DEVENS -- Work began July 30 to build a $1.6 million underground "slurry" wall in an effort to stop the migration of naturally-occurring arsenic into Plow Shop Pond in Ayer. The wall is being built along the northwest quadrant of the 84-acre former Fort Devens Shepley's Hill Landfill site.
Work on the slurry wall, "Phase 1" of the current remediation project, is scheduled to conclude at the end of September. Sovereign Consultants from New Jersey is now constructing a 30 inch-wide barrier wall. The wall will stretch 850 feet and will prevent arsenic-contaminated groundwater from discharging into Plow Shop Pond within the area known as Red Cove. Surface water from the pond passes under West Main Street in Ayer and connects with the Nonacoicus Brook and Nashua River.
Tom Hevner of Sovereign said his crew is digging to the "top of the rippable layer" of bedrock located approximately 52 feet deep in some areas. As material is removed, the hole is then immediately backfilled with low-permeability soil-bentonite material that will mitigate the discharge of contaminated groundwater by diverting the flow of groundwater away from Plow Shop Pond.
Phase 2 calls for the pond sediments located within the Red Cove area to be dried and for sediment to be excavated and removed for off-site disposal. Sediment samples taken from Plow Shop Pond have revealed that arsenic has migrated into the pond from the vicinity of the landfill. This phase of the project will be completed by next summer.
A 1995 record of decision outlined a process to contain waste contaminants from leaving the landfill and seeping into the groundwater and further aggravating the arsenic situation. The goal was to ensure the health and safety for humans and the ecology of Plow Shop Pond and beyond.
The remedy includes landfill cap maintenance, monitoring of groundwater and landfill gases, and restricted access to the groundwater flowing through the landfill. Another contingency remedy required was the construction of a groundwater pump-and-treat system or arsenic treatment plant (ATP) at the north end of the landfill that would prevent arsenic-contaminated groundwater from migrating off-site, beyond the landfill boundary and toward West Main Street in Ayer. The ATP was built in 2005.
Its goal was to extract arsenic from the groundwater around the northern quadrant of the landfill via a microfiltration process. The resulting wastewater is then discharged to the Devens wastewater facility for treatment.
Bob Simeone, environmental coordinator for the U.S. Army Base Realignment and Closure (BRAC) division, said "nothing's 100 percent" in terms of water treatment. Simeone said the Army will soon seek to shut down the pump house in 2013 in favor of "monitored natural attenuation" in which nature runs its course in filtering the arsenic from the groundwater without intervention. The slurry wall barrier is meant to assist in that underground filtration process.
"We believe the pump-and-treat approach is not a sustainable remedy because it would have to run in perpetuity," said Simeone. "We don't believe it's technically beneficial."
Simeone said the pump station has made only "a very little dent" in the amount of arsenic found in the groundwater. In preparing its proposed changes to the record of decision, the Army also suggests enhanced land-use controls would also serve as a more effective and sustainable remedy for the arsenic situation.
Since the base closed in 1996, there has been some $155 million spent on the environmental cleanup at Fort Devens. The annual cost to operate and maintain the Shepley's Hill Landfill remedy is $1 million. The Army intends to transfer ownership of the 118-acre landfill parcel to MassDevelopment. The land is zoned for open space and recreation uses. The site is currently restricted, however, from public access.
On Aug. 30, various stakeholders in the project toured the job site, including state Sen. Jamie Eldridge and members of the People of Ayer Concerned about the Environment.
The Shepley's Hill Landfill served historically as a household waste and construction debris dump for the former Fort Devens Army base from as early as 1917. Operations at the landfill ceased in 1992 and the landfill was capped in four phases from 1986 through 1993.
The landfill also contains ash and debris from a 1940s incinerator that was located at the end of Cook Street on what's now known as the Devens Regional Enterprise Zone. The landfill also contains two asbestos "cells" containing approximately 7 tons of asbestos construction debris.