The effects of suspensions of uncontaminated natural sediments do not seem to 

 differ significantly from those of inert clay minerals. Experiments with con- 

 taminated natural sediments indicate a much greater potential for adverse 

 impact than would be associated with uncontaminated sediment. Sensitive spe- 

 cies are killed more easily at warmer temperatures or if dissolved oxygen is 

 reduced." 



Turbidity has the greatest potential for damage in soft freshwater where 

 it is extremely persistent. Hard water (200 mg/1 or greater of total dissolv- 

 ed solids) and saltwater induce flocculation and consequent rapid settling 

 (Wechsler and Cogley 1977). Later resuspension of dredged material can occur 

 and cause slight to moderate turbidity problems (Vitter 1972, National Marine 

 Fisheries Service 1976). 



Synergistic and antagonistic effects of suspended particles, toxicants, 

 dissolved oxygen, and other constituents of dredged material and the receiving 

 waters complicate the evaluation of impacts. 



The following discussion of techniques of controlling dispersion of 

 dredged material at open-water disposal operations is summarized from the re- 

 port of Barnard (1978). Normally about 1% to 3% of the material discharged is 

 suspended in the water column. The rest of the slurry descends rapidly to the 

 bottom where it may remain in a mound or it may become fluid mud and move in a 

 lateral direction or downslope. According to Barnard (1978: 3), laboratory 

 studies (but not actual field observations) indicate water-column turbidity 

 can be controlled to a great extent by using different discharge configura- 

 tions. "The simple open-ended pipeline, discharging above and parallel to the 

 surface, will maximize the dispersion of the slurry throughout the water col- 

 umn and produce a thin, but widespread fluid mud layer. In water depths in 

 excess of 2 m, (6.5 ft) the dispersion of the material in the water column can 

 be decreased by vertically discharging the slurry through a 90-degree elbow at 

 a depth of 0.5 to 1 m (1.5 to 3 ft) below the water surface. Most water-column 

 turbidity can be eliminated by using a submerged diffuser system at the end of 

 the pipeline. This latter discharge configuration also maximizes the mounding 

 tendency of the fluid mud d'^edged material , thereby minimizing its areal cover- 

 age over the disposal area-'. 



Silt curtains can sometimes be used to control near-surface turbidity but 

 not fluid mud (Figure 3). Turbidity levels in the water column outside the 

 curtain, under certain conditions, can be as much as 80% to 90% lower than 

 levels inside or upstream from the curtain (J.B.F. Scientific Corp. 1978). 

 However, silt curtains are only effective in quiet waters. Effectiveness drops 

 rapidly as currents, waves, and tides increase. Use of silt curtains is not 

 recommended where current velocities exceed 50 cm/sec or about 1 knot (Barnard 

 1978). 



Release of toxicants: Several studies indicate that (with some excep- 

 tions) there is not a significant release of potential toxicants, e.g., oils 

 and greases, pesticides, PCBs and heavy metals, into the water column during 

 the discharge of dredged material (May 1973b, Fulk et al . 1975, Chen et al. 

 1976, Lee et al . 1977, Schroeder et al . 1977). The common exceptions are 

 ammonia, phosphorous, manganese, and iron. Burks and Engler (1978) have sum- 

 marized DMRP laboratory investigations of releases of contaminants to the 



26 



