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DEVENS — The Army contractor working on remediation of arsenic-laden groundwater flowing from the Shepley Hill Landfill is developing a revised plan that focuses on deeper exploration in the Nonoacious Brook area and in Plow Shop Pond’s Red Cove.

Marc Grant, program manager for AMEC, told the Restoration Advisory Board last week his firm is developing a two-phase approach that will better delineate the boundaries of an arsenic plume some 50 to 60 feet under West Main Street, Ayer, and fill in gaps in data for the pond.

Once the boundaries of the West Main Street plume are known, a permanent monitoring well will be drilled.

“One of the goals (or) at least part of all (solutions) is to gain some kind of a time horizon on how long (all) remedies must operate, and also determine if something (permanent) can be put in its place,” said Robert Simeone, the Army’s environmental director.

The conceptual work plan is expected to be revised in two weeks and the easier field work will begin in March.

Grant said the plume of arsenic under West Main Street has solidified and is not moving toward Nonoacious Brook, which empties into the Nashua River.

“Water flows through it, but somewhere along the line it solidifies and comes to the surface,” he said. “We’re trying to find out where. We found some we believe is coming from the landfill, but that’s not easy to determine.”

Water may, for example, flow through cracks in the bedrock that surrounds Plow Shop Pond. The revised plan includes seismic testing — “thumping” the ground and reading sound waves as they travel upward, Grant said.

Borings will be done in soil on the landfill side of West Main Street. Records of the phases by which the landfill grew are available. When compared against the thicknesses of waste in the borings, valuable information can be gained for remediation, Grant said.

Simeone said the EPA has suggested use of old aerial photos as well, for example in the area of an Arm incinerator that was used from 1919 to 1940. Whether or not the ash is affecting what is seen in Grove Pond water remains to be seen.

“You can’t punch a million holes, but you can choose the best overall spots,” Grant said.

Both the Army and Mass DEP have data gathered from temporary monitoring wells on the West Main Street side of the landfill. Most data is water-related. The field work in March will also sample soil from the landfill and test it for reaction to water and other elements to determine if it will continue to affect the pond.

The pond

AMEC’s draft report regarding sediment and water quality in the western half of Plow Shop Pond that focuses on Red Cove (southwest near the landfill) and Tannery Cove (on the north side near the former leather tannery) is due in January and a final in June. A final field study that includes a detailed evaluation that has been compared to Superfund criteria is expected in November 2010.

Grant said preliminary test results are consistent with measurements done in the past two years, showing arsenic bound to reddish iron flock in Red Cove, heavy in some spots and thin in others. Highest levels of chromium and mercury are found in Tannery Cove.

Simeone said the highest concentrations of chromium and mercury is highest near the railroad tracks. Mercury seems to be concentrated at the channel beneath the tracks that connects Plow Shop and Grove ponds. Levels are higher on the tannery side of the pond. Tanners used mercury salt to preserve and sanitize hides.

Groundwater from the south side of the Shepley Hill Landfill seems to be running off the edge of the membrane covering the landfill cap and into the pond. Tests near the cap indicate the water matches what’s in the pond, Grant said. The treatment plant at the site is removing about one third of the water that would be flowing into Red Cove.

Preliminary findings indicate human risks to waders and “subsistence fishermen” (those who eat the fish) in Plow Shop Pond if the activity is conducted over many years.

BMI (Bentic Macro Invertebrates, or “bugs in the mud”) are widely impacted by metals and ammonia. Risks to mammals and birds come from mercury, and arsenic content presents a cancer risk to humans in the North side of the pond and Red Cove.

Simeone re-emphasized that the Army will not be active in remediation of risks on private property surrounding the pond. Fishing and swimming has been closed for some time.

P.A.C.E. (People of Ayer Concerned about the Environment) member Laurie Nehring asked how arsenic levels around Shepley Hill compare with other contaminated landfills. Ginny Lombardo of Mass DEP said arsenic growth appears in others in New England, but “not in the 4,000’s (4,000 parts per billion) like we have here.”

Simeone said there are other sites in New England “in the 100’s,” and one has been in the 5,000 to 6,000 parts per billion range but is now about 3,000.

“The pump and treat (plant on Devens) is holding steady at 3,000 and (levels in) down gradient wells are dropping. The main content of the (arsenic) plume (around West Main Street) is at 1,000,” he said.

Simeone said the Army will probably talk with Ayer officials in the near future about creating regulations to prevent the drilling of private wells in the affected area.