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Chemical Contaminants

Based on the list of impaired and threatened waters that states submitted to the U.S. Environmental Protection Agency in 2012, 74 percent of the 92 tidal segments analyzed in the Chesapeake Bay were partially or fully impaired by chemical contaminants.

Chemical Contaminants

Chemical Contaminants (2012)

Map: Chemical Contaminants (2012)

Date created: Oct 18 2013 / Download

Tidal waters that are impaired for part or all of the indicated Bay segment by toxic chemicals based on each state's implementation of the Clean Water Act.

July 22, 2011

Fish biologist Vicki Blazer with the U.S. Geological Survey (USGS) brings her team to the Shenandoah River in Front Royal, Va., to collect and study smallmouth bass, a species in which intersex characteristics have been linked to chemical contaminants in the Chesapeake Bay region.

Intersex conditions occur when exposure to chemicals disrupts the hormonal systems of an animal, leading to the presence of both male and female characteristics in an animal that should exhibit the characteristics of just one sex in its lifetime. In the case of smallmouth bass, male intersex fish are found with immature eggs in their testes, which indicates exposure to estrogenic and anti-androgenic chemicals.

“The sources of estrogenic chemicals are most likely complex mixtures from both agricultural sources, such as animal wastes, pesticides and herbicides, and human sources from waste water treatment plant effluent and other sewage discharges,” says Blazer, who first discovered intersex characteristics in fish while studying fish kills in the South Branch of the Potomac River and the Shenandoah River.


Pesticides, pharmaceuticals, polychlorinated biphenyls (PCBs) and other chemical contaminants can be found in the sediment and tissues of fish in the Chesapeake Bay. These contaminants—also known as toxics—can harm human health and affect the survival, growth and reproduction of fish and wildlife.

Toxics enter the food web when small, bottom-dwelling organisms take them up while feeding or through direct skin contact. Contaminated organisms pass toxics on to the larger fish, birds and people that eat them. By analyzing the tissues of certain fish species, scientists can estimate the overall presence of toxics in the ecosystem.

While toxics are often seen as a local problem occurring in “hot spots” or “regions of concern” (like Baltimore Harbor or the Anacostia and Elizabeth rivers), this indicator shows that chemical contamination can pose a problem outside of these local areas.


In 2014, the Chesapeake Bay Program adopted a goal to ensure the Chesapeake Bay and its rivers are free of the effects of toxic contaminants on living resources and human health.

Short-term trend (2006-2012)

A comparison of the 2006, 2008, 2010 and 2012 303(d) assessments by Maryland, Virginia and the District of Columbia shows an increase in the portion of the Chesapeake Bay impaired by chemical contaminants. In 2006, 66.3 percent of the tidal segments analyzed were partially or fully impaired. By 2012, this number increased to 73.9 percent.

Change from previous reporting year (2010-2012)

Between 2010 and 2012, the portion of the Bay partially or fully impaired by chemical contaminants rose from 72.2 percent of segments analyzed to 73.9 percent of segments analyzed.

Between the two reporting years, the number of segments analyzed rose from 89 to 92. This better aligned the data’s segmentation with that used in the Bay’s “pollution diet,” or Total Maximum Daily Load (TMDL). The additional segments add more detail and may more accurately capture the extent of tidal impairments. But they were not listed as impaired, and may have slightly deflated 2012’s percentage relative to previous years. The increase in impaired segments that took place between 2010 and 2012—from 65 to 68—are the result of revisions in listing geography rather than new data.

While our partners are working to reduce the impacts of toxics on our environment, there may be little positive change in the short term. Even if fewer toxics are delivered to rivers and streams, many impaired waterways have persistent problems with the accumulation of PCBs and other toxics in fish tissue. The presence of these toxics in affected fish will not decrease quickly.

Additional Information

Collecting data

This data is based on the 303(d) assessments—or lists of impaired and threatened waters—that Maryland, Virginia and the District of Columbia submit to the U.S. Environmental Protection Agency every other year.

Virginia and Maryland use different contaminant concentration thresholds and different fish species to determine whether a body of water passes the “fishable” designation of use. For this reason, listing variation can occur between adjacent waters. For example, the Middle Chesapeake Bay segment covers portions of both Maryland and Virginia. Virginia has assigned this segment “impaired” due to PCBs, but Maryland has not. Therefore, the segment is considered “partially impaired” and is included in the overall count of PCB listings in major tidal waters.

Detailed information about the extent and severity of toxics in the watershed can be found in a technical report released in December 2012 by the EPA.

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