landfill cap has been neglected,
causing its deterioration well below the original DEP Permit
specifications. As a result, large amounts of water are infiltrating
through the clay layer (cap) that was supposed to be impermeable. This
generates a large amount of leachate which leaks into the
groundwater and pollutes the surrounding wetlands. Urgent measures
are required to prevent further deterioration of the cap.
The DEP letter of June 25, 1985 approved the Old Landfill Closure Plan, with a 12 inch thickness of the impermeable clay layer (cap). In response to a request from the town, the DEP agreed in a letter of August 26, 1985 to allow the town to decrease the cap thickness to 8 inches to reduce costs on the following conditions:
particular care is exercised to ensure that the layer is at least 8 inches in depth after compaction.
The cap’s maximum coefficient of permeability should remain less or equal to 1x10-6 cm/sec.
What was Actually Installed
Project Manual for Sanitary Landfill Closure was used by the
Town as an engineering document for the contractor hired to do the
work. This document states that:
semi-impervious soil that contained a mix of clay with silt and sand was used as a substitution for an impermeable clay layer required by the DEP Closure Permit
the document instructs the contractor to achieve the permeability coefficient of the clay layer to be 1x10-5 - a value that exceeds the maximum allowed value in the 1985 Closure Permit by 10 times.
DEP Comprehensive Site Assessment Results
The DEP test report of 2007 shows (on page 5) that the clay cap has deteriorated well below the original design specifications:
the impermeable layer was less than
6 inches in 9 of
the test holes, with a minimum thickness of only 4 inches, which is half
of the original design specification and is much less than the 18
inches minimum required by today's standards.
the average hydraulic connectivity (coefficient of permeability) of the impermeable layer is 1.5x10-4, which is 150 times more than the original design specification approved by the DEP in 1985 and 1500 times more than the current standards.
This results in water infiltrating the landfill, which generates a large amount of leachate leaking into the groundwater.
The DEP Comprehensive Site Assessment report of 2010 states (on page 3) that:
“940 gallons/acre/day of leachate are produced by precipitation infiltrating through the existing landfill cap” which is supposed to be impermeable.
The plume resulting from the leakage flows westward and causes elevated levels of lead, mercury, arsenic, copper, iron and cyanide that exceed WQC and MassDEP GW-1 standards, as well as the SSC guidelines.
While the GW-1 standard is for the “Potential Drinking Water Source Area” and exceeding it may not presently represent a significant risk to human health, there is a trend toward increasing levels of contaminants in the environment.
A Comparison of the DEP 2010 data with the 2007 DEP report reveals a 5-fold increase in arsenic level and a 3-fold increase in mercury level in 3 years in sediment samples at the Gull Pond and Hop Brook wetlands.
The 2007 DEP report also reports lead and chromium exceeding MassDEP GW-3 standard levels in wetlands adjacent to Hop Brook Drive near the railroad.
GW-3 standards “apply to all groundwater in the Commonwealth”
and are “intended to address the adverse ecological effects that
could result from discharge of oil or hazardous material to surface
water” that “pose a significant risk of harm to aquatic
How do we address
The current state of the landfill cap requires urgent attention. The cap is sagging due to differential settlement, which is supposed to be addressed by the planned regrading.
However, the problems listed above may require more than a simple regrading in order to prevent further deterioration. A more permanent solution including the installation of an additional impermeable membrane may be necessary.
Adding an additional
load of thousands
of tons of equipment on top of the already fragile landfill cap will
only accelerate its deterioration, increase the leaching of
contaminants into the groundwater, and prevent the periodic
maintenance necessary to maintain the integrity of the cap.
for scans of the DEP documents
mentioned on this page.