The Dover Municipal Landfill (Site) 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 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 NHDES (Agencies) 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. In addition, the Agencies required that the PRPs investigate and remediate a portion of the landfill ditch and swale in an effort to mitigate short-term risks associated with contaminated sediments contained therein.
The primary component of this remedial action involved removal and off-site disposal of sediment contaminated with arsenic and volatile organic compounds (VOCs). This interim remedial action was completed in November 2000. Final remedial action of the ditch and swale areas will be coordinated with implementation of a final remedy.
The TZD operations began in December 1997 and continued through November 2001 with a final report submitted December 31, 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 PRPs proposed the alternate remedy in response to many uncertainties the Agencies raised regarding the TZD and full-scale implementation of enhanced bioremediation technology at this Site. The Agencies agreed to allow the PRPs to evaluate the alternate remedy in a Revised Focused Feasibility Study (RFFS). Pending review and modifications to the RFFS, the Agencies made the determination 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. Given that the sparge trench is innovative, in that it is relatively unproven at the scale and setting proposed, it was to be piloted before full-scale implementation. The amended consent decree/statement of work (SOW) 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 will be designed and implemented in the Southern Plume and monitored natural attenuation will be implemented in the Eastern Plume.
Pre-design investigations that followed the 2004 AROD included: (1) focused sediment sampling to evaluate ecotoxicity in the Cocheco River east of the Landfill; (2) surface water sampling, discrete groundwater sampling and soil boring activities to evaluate the origins of a hot-spot of volatile organic compounds located in the northwest portion of the Landfill; (3) discrete water sampling and soil boring to evaluate conditions in the area between the Landfill and the Bellamy Reservoir {Southern Plume}; (4) groundwater sampling to evaluate conditions associated with potential vapor intrusion near residents along Tolend Road, and; (5) discrete groundwater sampling to evaluate conditions in the interior of the Landfill to facilitate the sparge trench design.
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 has been collected from the work carried out for the Source-Control Pre-Design Investigations required by the Amended ROD. The SC-FFS concludes that modifying the source control remedy from an air sparging trench to an extraction well system with offsite treatment at the Dover POTW is preferable for numerous reasons detailed in the report. The concept of an extraction system is not new for this site. It was an element of the 1991 ROD remedy and remains so in the 2004 AROD contingent remedy. 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 is scheduled to be completed in 2010 with implementation in 2011. The City of Dover is planning to extend the sewer main to the Site during the summer of 2010, which will convey extracted groundwater to the POTW. The pump and treat remedy implemented in the Southern Plume and the air sparge/soil vapor extraction system installed in the northwest landfill hot-spot will continue to be operated on a seasonal basis.
The City of Dover has also begun preparations to enhance the landfill surface to better support vegetation. To this end, the City will use state-approved Class A Biosolids generated from the Dover POTW combined with street-sweepings, catch basin materials and leaf compost to enhance the existing landfill cover materials. The City will then seed the amended areas with native grasses to encourage a durable vegetative cover.