Exide Group Inc. now estimates it will have to dredge 27,600 cubic yards of lead-contaminated sediment from Mill River -- an increase of 29 percent over an earlier estimate -- and the state Department of Energy and Environmental Protection has extended public comment on Exide's proposed cleanup plan to Feb. 28.
But the town Conservation Commission on Thursday night mostly focused on another part of the cleanup plan -- Exide's proposal to discharge sediment-cleansed water back into Mill River during the dredging operation -- because the deadline for public comment on that application is Feb. 7.
Thomas Steinke, director of the Conservation Department, discussed a five-page report that he wrote in response to Exide's application for a National Pollutant Discharge Elimination System permit, and the commission voted to approve sending Steinke's report to Daniel C. Esty, commissioner of the DEEP.
"You have our go-ahead on this, Tom, to get this letter off and you can sign it on behalf of the commission," Kevin Gumpper, chairman of the Conservation Commission, said toward the end of the 90-minute meeting in Sullivan-Independence Hall.
Exide manufactured automobile batteries at its 2190 Post Road factory from 1951 to 1981, polluting the adjacent Mill River with lead. In 2006, Exide demolished its factory and removed lead-contaminated soil from its 6.25-acre property. It's now proposing to dredge lead-contaminated sediment along 4,000 linear feet of Mill River in response to an order from the DEEP.
On Thursday night, Steinke told the Conservation Commission that he suggested Exide use a cofferdam in areas with the greatest concentrations of lead, but Exide declined to do that. The cofferdam would involve sectioning off and dewatering parts of the river for dry dredging, instead of using silt curtains to section off an area in wet dredging. Steinke said re-suspended sediment could "billow out" of silt curtains. "The potential exists that they're going to have a real problem," he said.
Steinke said Exide also intends to dredge and discharge sediment-cleansed water back into the river during spawning periods, according to its remedial action plan. The water would be discharged back into the river at an average of 435,000 gallons a day, according to Steinke's report.
Shellfish can't be protected during the dredging operation, according to Steinke. "All shellfish you have in there will be dredged. Everything will be taken out, they're cleaning out the bottom. There's not going to be anything there for a long time," he said. Sediment would be excavated to a depth of one to three feet in most parts of the river, but a few areas will be dredged to a depth of five feet, Steinke said.
He was concerned about the effect of Exide's dredging operation on alewives and blueback herring, which are collectively known as "river herring."
"It is a relic population that I don't think should have any more stress on it than there is now," he said. His report says the federal government is evaluating whether river herring should be listed under the Endangered Species Act.
Before Exide discharges water back into Mill River during the spawning season, which is March to June for river herring, Steinke said the company should first demonstrate, through acute toxicity test results, that the discharge will not do any harm. If Exide doesn't submit such data, Steinke proposed that Exide offer mitigation, such as restocking shellfish beds after the river is dredged. He said Exide does not plan to restock shellfish beds and the DEEP has not required it.
Steinke said water discharged during the spawning season also should be continuously tested to ensure its temperature, pH, salinity and dissolved oxygen are consistent with river water.
Ed Jones, the town's inland wetlands compliance officer and open space manager, said river herring are "very temperature sensitive." Steinke said the state fisheries biologist opposes the dredging operation taking place more than 12 hours a day to limit its impact during the spawning season, but Exide's application for a NPDES permit calls for at least 15 hours a day. Steinke said the increase to 15 hours a day may be due to Exide's revised estimate of dredging 27,600 cubic yards of sediment, instead of 21,440 cubic yards.
Exide needs an NPDES permit to discharge sediment-cleansed water back into the Mill River, and Steinke said Exide also should have that permit for potential discharges from the containment structure used in wet dredging.
But Steinke said he's virtually alone in believing Exide needs an NPDES permit for that. "No one shares my opinion on this one," he said. "Everyone looks at the NPDES program as a conduit, as a straight pipe."
If Exide doesn't apply for the second NPDES permit, it should be required to use cofferdams when dredging highly contaminated areas, according to Steinke's report. Barring that, Exide should have to dredge those areas during non-spawning seasons, Steinke's report says. If none of those conditions are required, Exide should "establish a robust compensatory mitigation program" that could include fish passage facilities and restocking shellfish beds, the report says.
The commission plans to discuss Exide's remedial action plan Feb. 7, according to Conservation Department staff.