There are in various parts of the world ponds or former ponds which have been completely filled by hazardous chemical wastes discharged by nearby industrial plants. Often these ponds are adjacent to or actually communicate with waterways and an especially egregious example of such ponds are the so-called tar ponds of Sydney, Nova Scotia which have received for over 80 years the toxic discharge from coke ovens of an adjacent steel mill. The sediment in these ponds is thixotropic by nature and thick enough to sustain the weight of a man walking on the surface. Removal of this material by surface diggers and trucks is undesirable not only because of the spongy nature of the terrain but such digging can release into the air carcinogous matter existing in some ponds, e.g. the Sydney ponds, and known as polynuclear aromatic hydrocarbons (PAH's). The presence of an adjacent waterway suggests dredging as a solution but a cutter head dredge is not acceptable not only because of the dense nature of the sediment but because a cutter head dredge would cause excessive turbidity thereby contaminating the adjacent waterway for possibly miles beyond the dredging site. An auger type dredge is therefore suggested and one such dredge intended for substantially the same purpose as the present invention is disclosed in the U.S. patent to Blackburn et al. U.S. Pat. No. 4,312,762. There a longitudinally extending auger is thrust directly into the sediment to transfer the sediment axially to and through a dredge pipe for disposal at a remote site. A problem with this arrangement is that the exposed rotating auger head is bound to create turbidity in the surrounding water and though this may be acceptable in a land-locked pond it could be disastrous if the sediment contained flowable toxic matter and the pond had open access to a substantially uncontaminated waterway, or the sediment contained gaseous poisons such as PAH.