveyance system could also be implemented much more quickly and at a significant cost savings. The agencies reviewed and approved this remedy change in a June 30, 2009 Explanation of Significant Differences.
The preferred remedy (extraction and conveyance system) design was completed in 2010 and largely implemented in 2011. The remedy has been fully operational since late spring 2012. The City of Dover also extended the sewer main to the site in 2011, as the component of the remedy that conveys extracted groundwater to the POTW.
The ditch surrounding the landfill was closed as the preferred remedy was constructed, effectively ending all off-site migration of landfill-impacted surface waters. The shallow groundwater that previously discharged to surface waters is now being captured by the extraction system and conveyed off-site for treatment at the POTW.
The air sparge and soil vapor extraction remedy installed in 2008 to treat the northwest landfill hot-spot area has been shut down to monitor for VOC rebound following groundwater concentrations reaching asymptotic levels. The system has removed approximately 43,000 pounds of VOCs since startup. Residual VOCs from this hot-spot will migrate toward the landfill toe and be captured and treated by the existing extraction and conveyance system.