...to the processing facility up to 10 miles away. Barge-mounted booster pumps positioned at every mile keep the flow at up to 6,000 gallons per minute, or about 150 cu yd of sediment per hour.

The pipelines from the dredges connect directly into the processing plant without a buffer pond, an unusual method that requires precise coordination between the dredges and processing plant to keep the slurry flowing smoothly. Computerized controls, expert operators, and good communication are needed to make the system work efficiently.

“Although hooking the dredges directly into the processing plant requires precise flow control, it is faster and cheaper than using a buffer pond,” says Steve McGee, another Tetra Tech vice president. “In this case, it also eliminates the problem of having an open pond hold contaminated material near stakeholders and the public.”

The cleanup is making extensive use of computerized mapping and project controls to define areas that need remediation, guide precision contour dredging, and verify the results. Before dredging started, engineers and scientists used a combination of GPS, multi-beam sonar, laser scanners and aerial orthophotography to map every inch of the river, shoreline and related structures in the 13.3-mile site. They also took more than 2,000 core samples of riverbed to identify the location, size and depth of PCB pockets.

A software program from Hypack Inc., Middletown, Conn., produced a three-dimensional grid map that is accurate to within inches. Color codings on the map show where to dredge and the depth of the contaminated sediment, which ranges from 6 in. to 10 ft deep.

“The mapping took a lot of effort and was not inexpensive,” says McGee. “But it helped us come up with an efficient approach that will save millions of dollars over the life of the project. Nobody has done this on such a large-scale remediation project before.”

As dredging continues, the information is updated daily, and color-coded screen displays guide dredge operators in cutting specific contours to remove exactly the right amount of sediment.

Greg Smith, one of dredging contractor J.F. Brennan’s project managers, says onboard GPS compensates for barge movements, so the dredge operators can be accurate to within centimeters.

“Neat-line contour dredging has never been done before on this scale, but if it’s done right, it minimizes the amount of clean material that’s removed unnecessarily and also minimizes the amount of contaminated material that gets missed,” says Mangrum. So far, three of four dredged areas have met standards on the first pass, and the one that didn’t was “really close,” he adds.

Normally, it would take 18 months to build a processing plant like the 247,800-sq-ft facility Tetra Tech is using at the downstream end. But this one was done in just nine months, thanks to design-build techniques and precise on-site coordination among more than a dozen contractors involved.

“At times, erectors were placing new steel next to electricians, who were mounting motors to beams that had been placed only 10 minutes before,” says Eric Schmidt of electrical subcontractor Van Ert Electric Co. Inc., Kaukauna, Wis., which worked under Miron Construction Co. Inc., Neenah, Wis., the plant’s general contractor. Despite having some 300 workers operating in such close proximity, there were no lost-time accidents throughout nine months and 360,000 man-hours of construction, the contractors report.

Process Flow

The incoming slurry first runs through a scalping screen that takes out rocks, wood chips, bottles, and other debris larger than 6 mm in diameter. Debris is sent to a landfill, and remaining sand, water and silt move on to separator cyclones that remove the sand from the remaining slurry. The first set of separators removes grains measuring...