Liquid sediment: behaviour of water gas process MGP tars in the aquatic environment

Gardiner Cross

Abstract

The same physical properties which made water gas tar a low value waste in the 1800s make it quite mobile today, both in surface water and groundwater. Subsurface migration of this tar is increasingly recognized as a major factor in site investigations. Transport by surface water can move significant quantities of tar over long distances, but has not been as widely reported in the literature. Three case histories from New York State’s MGP remedial program illustrate the problem:

At Plattsburgh, tar moves through the subsurface from a former lagoon, accumulating in pools on the bed of a vigorously flowing river. Tar balls are periodically swept away by the river, and can occasion-ally be observed bouncing along the riverbed. The tar is still a visible liquid over 1 km downstream, where it is redeposited in a delta. At Gloversville, small volumes of tar enter a stream in a reach with temporary sediment traps such as fallen logs and boulders. However, the principal accumulation is behind a bridge abutment 1 km downstream. At Oneida, the tar moves a few hundred m in a low-gradient drainage ditch. Once redeposited, it is still sufficiently liquid to penetrate up to 40 m laterally into the river bank, thus forming a secondary NAPL plume detached from its source.

Recognition of water gas tar’s behavior as a ‘liquid sediment’ is essential to properly characterizing this important class of hazardous waste site.

Key words: carbureted water gas, manufactured gas plant, PAH contamination, tar, sediment contamination