River dredging

Fluvially dispersed tadings - deposited on soil during flooding, river dredging, etc. Transported ore separates - blown from conveyance, etc. onto sod Ore processing — cyanides, range of metals... 

One of my first jobs out of engineering school was building river dredging equipment in South America. Because of the prohibitive cost of importing equipment, we reconfigured a lot of used and old equipment. Using a systems approach, coupled with hazard analyses, helped us to understand the implications of our contraptions on the human operator (our customers). It was easy and fast to apply. 

Rhee, G.-Y., B. Bush, M.P. Brown, M. Kane, and L. Shane. 1989. Anaerobic biodegradation of polychlorinated biphenyls in Hudson River sediments and dredged sediments in clay encapsulation. Water Res. 23 957-964. 

The major release of chlordecone to sediments, however, occurred indirectly as a result of waste water discharges, runoff of contaminated soil, and direct disposal of solid wastes at a production facility in Hopewell, Virginia. An estimated 10,000-30,000 kg (22,000-66,100 lb) of chlordecone is associated with bottom sediment in the James River estuary (Huggett and Bender 1980 Nichols 1990). This sediment serves as a reservoir for future release of chlordecone via resuspension of sediments resulting from storms or dredging activities (Lunsford et al. 1987). 

Tunsford CA, Weinstein MP, Scott L. 1987. Uptake of Kepone by the estuarine bivalve Rangia cuneata, during the dredging of contaminated sediments in the James River, VA. Water Research 21 411-416. 

If river cleanup is involved, including dredging, the first estimate of the costs likely to be incurred is on the order of 6.6 Million per kilometer. This includes dredging the bottoms, cleaning and excavating the sides, and thermally treating the sludges after they are dewatered. The sediments will be pumped out at about 2% solids and have to be put into a contained area, the liquid treated, and the solids dried and excavated. 

Sediments Erosion from farming, forestry, mining, and development river diversions coastal dredging and mining Reduces water clarity and changes bottom habitats clogs fish gills carries toxins and nutrients. 

Now the New York State Department of Environmental Conservahon and some environmental groups have advocated dredging the PCB-contaminated river bottom and transferring the PCB-containing sediment to a landhU site. Even though the cleanup costs, now estimated to run as high as 300 nuUion U.S. dollars, are acceptable to U.S. tax payers, a landfill site to receive the PCB-contaminated sediment shU cannot be found because of pubhc resistance [50]. 

Sediment samples were collected with a dredge-type sampler from a boat and also with the aid of a diver. One quart glass jars with aluminum foil-lined caps were used for the sediment samples after collection they were placed in a box containing dry ice. The composition of the river bottom sediments varied from coarse sand (>600m) in the center of the river to coarse and fine silt toward the banks. 

Sedimentation and siltation can have a number of harmful effects on the environment. For example, as sediments accumulate in a river, stream, harbor, hay, or other body of water, they may disrupt transportation on the water and limit its use for swimming, hshing, boating, and other recreational activities. In some bodies of water, dredging has become a virtually nonstop operation in order to keep them open to shipping and other commercial and recreational operations. The U.S. Army Corps of Engineers alone dredges more than 78 million cubic yards (60 million cubic meters) of sediment each year at an annual cost of nearly 200 million. 

Table 8 Calculated annual input of TEQ in the Dutch coastal zone from the rivers, atmospheric emission and disposal of dredged material (period 1999-2005). For the TEQ-lev-els in sediment, the ecotoxicological risk is estimated as % Potentially Affected Fraction of species (PAF) and calculated based on the SSD for chronic dioxin-toxicity (Figure 6). 

Sediments in the Mississippi River were accidentally contaminated with a low-level radioactive waste material that leaked from a nuclear power plant on the river. Pore water concentrations of radioactive compounds were measured following the spill and found to be 10 g/m over a 2-mm depth. The water contamination was 30% radioactive cesium ( Cs), with a half-life of 30 years, and 70% radioactive cobalt ( °Co), with a half-life of 6 years. Objections by the local residents are preventing clean-up efforts because some professor at the local state university convinced them that dredging the sediments and placing them in a disposal facility downstream would expose the residents to still more radioactivity. The state has decided that the sediments should be capped with 10 cm of clay and needs a quick estimate of the diffusion of radioactive material through the clay cap (Figure E2.8.1). If the drinking water limit (10 g/m ) is reached at mid-depth in the cap, the state will increase its thickness. Will this occur ... 

Metal reclamation of sediments uses many of the same approaches as for soils, except that sediment access is often more difficult. Once removed from the bottom of a lake or river, sediments can be treated and replaced, or landfilled in a hazardous waste containment site. The actual removal of sediments involves dredging. This can pose serious problems since dredging includes the excavation of sediments from benthic anaerobic conditions to more atmospheric oxidizing conditions. This can result in increased solubilization of metals, along with increased bioavailability (see Section 10.3) and potential toxicity, and increased risk of contaminant spreading (Moore, Ficklin Johns, 1988 Jorgensen, 1989 Moore, 1994). There are ongoing discussions as to whether it is more detrimental to remove sediments, whether for treatment or removal, or simply to leave them in place. 

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