Ports & Dredging

The Problem

• Ports must be dredged at the time of construction, expansion or deepening, and periodically for maintenance. The dredged sediments are most commonly dumped in nearby marine waters, but they are often severely contaminated with a variety of harmful chemicals.
• Sediments in port areas become contaminated from port activities, the surrounding urban development, and sources upstream and upwind. Over time these contaminants build up to concentrations that degrade the marine environment around the port and make the sediments unsuitable for dumping in other marine environments.
• In the US, between 2 and 3 hundred million cubic yards of sediments are typically dredged each year; this is down from a high of more than 6 hundred million cubic yards in the late 60s. Some of the reduction in volume is due to difficulties and delays in getting permits for disposal as the dumping of contaminated sediments into marine environments is prohibited
• Determining whether dredged sediments are legally too contaminated to dump is a controversial process. The Army Corps of Engineers considers 5-10% of all dredged materials to be contaminated; however, they only analyze sediments if they think they have reason to believe the sediments might be contaminated. For the permitting process one set of procedures involving chemical analyses and biological lab tests is used. However, different methods of assessment using field data and lab tests as conducted by the EPA and NOAA suggest that a much higher percentage of sediments in port areas are likely to be contaminated.
• If it is demonstrated that contamination of the sediments is too great to legally permit open water dumping, viable alternatives should be found. Nevertheless, a great deal of contaminated sediment is being dumped at designated sites in open waters. Sometimes cleaner sediments are dumped on top but the effectiveness of this mitigation (called capping) is limited. Some ports have opted to construct confined marine disposal sites -- shallow water disposal areas that are more or less walled off from the surrounding marine environment. Other proposed methods include the decontamination of sediments hy biological or chemical treatment and destruction by incineration in specially constructed and regulated incinerators on shore. The effectiveness of each of these methods varies considerably, depending on the contaminants and where the sediments lie.

The Causes

• 95% of all US foreign trade is waterborne and passes through US ports. The design trend of cargo ships is toward ever bigger and deeper-draft mega-vessels (called "very large capacity carriers"), so that most ports are compelled to dredge ever deeper in order to accommodate new generations of ships. About 75% of port dredging in the US is for maintenance, while the remainder is related to new construction, riotably expansion or deepening.
• Ports are most often located in embayments at the outflow of one or more rivers which carry sediment loads and dissolved contaminants that end up on the bottoms of bays. The amount of erosion upstream is multiplied -- often many times beyond natural quantities -- by human activities such as agriculture, forestry, and construction: so sediments may accumulate at an abnormally rapid rate in the placid waters of embayments at the river mouths. Significant repeated dredging is thus required to maintain ports.
• Upstream contamination originates from ongoing sewage and industrial discharge, agricultural and urban runoff as well as historical spills into port areas via rivers. Air emissions from automobiles and industry, and even from the ships themselves, are additional sources of pollutants that settle on the water and make their way into the sediments. Finally, runoff and discharges from the surrounding urban and industrial buildup that invariably develop around ports add to water and sediment contamination
• Port-associated activities cause some of the contamination: highly toxic antifoulants on ship hulls leach into the water; port industries and shipyards use toxic materials that wash into the water; contaminated bilge and ballast waters are flushed from ships and boats; and cargo handling accidents and spills are consistently sources of sediment contamination.

The Context

• The disposal of dredged materials is addressed by the Ocean Dumping Act, which prohibits them from being dumped in ocean waters if it is determined that they will cause environmental degradation. To be acceptable for dumping into the marine environment, sediments must not contain any of several chemicals prohibited by national and international laws in more than 'trace' amounts. However, the term 'trace' is ill defined. Even if they are not contaminated with any of these prohibited substances; they must pass biological laboratory assessments to determine whether these are likely to adversely affect marine life. There is a great deal of controversy over whether the law is adequately enforced.
• The prevention of contamination in sediments is addressed indirectly by industrial discharge limits under the Clean Water Act, air emission limits under the Clean Air Act, and various government programs to reduce polluted runoff. Source reduction, then, is a principal element of long term strategies for the reduction of contamination in port sediments. Decontamination technologies remain in the developmental stage.
• The issues surrounding port dredging are exacerbated by the absence of a national planning system which could allocate appropriate types of ships (i.e. size and draft) to each port, based on environmental considerations and dredging needs. Because ports are competitive businesses, they vie for ship traffic instead of cooperating to distribute it according to harbor and environmental capacities.

Further Reading

Environmental Protection Agency. 1996. The National Sediment Quality Survey: A report to Congress on the extent and severiy of sediment contamination in surface waters of the United States. EPA-823-D-96-002. EPA Office of Science and Technology, Washington, DC.

National Research Council. 1997, Contaminated Scdimcnts in Ports and Waterways Clcanup Strategics and Tcchnologics. National Academy of Sciences, Washington, DC.

National Oceanic and Atmospheric Administration. 1998. Sediment Toxiciy in US Coastal Waters.