All of the sediment cleanup techniques currently under consideration for the Lower Passaic include excavation dredging. This means using a crane to lower a clamshell-shaped bucket through the water column to the river bottom. When the bucket reaches the bottom, it closes and brings the dredged material up through the water to the surface and is then placed on a barge.
The amount of accumulated debris at the Lower Passaic river bottom makes this kind of dredging the only viable alternative here. Any attempts to use hydraulic dredging, where sediment is pumped up from the river bottom through a large tube, would quickly fail by becoming clogged with debris. There are at least 20 cars, pieces of old barges, trash and other material that need to be physically removed during the dredging process.
Even using the best technology available, dredging is a messy business as some of the contaminated material that is picked up by the dredge bucket will leak out into the river. Therefore, it is best to confine dredging efforts to the areas of highest contamination, while allowing areas of lower contamination to recovery naturally – as they have been in recent years.
Dredging in the Lower Passaic poses some special challenges. There are 17 bridges in this stretch of the River and numerous communities along the river banks that will be affected by any dredging project. Low clearances under many of these bridges mean either that they will have to be opened (potentially causing traffic problems) or smaller barges will need to be used (slowing the pace of work). There is also the issue of processing the dredged materials. Once they are removed from the River they must be drained of excess water. This happens at a processing facility somewhere along the River. At this time, there is no approved facility that can neutralize the remaining material, so it will have to be shipped to another location through this heavily urbanized area.
Because of these challenges, it is likely that some of the cleanup plans being discussed, that involve removing millions of tons of sediment from the river bottom, would take 15 years or more to complete. Another reason to consider targeted dredging in this case.