Pump and Treat and Containment of Contaminated Groundwater at the Sylvester/Gilson Road Superfund Site, Nashua, New Hampshire

Site Name:

Sylvester/Gilson Road Superfund Site


Nashua, New Hampshire

Period of

Status: Completed
Period of Operation: December 1981 through December 1996 (1981 through 1986 for hydraulic control and 1986 through 1996 for remediation), followed by monitored natural attenuation. Performance data collected through December 1995.


Full-scale cleanup

Pump and Treat; Vertical Barrier Wall; Cap; and Soil Vapor Extraction

- Groundwater was extracted using 14 wells, located on site, at an average total pumping rate of 265 gpm
- Extracted groundwater was treated with addition of chemicals (lime slurry), flocculation, clarification, mixed-media pressure filtration, air stripping (at elevated temperature [175°F]), and biological treatment (biological treatment was used for only 50 of the 265 gpm extracted)
- Treated groundwater was reinjected on- and off-site through recharge trenches
- A slurry wall, 4 ft wide, 4,000 ft long, and as much as 100 ft deep, encloses the 20-acre site
- A 40-mil HDPE synthetic cap covers the area inside the slurry wall
- The SVE system included 66 wells and a boiler/incinerator for destruction of VOCs

Cleanup Authority:
CERCLA Remedial
- ROD Date: 7/29/82 and 9/22/83
- ESD Date: 7/10/90 and 9/23/02


EPA Point of Contact:
Darryl Luce, RPM
U.S. EPA Region 1
JFK Federal Building
1 Congress Street
Boston, MA 02203
(617) 573-5767

State Point of Contact:
Kenneth Kettenring
6 Hazen Drive
Concord, MA 03301
(603) 271-4060

Weston (construction oversight)
Veolia Water North America (O&M)
Contact: John Fritsch
57 Gilson Road
Nashua, NH 03062
(603) 249-9840

Chlorinated solvents; volatiles - nonhalogenated; and heavy metals (selenium)

- Maximum concentrations detected in 1980 included methylene chloride (122,500 mcg/L), chloroform (81,000 mcg/L), tetrahydrofuran (1,000,000 mcg/L), methyl ethyl ketone (80,000 mcg/L), and toluene (140,000 mcg/L)

Waste Source:
Waste disposal, drum burial, waste storage

Type/Quantity of Media Treated:
- 1.2 billion gallons treated as of December 1996
- LNAPL (toluene) observed in several monitoring wells on site
- Depth to groundwater was not provided for this site
- Extraction wells are located in 3 hydrogeologic units which are influenced by a nearby surface water
- Hydraulic conductivity in the upper unit ranges from 30 to 50 ft/day

Purpose/Significance of Application:
ACLs have been attained for all contaminants except chlorobenzene. The primary issue at this site is with a contaminant (arsenic) for which cleanup levels were not established.

Regulatory Requirements/Cleanup Goals:
- The remedial goals for this site were set as alternate concentration limits (ACLs) within the containment structure. ACLs were set at 10% of the maximum concentration detected and consisted of the following: vinyl chloride (95 mcg/L), benzene (340 mcg/L), chloroform (1,505 mcg/L), 1,1,2-TCA (3 mcg/L), MEK (8,000 mcg/L), chlorobenzene (110 mcg/L), methylene chloride (12,250 mcg/L), toluene (2,900 mcg/L), 1,1-DCA (81 mcg/L), trans-1,2-DCA (1,800 mcg/L), 1,1,1-TCA (200 mcg/L), methyl methacrylate (350 mcg/L), selenium (2.6 mcg/L), and phenols (400 mcg/L).

- Risk-based concentration levels were set for groundwater outside of the containment structure.

- A performance goal for the remedial system was to prevent the contaminant plume from further migration.

- As of December 1996, the remedial action appeared to have attained ACLs for all contaminants except 1,1-DCA and 1,1,2-TCA. In 2002, the ACL for 1,1-DCA was adjusted to 81 µg/L and 1,1,2-TCA was adjusted to 3 µg/L. Following this adjustment, all ACLs had been attained. As of spring 2004, ACLs for all contaminants continued to be met with the exception of chlorobenzene, which was detected slightly above its ACL of 110 ppb. According to the most recent 5-year review (September 2004), chlorobenzene levels are declining and are expected to reach their ACL in the near future.

- From 1986 through 1996, the system removed approximately 430,000 pounds of contaminants from the groundwater.

- A net inward flow into the containment structure has been maintained, thus reducing downward migration of contaminants.

Cost Factors:
- Actual costs for the remedial application at this site were $29,700,000 ($9,100,000 in capital and $20,600,000 in O&M), which corresponds to $25 per 1,000 gallons of groundwater extracted and $69 per pound of contaminant removed.

- The high O&M costs for this site were attributed to the 300 gpm treatment system and the number of staff required to operate it. For many years, the site was staffed with 15 full-time personnel who operated the site 24 hours/day.

The Sylvester/Gilson Road site is a 2-acre site. Approximately six acres of the site was used as a sand borrow pit for an undetermined number of years. Illegal dumping was first discovered in 1970. Although the total amount of hazardous waste disposed at the site had not been determined, documents show that approximately 900,000 gallons of hazardous waste were discarded at the site during a 10-month period in 1979. It was estimated that the site was used for hazardous waste disposal for five years. In 1981, initial remedial investigations by the state showed high concentrations of heavy metals and organic compounds in the groundwater under the site. A ROD for this site was signed in July 1982 and a supplemental ROD in September 1983. ESDs for this site were signed in July 1990 and September 2002.

The remedial application at this site consisted of a pump and treat system, vertical barrier wall, cap, and soil vapor extraction system. Groundwater was extracted using 14 wells, located on site, and treated with addition of chemicals, flocculation, clarification, mixed-media pressure filtration, air stripping, and biological treatment. A slurry wall encloses the 20-acre site, and a HDPE synthetic cap covers the area inside the slurry wall. To address an area with LNAPL (toluene) that was identified part-way through the application, a SVE system was installed that included 66 extraction wells. ACLs have been attained for all contaminants except chlorobenzene, which is expected to achieve its ACL in the near future.
According to the third Five Year Review for the site, the current concern is the presence of arsenic in groundwater, surface water, and sediments. Arsenic was not an original contaminant of concern and was not part of the sampling strategy during pump and treat operation. The review recommended expanding the boundaries of existing institutional controls to encompass all areas where groundwater is contaminated with arsenic. Ecological risks due to elevated arsenic concentrations in sediments are being evaluated.