Designing the workplan for dredging toxic sediment next to Pier 1 at Alameda Point required precision so as not to undermine the stability of the concrete posts supporting the roadway that passes along the pier area. The ground under the water slopes down nearly 40 feet from the cement parking slab adjacent to Wharf Road. Six-foot sediment core samples were obtained during investigations. The varying depths of contamination were charted and used to plot a computer program showing a slope profile that not only would accomplish cleanup, but also maintain the stability of Wharf Road. This means that in some case they are dredging deeper than the contamination.
Dredging toxic mud at the pier area, in what is known as Site 24, got underway in early January 2012. The Maritime Administration had to temporarily vacate this berth. Prior Navy activities east of the pier area, which used solvents, paints, sandblasting materials, and hydrocarbons such as fuels and lubricants, led to contamination when waste products, including pesticides, were disposed of down three storm water drains. The contamination chemicals of concern are cadmium, pesticides, lead, and PCBs. The dredging process requires two methods – vacuuming mud from under the wharf road that is supported by concrete posts, and dredging with a clamshell scoop in the open water.
Vacuuming mud under roadway
The first stage of work, now completed, was vacuuming mud from under the roadway. The specially built dredge pump, equipped with an agitator where the hose contacts the mud, was held in position by a barge-mounted crane that was custom built for this application. The mud went through a large hose, across the roadway, into a drainage basin and into special geotextile tubes that retain the mud.
The water that drained off of the mud, however, was too muddy to immediately send to a filtration system. It was first pumped into a large above ground pool of water in order to dilute it. From there the water was pumped through a series of filtration tanks. The water is being used for dust control and can also be pumped back into the harbor.
Open water precision dredging
The open water dredging is conducted from a barge using a special clamshell scoop that allows virtually no mud to drip out of the jaws when hoisted out of the water. This helps to minimize dispersing fine contaminated sediment in surrounding water, which could necessitate repeat dredging. The scoops of mud are held in the air for about 30 seconds to drain the water before being hoisted over to a dump truck. The trucks are driven a few hundred yards to a special drying pad to dump the mud. As the dredge barge moves out of arms length of the wharf, it will have to deposit the mud in a hopper barge that will then be moved next to the wharf where the mud will be unloaded and put into the dump trucks.
The operator of the dredging rig has a real time computer picture of the contour of the underwater ground surface. The image is aided by positioning sensors on the scoop. This allows the operator to follow precisely the slope design. There is also a floating curtain to keep any disturbed contamination within the work area. At the curtain boom and outside the work area are two floating rafts that detect turbidity, or muddiness, in the water. These rafts send real time measurements to the dredge operator. If the turbidity exceeds a certain limit, the dredging is temporarily halted.
Although the Navy periodically dredged the berthing areas for ship and submarine access when the base was open, their dredging was not able to get close to the wharf or under it, leaving the current legacy of contamination. In the health risk assessment conducted by the Navy, they used the Least Tern as an indicator species since they are the most sensitive birds to forage here, and protecting them would therefore protect all other birds. Fish consumption by humans was also used to determine that remediation was warranted. The investigation process that led to this dredging project began with sediment core samples collected in 2005 and 2006. About 4,000 cubic yards of mud will be dredged.
Alameda Point Environmental Report 