All of the evidence to date indicates that sludge material reached the bottom in significant amounts slightly west of the rectangular surface discharge area and that it had significant effects on benthic metabolism, megafaunal diet and infaunal species composition. The presence of sludge in sediments at LEO-2500 has been independently confirmed by several different investigators. In 1989 and 1990, Bothner et al. (1994) found that silver levels were significantly elevated at the dumpsite relative to an unaffected reference area.
Takada (1994) reported elevated levels of linear alkylbenzenes (LABs) and coprostanols during the same period. Hill et al. (1993 and submitted) reported significant elevation in the bacterium Clostridium prefringens at the dumpsite in 1990 relative to reference areas. Clearly, at this time, there was a strong sludge signal in the sediments within the dumpsite region. This impact was also reflected in biological samples from the area. Van Dover et al. (1992) showed that several megafaunal taxa (the sea urchin Echinus affinus and the holothurian Benthodytes sanguinolenta) incorporated sludge-derived material in their diet, resulting in substantially different stable isotope ratios in their tissues. Grassle (1991) found elevated macrofaunal densities at the dumpsite compared with control areas, including increases in the polychaetes Notomastus latercius, Notomastus sp. 2 and Tharyx kirkegaardi and the isopod Oecidobranchum plebejum. All of this data indicates that the sludge deposition area had been clearly impacted at the height of sludge dumping activity around 1990.
In late 1991 and early 1992, however, sludge dumping activity was curtailed and when we returned to the area in late summer 1992, cessation of dumping had resulted in measureable improvements within the dumpsite. Silver levels in fluff sediment samples at the dumpsite were lower than in 1990 and 1991, and in fact continued to decline in 1993 samples (Bothner, pers. comm.). Although oxygen penetration depth was still reduced and oxygen consumption elevated at the dumpsite relative to reference areas in 1992, the effect had largely abated by 1993 (Sayles et al., in press). Lamoureux et al. (in press) found that although PCBs and PAHs were still noticeably elevated in 1992, levels of LABs were substantially reduced relative to earlier samples at the dumpsite. Van Dover (pers. comm.) has found that isotope levels in megafauna collected in 1992 were shifting back towards reference levels, and infaunal data indicate that total densities, as well as densities of the species listed above, are no longer markedly different from reference sites (Grassle, Snelgrove and Petrecca, in prep.). Thus, the effects of sludge dumping appeared to be abating in 1992, at least in the vicinity of the dumpsite. At present, it appears that the situation in the dumpsite region is improving.
Unfortunately, there is an additional chapter to the story. Preliminary samples from 50 nautical miles south of the dumpsite indicate that levels of silver have increased from 1992 to 1993 and that oxygen metabolism in this site remained elevated in 1993 even after levels at the dumpsite had declined toward reference levels (Sayles et al., in press). Preliminary infaunal data also suggest that densities at the 50-nm site have risen since 1990(Grassle, Snelgrove and Petrecca, in prep.). Thus, the recovery at DWD-106 may be accompanied by a deterioration of habitats to the south, as a consequence of transport of materials in a southerly direction during periods of resuspension.
Because ALVIN was fully booked with NSF-funded programs, we were unable to return in 1995 despite a recommendation for support from the MAB Center. Based on the results described above, we feel it would be extremely desirable to return to the region in 1996. Our evaluation of recovery of the dumpsite per se is still incomplete; during our most recent visit, there were still elevated levels of some variables. An additional set of data from 1996 would greatly improve our interpretation of the data collected in 1992 and 1993. In addition, it is very important to evaluate the preliminary evidence that material may simply be shifting to the south. Dilution of material during sinking and stability on the bottom once it arrives are two major arguments in favor of deep-ocean dumping. Given that our data indicate that dilution does not completely abate sludge dumping effects, it is now important to evaluate the mobility and impact of material after dumping has terminated. The sludge-dumping that occurred from 1986 to 1992 created a unique opportunity for deep-sea scientists to evaluate the impact of sludge disposal over the deep ocean. Now that dumping activity has ceased, another unique opportunity exists; to evaluate the long-term effects of dumping and the recovery of the deep-sea ecosystem.
Fry, V.A., and B. Butman. 1991. Estimates of the seafloor area impacted by sewage sludge dumped at the 106-mile site in the Mid-Atlantic Bight. Mar. Environ. Res. 31:145-160.
Grassle, J.F. 1991. Affects of sewage sludge on deep-sea communities (abstract). EOS, Trans. Amer. Geophys. Union 72, p. 84.
Hill, R.T., I. Knight, M. Anikis, W.L. Straube, and R.R. Colwell. 1993. Benthic distribution of sludge indicated by Clostridium perfringens spores at a deep-ocean dumps site. Appl. Environ. Eng. 59, 47-51.
Hill, R.T., W.L. Straube, A.C. Palmisano, S.L. Gibson, and R.R. Colwell. Submitted. Distribution of sewage indicated by Clostridium perfringens at a deep-water dumpsite after cessation of sewage disposal. Appl. Environmental Microbiol.
Lamoureux, E.R., B.J. Brownawell, and M.H. Bothner. In press. Linear alkylbenzenes as tracers of sewage sludge derived inputs of organic matter, PCBs and PAHs to sediments at the 106-Mile Deep Water Disposal Site 106. J. Mar. Environ. Engr.
Sayles, F.L., S.P. Smith and J.E. Goudreau. In press. Deep ocean sludge disposal, sediment oxygen consumption and sediment redox profiles at Deep Water Municipal Dump Site 106. J. Mar. Environ. Eng.
Takada, H., J.W. Farrington, M.H. Bothner, C.G. Johnson and B.W. Tripp. 1994. Transport of sludge-derived organic pollutants to deep-sea sediments at Deep Water Dump Site. Environ. Sci. Tech. 28, 1062-1072.
Van Dover, C.L., J.F. Grassle, B. Fry, R.H. Garritt, and V.R. Starczak. 1992. Stable isotopic evidence for entry of sewage-derived organic material into a deep-sea food web. Nature 360, 153-156.