Between 1986 and 1992, approximately 8 million tons of sewage sludge were discharged annually at the surface in water depths of 2500 meters. Model calculations and field measurements were undertaken to determine the fate of this material on the seafloor.
The U.S. Geological Survey (USGS) has been a major participant in a multidisciplinary and multi-institutional program designed to determine the distribution of sludge on the seafloor, the level of contamination in bottom sediments, and its effects on the benthic ecology. The program utilized the latest technology for mapping and sampling the seafloor. The 3-man submarine ALVIN and the unmanned robot JASON have provided direct observation of the seafloor for sample site selection. The majority of fundin g for this study is through the New York Bight National Undersea Research Center of the National Oceanic and Atmospheric Administration (NOAA).
USGS scientists used the most recent information on ocean currents and the settling velocity of sewage sludge particles to predict the distribution and amount of sewage sludge deposition on the seafloor.
The mathematical model predicted deposition of the most rapidly falling sludge, about 23%, on the seafloor to the southwest of the dumpsite with a flux of 60 mg/m2/day along the western edge of the dumpsite and a flux of 1 mg/m2/day 350 kilometers to the southwest. The remainder would be carried by the water column to even greater distances. Little or no deposition of sludge particles was predicted to occur on the continental shelf.
USGS researchers use bathymetric, sidescan sonar, and seismic reflection surveys to characterize the morphology and geology of the seafloor and to provide new interpretations about the processes that transport sediment in this area.
These surveys identify specific locations for sampling with the submersible ALVIN and coring from surface ships. They also indicate that major landslides and other mass movement events have occurred in the dumpsite region in the past. Large boulders of Eocene chalk (4 meters in diameter) observed 20 kilometers down slope of their outcrop location support this interpretation. Local depressions identified by the surveys have been specifically targeted for sampling because they may preferentially accumulate sludge.
The concentration of silver in sediments and in suspended matter is a sensitive tracer of sludge deposition.
Because of its use in photography, the concentration of silver in sewage sludge is typically 150 times higher than in deep sea sediments. The concentration of silver in surface sediment near the dumpsite is 20 times higher than background, and the flux of silver, as determined using sediment traps, is in agreement with the particle deposition model. The distribution of silver is similar to that of other indicators of sewage such as linear alkylbenzenes (a component of detergent) and Clostridium perfringens (a bacterium spore characteristic of sewage). These other indicators have been studied by co-investigators from the Woods Hole Oceanographic Institution and the University of Maryland.
Sediment mixing by benthic organisms and resuspension by bottom currents are two processes which dilute and disperse contaminants that initially settle on the seafloor.
Contaminants introduced at the sediment surface since 1986 have penetrated to a depth of 5 centimeters below the seafloor as a result of rapid vertical mixing by organisms living in the sediments. This mixing dilutes material added to the sediment surface, possibly an environmental advantage. However, it also transports contaminants below the level of resuspension thereby increasing the potential for long-term accumulation. During a 10-month instrument deployment in the dumpsite at 2400-meter water depth, bottom currents were strong enough to resuspend sediments on seven occasions. Currents suggest transport of resuspended sediments to the southwest.
The increased flux of sludge-derived organic matter has caused measurable changes in the benthic ecology near the dumpsite.
Dr. J.F. Grassle of Rutgers University, and co-investigators for biological studies have measured differences in areas impacted by the sludge accumulation compared to control areas including: 1) two additional "opportunistic" species of benthic polychaete worms; 2) a tenfold increase in abundance of urchins, starfish, and sea cucumbers (organisms which feed on the sediment surface); and 3) ingestion of sludge-derived organic matter by sea urchins, based on uptake of sulfur isotopes. The cessation of sludge disposal at the 106-Mile Dumpsite in July, 1992, provides a unique and valuable opportunity to examine the long-term dispersal and effects of waste material in the deep-sea environment.