Transport of Contaminants

Research and monitoring have demonstrated how contaminants move into and through the estuary, becoming resuspended and moving with currents and tides. We know that:

• Water circulation and sediment resuspension, recycling and movement are key processes in the transport of contaminants.
  
  
• Natural estuarine processes - physical, chemical and biological mechanisms - and human activities such as dredging redistribute both recent and historical inputs of chemical contaminants.
  
  
• Environmental conditions such as anoxia in bottom waters and sediments affect the availability, transformation and degradation of potential contaminants. During anoxic events, reducing metals such as arsenic and manganese are generally released from sediment, while non-reducing metals such as cadmium, copper and zinc tend to be fixed in sediment. When water is oxygenated, the reverse occurs: arsenic and manganese are captured by the sediment, while copper, cadmium and zinc are released from the sediment.
  
  
• Biological processes, including uptake, metabolism and transformation, can transport contaminants through the estuary. Benthic organisms, for example, may take up toxic compounds from sediments, transform and concentrate them, thus incorporating them into food web. In short, toxic substances are not inert, and as a result of biological processes their form, toxicity and availability can change dramatically.
  
  
• Recently arrived contaminants are more geochemically and biologically reactive than are historical inputs, which are more likely to be bound up in mineral phases and therefore less biologically available.
  
  
• Fisheries and fisheries management, as critical components of Bay food web dynamics, can also play an important role in the cycling of contaminants. The widespread loss of oysters, for instance, has evidently reduced the biological filtering capacity of the Bay, which has important implications for the movement of contaminants through the estuary, and for the estuarine food web.
  
  
• Nutrient enrichment has resulted in shifts in primary production that favor algal blooms over submerged aquatic plants; these shifts represent a key change in fundamental processes of metabolism and transport of chemical contaminants.

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