Asarco may dig a 940-foot-long, 5-foot-wide and 32-foot-deep trench, then fill it with bentonite, in an attempt to contain one of the numerous sources feeding an underground arsenic plume at its former lead smelting plant in East Helena.

This "slurry wall" would be rectangular in shape -- 220 feet by 250 feet -- and installed around the smelter's acid plant sediment drying area, near the southern edge of the Asarco facility. The rectangle would be capped with 6 inches of topsoil, 18 inches of clean sand, silt or clay and two other liners as part of the effort to keep water from seeping through the contaminated soils.

"We want to both contain what's inside of this area and also to prevent groundwater from flowing through it -- it sort of forms an encapsulated area," said Sharon Kercher, hazardous waste director for the Environmental Protection Agency. "When groundwater hits the wall, it will flow around it," not through the contaminated area.

The $1 million project is being called an interim measure at this point, but Kercher said that if all goes according to plan, it will be a permanent arsenic control measure at the East Helena plant.

"We've looked at possible alternatives and haven't come up with one that's as effective and as cost-effective as this measure," Kercher said on Wednesday. "If it works -- and we think it will -- then we'll be leaving it in place permanently."

Tom Aldrich, Asarco vice president of environmental affairs, said they've worked closely with the EPA and the Montana Department of Environmental Quality to develop this alternative, and agrees with Kercher's assessment that this is the best possible alternative.

He doesn't expect that the excavation work will turn up any unknown substances at the 100-year-old facility.

"We have so many holes around there, we feel very confident that we know what we're getting into," Aldrich said.

They anticipate that the work done this fall or winter, depending on the weather.

Arsenic occurs naturally in low levels in water in the Helena area, but also is a well-known byproduct of the lead smelting process. In addition, Asarco employees routinely sprayed arsenic-laden water on its grounds and buildings to keep dust down, or stored it in one of two man-made lakes on the property.

The arsenic percolated into the groundwater. Recently, below the lead smelter plant's surface, arsenic has been measured in groundwater as high as 34 parts per million -- more than 3,000 times the federal drinking water standard of 0.01 ppm. In the past five years, the 600-foot-wide plume of arsenic contaminated groundwater has migrated off-site, with levels of up to 12 ppm found in groundwater below the city of East Helena.

The plume is estimated to be moving anywhere from as fast as 300 feet per year to as slow as 265 feet in 50 years. But state, federal and Asarco officials all have said that since the plant no longer is operating, the plume's movement should slow considerably.

The slurry wall should slow the plume's off-site movement, said Denise Kirkpatrick, a solid and hazardous waste specialist with the Montana Department of Environmental Quality.

"This is proven technology that works," for containing arsenic in one spot, Kirkpatrick said. "It should address the arsenic that this site contributes, although it will not address anything off-site."

Asarco and the EPA are tracking the off-site plume's arsenic levels through monitoring wells, and say that it hasn't affected any residential supplies.

Kercher noted that the slurry wall is a "passive" corrective measure, as opposed to an active action, which is the preference of the EPA.

"This is a source-control remedy. We're trying to isolate and contain the source," Kercher said. An active measure could involve pumping and treating the groundwater, which is a much more expensive remedy.

A second part of the arsenic containment effort involves installing a similar slurry wall near Asarco's dross and speiss processing plants. That effort is in the preliminary planning stages, and probably wouldn't be installed until next summer.

Asarco also is expected to place a second "permeable reaction barrier" -- or PRB -- near the northern edge of the plant. The PRB involves digging a trench and filling it with iron filings, which bond with the arsenic and keep it from migrating.

Kercher said the EPA doesn't anticipate any problems when digging into the contaminated soil, and that any dirt from the trench would be put in a second high-tech hazardous waste landfill to be constructed south of the plant.

The slurry walls and PRB are part of the ongoing demolition and cleanup effort at the 100-year-old facility, which "temporarily" closed in April 2001. Asarco has removed most, if not all, of the heavy equipment from the plant, and is in the process of demolishing more than a dozen buildings at the facility.

Aldrich said he hopes the public will look over the plans, adding that the work done already shows that Asarco is serious about cleaning up the site.

"One of the advantages of the public comment period is that I think the citizens of Helena and East Helena can see that we are making progress at the site," Aldrich said. "Everybody has the same goal in mind -- to figure out what we have to do there, then move on."

Public may review EPA proposal

Copies of the conceptual remedy document are available for public review at the EPA office in the Great Northern Area, 10 West 15th St. in Helena; at the East Helena City Hall, 306 E. Main St.; and at the Lewis and Clark Library, 120 So. Last Chance Gulch in Helena.

Comments on the proposed trenching plan must by received by Friday, Oct. 20. They should be sent to Linda Jacobson, US EPA Region 8, 999 18th Street, Suite 300, Denver, CO 80202-2466; faxed to 303/312-6409; or e-mailed to jacobson.linda@epa.gov

Jacobson can be reached by phone at 303/312-6503.