The Gilson Road Hazardous Waste site, also called the Sylvester site, is a seven-acre sand and gravel pit that was used for disposal of hazardous liquids and solid waste from the late 1960s through 1979. Over 900,000 gallons of fluids containing a variety of toxic organic and inorganic compounds were disposed at the site, contaminating the soils and groundwater. The contaminated groundwater plume migrated to the northwest and threatened drinking water wells of nearby homes. The leading edge of the contaminated groundwater plume flowed into and under Lyle Reed Brook, and was carried downstream to the Nashua River, and then to the Merrimack River. This posed a potential threat to public water supplies.
Several emergency actions were undertaken after discovery to ensure protection of public health and the environment: 1) a security fence was placed around the entire site by the city of Nashua in May 1980; 2) over 1,300 drums were removed and sent to an approved hazardous waste facility for treatment and disposal; 3) after the signing of the Record of Decision (ROD) in July 1982, the EPA installed an emergency groundwater interception and recirculation system to impede the migration of contaminated groundwater to Lyle Reed Brook; 4) in late 1982, a bentonite slurry wall was constructed around the site to inhibit migration of contaminated groundwater from the site, and an impervious cap was placed over the site to prevent the infiltration and contamination of rainwater; and 5) the city of Nashua extended municipal water to the surrounding area in 1983.
In the fall of 1983, EPA issued a supplemental ROD that authorized construction of a $5.4 million groundwater treatment facility and established cleanup goals for the site.
Construction of the facility began in the spring of 1984, and it was completed in 1986. Between 1986 and 1996, the facility treated groundwater at a rate of approximately one-half million gallons per day. Inorganic constituents were initially precipitated from the water and dewatered in a filter press before placement in an on-site lined landfill. Volatile organic carbons (VOC) were then stripped from the groundwater and destroyed by incineration. The processed water was then pumped back into groundwater recharge wells located within the bentonite slurry wall containment. More than one billion gallons of groundwater was pumped through the treatment facility during its operational life, and more than 430,000 pounds of contaminants were removed.
In 1995, EPA and DES determined that clean-up goals had been achieved, and in January 1996 groundwater treatment ceased. The facility was maintained in an operational condition for five years following shut-down. Discussions between EPA and DES officials led to the conclusion that the treatment plant was obsolete, and that less expensive alternative remedial technologies exist if contaminant levels ever rebound and additional treatment were necessary. Pumping and recharge wells were left in place, but the plant was decommissioned, and treatment equipment was dismantled and removed by August 2001. The second of the two on-site landfill cells was closed during the summer of 2005.
DES has continued to monitor groundwater and surface waters both within the containment area and offsite. The data show a continued downward concentration trend of contaminants associated with the original plume.
It has been found that sediment in Lyle Reed Brook contains elevated levels of arsenic, though arsenic is not a contaminant historically associated with the contaminated plume. The arsenic has likely been mobilized from the natural soils of the area to a greater extent than would otherwise occur under ambient conditions due to the chemical dynamics of the site. However, sampling performed since 2004 indicates that Lyle Reed Brook is in compliance with current New Hampshire surface water quality regulations. Further, an evaluation by DES of sediment quality found that arsenic containing sediments do not pose a significant risk to benthic organisms or to human health (Evaluation of Sediment Quality to Support an Ecological Risk Assessment at the Gilson Road Superfund site, DES, Concord, NH, July 2004).
EPA's fourth five-year review was signed on September 14, 2009, and found the remedy protective of human health and the environment in the short-term. However, in the long-term, the remedy was found not to be protective due to future potential exposures to site contaminants through the possible use of groundwater outside the current GMZ, future exposure to site contaminants via migration to indoor air, and possible future failure of the slurry wall and/or cap over the former waste disposal area to perform as designed.
DES will ensure the remedy remains protective in the future by expanding the GMZ and renewing the Site's Groundwater Management Permit and/or facilitating the incorporation of the GMZ into a city ordinance. In addition, in 2012, DES contracted with an environmental consultant to evaluate the potential for contaminant vapors to migrate to indoor air in nearby residential neighborhoods; the subsequent report concluded that there was no risk of vapor intrusion. Finally, DES is working with an EPA research group in Ada Oklahoma to evaluate the current and long-term functionality of the cap and slurry wall; reporting is scheduled for 2013.