83 



the Gulf Stream near Cape Hatteras. The current will move waste, as it is being 

 diluted, at a rate of about 5 nautical miles per day. Along the line of current flow, 

 the closer of the indicated alternative sites to the 106-mile site is 30 nm (site center 

 to site center); the more distant site is 80 nm away. Water and waste at the closer 

 and more distant site would require 6 and 15 days, respectively, to reach the 106- 

 mile site. In either case, this would be sufficient time to dilute sludge by a factor of 

 one million. However, on average, the additional travel time from the farther site 

 would yield significantly greater dilution of sludge. As discussed previously, a ten- 

 fold increase in dilution requires a tenfold increase in time. If 4 days are required 

 for a million-fold dilution, 40 days would be needed to achieve a ten-million-fold di- 

 lution. On that time scale entrainment into the Gulf Stream will occur regardless of 

 which of the three sites is used. 



For these reasons we do not see a compelling need to use either alternative site 

 even though we have no technical objection to their use. If the objective is to 

 achieve a million-fold dilution of sludge before it even enters the 106-mile site, the 

 further of the two site is slightly preferable. Another option that could be consid- 

 ered is for sewage sludge to be dumped instead at a site south of the 106-mile site. 

 On average, water from that site would flow away from the 106-mile site and the 

 106-mile site could receive industrial waste in waters generally free of sludge. 



Background Information on the Dilution of Sewage Sludge in the Marine 



Environment 



Dilution of neutrally buoyant, primarily liquid wastes (sewage sludge and most in- 

 dustrial wastes) once discharged to the marine environment occurs in three phases. 

 The first phase is referred to as initial mixing and is due to the discharge process. 

 The second phase is slow dilution under oceanic conditions due to low level, oceanic 

 turbulence. The third phase, also due to oceanic processes, is rapid dilution under 

 the influence of storms. 



Dumping liquid wastes from barges initially dilutes the wEistes by a factor directly 

 proportional to the distance the barge travels per unit of waste discharged. Common 

 practice is to discharge industrial wastes at deep ocean sites at a rate of about 80 

 m^ of waste per km of dumping track. Typically, barges carry about one million gal- 

 lons (3785 m^) and discharge it over 25 nautical miles (46 km). Dumping at that rate 

 achieves initial dilution factors of about 5000. This initial dilution is higher than is 

 achievable with discharge pipes (basically because pipes do not move) and is higher 

 than sludge receives now in the New York Bight (where discharge is more than 80 

 m^ per km). 



The extent of initial dilution is controllable. If, for example, one million gallons 

 were dumped over 2.5 miles the initial dilution factor would be 500, or if the dump- 

 ing distance was 250 miles the initial dilution factor would be 50,000. However, it is 

 impractical to dump over 250 miles because — at a typical vessel speed of 5 knots — it 

 would require 50 hours (two days) to release a single load of waste. Initial dilutions 

 of one million are for all practical purposes impossible; this would require dumping 

 one million gallons over a distance of 5000 miles. 



Initial dilution results in the creation of long, thin ribbons of waste in the ocean. 

 Remotely-sensed images of waste plumes show these ribbon-like structures of freshly 

 dumped wastes. Oceanic mixing under calm conditions forces these ribbons to widen 

 at a relatively slow rate, so that about 10 hours are required for initial dilution to 

 be increeised by a factor of 10 (e.g. increasing an initial dilution factor of 5000 to a 

 factor of 50000). Under calm conditions, a further 10-fold dilution would require 

 about 4 days. However, very rapid dilution of wastes does occur under the influence 

 of storms. This has the effect of tearing the ribbons into small pieces, each of which 

 is mixed into the surrounding seawater more quickly than the larger ribbon could 

 be mixed. As as point of reference, one can expect wastes to be diluted by a factor of 

 about 100 times beyond the initial dilution within a week or less of their being 

 dumped. Therefore, if the initial dilution factor achieved by the dumping rate is 

 5000, that waste will have been diluted approximately one million times in about 

 one week. 



A limitation to the achievable dilution is the availability of water. It has been 

 estimated that at the 106-mile site flow of water available for diluting wastes is 

 10 ^°m^ or about 2.5 trillion gallons per day. This estimate of water renewal rate is 

 low because it is derived from the average current speed and distance across the 



