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Chesapeake Bay News


Bay Program Updates Toxics of Concern List

The Bay Program Toxics Subcommittee has updated its list of Toxics of Concern, ranking the toxic organic chemicals in the Chesapeake Bay with the most potential for harm. PCBs topped the list, followed by PAHs and organophosphate pesticides. Organochlorine pesticides and five other organic toxics are also included in the list.

The 2006 Toxics of Concern list prioritizes organic chemicals based on:

  • Estimates of loads to the Bay
  • Presence in the Bay
  • Eco-toxicological properties
  • Any fish advisories or Clean Water Act 303(d) impairment designations in the Bay and its tidal tributaries due to these chemicals

The original Toxics of Concern list, which was completed in 1991, identified and documented chemicals that were adversely impacting or had the potential to impact the Bay. The list was subsequently refined in 1996 and 2000 prior to this latest update.

The 2006 Toxics of Concern list is based on the same chemical ranking system used for the 1996 list, incorporating chemicals' source, fate and effects of exposure. Also, like the 2000 list, fish consumption advisories and 303(d) impairments were considered for the 2006 revision.

Improvements in the 2006 chemical ranking system include:

  • Persistence, bioaccumulation and toxicity (PBT) adjusted loadings estimates.
  • Frequency of detection in the Bay's tidal rivers.
  • State management outcomes, including fish advisories and 303(d) impairments.

The Toxics of Concern list is used by the Bay Program Toxics Subcommittee to help develop strategies to address the most problematic toxic organics in the Bay and its tributaries. It is not a complete list of all chemicals that may impact the Bay or its watershed. Some organics could not be included due to data gaps. Also, metals, such as mercury, are not included in the list because assessment guidelines comparable to those used for organics are not currently available.


Although PCB manufacturing was banned in 1977, PCBs can build up in bottom sediments and persist for many years; therefore, historic discharges of PCBs can still affect the Bay today. Also, when old PCB-containing equipment that is still in use fails, PCBs can flow into the nearest stream or river via stormwater.


PAHs are formed when coal, gasoline and fuel oil are burned and are a major component of tar and asphalt. The most rapid increases of PAHs in river bottom sediments are found in watersheds with increasing development and motor vehicle traffic.

Organophosphate Pesticides, Organochlorine Pesticides

Organophosphate pesticides are mostly herbicides and insecticides used in agriculture. Organochlorine pesticides, such as DDT, are no longer widely used but persist in the environment.


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