Polluted Spoil : Disposal of polluted spoil, material containing high 

 concentrations of heavy metals, pesticides or other contaminants susceptible 

 to resuspension when dredged, poses a significant threat to estuarine areas. 

 Consequently, bottom materials to be dredged should be analyzed for the 

 type and content of pollutants. If spoils are found to be polluted they 

 must be handled as prescribed by pollution control agencies. Several 

 methods of treating spoil to remove contaminants and to improve the quality 

 of effluent from dewatering operations at disposal sites are available. 

 Treatment methods include flocculation, filtration, aeration, incineration, 

 chemical processes, and sewage plant treatment. These are all very costly. 



Mitigation : Creation of habitat through disposal of dredge material 

 offers a constructive use of some dredge spoil material. For example, 

 islands of spoil can absorb large volumes of materials and become desirable 

 habitats for shorebirds or utilized to create marshes. If clean, coarse 

 dredge spoil is available (such as coarse sand), islands may become 

 productive intertidal flats [39]. Spoil placement and its effects on 

 hydrology of the system, island erosion and other factors must be 

 evaluated. 



Combined Effects : The Mississippi River-Gulf Outlet in coastal 

 Louisiana serves as an example of the combined effects from dredging of 

 navigation channels. Construction of the channel destroyed 23,606 acres 

 of marsh and shallow nursery areas of aquatic life--17,058 acres by 

 spoil deposition and 6,548 acres by deepening. It has greatly altered 

 the hydrology and water chemistry of the adjacent estuarine areas. The 

 large cross-section of area provides an avenue of ingress and egress for 

 runoff and tidal waters. Saltwater intrusion is a major problem. Changes 

 in salinity are well -documented. Recording stations in the vicinity of 

 the channel show significant changes (Figure 11) after the channel was 

 completed in 1962 [42]. 



Gagliano has summarized negative ecological effects of canal dredging 

 in the Gulf coast region as follows [42]: 



Direct loss of productive habitat through dredging; 



Direct loss and/or reduction in habitat quality through spoil 



disposal ; 

 Saltwater intrusion causing faunal and floral changes; 

 Increased storm-generated surge; 

 Accelerated erosion resulting from increased length of land-water 



interface; 

 Increase in runoff rate resulting in loss of freshwater storage; 

 Modification of runoff pattern, often creating fresh water 



deficient areas; 

 Accelerated erosion resulting from increase in tidal prism volume; 

 Changes in circulation patterns in bays and sounds; 

 Accelerated erosion resulting from boat-generated wash; 

 Accelerated erosion along unstable canal banks; 

 Alteration and/or disruption of longshore drift of sand; 



94 



