Toxic Contaminants

How much toxic contamination is in shellfish tissue and how has it changed over time?

Most concentrations of measured metals and organic chemicals in blue mussel tissue from
1991-2016 are declining or not changing. Mercury and PCB levels remain high enough to merit continued concern. Many new contaminants have been introduced to the estuary, such as pharmaceuticals, perfluorinated compounds and brominated flame retardants, and they are not being consistently monitored.


Toxic and persistent contaminants such as PCBs (polychlorinated biphenyls), mercury, and DDT (dichlorodiphenyltrichloroethane) can accumulate in the tissue of filter-feeding mussels, clams, oysters and other marine biota and seafood. Tracking contamination in mussel tissue offers insight into changes in contaminant levels in our estuarine and coastal ecosystems.
Zero percent of sampling stations in the estuary have shellfish tissue concentrations that exceed levels of concern and no increasing trends for any contaminants.
The Gulfwatch Program uses blue mussels (Mytilus edulis) to better understand trends in the accumulation of toxic and persistent contaminants, including metals, pesticides, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs). The use of many of these contaminants has been banned or is limited, so trends are expected to be stable or decreasing. At Dover Point, concentrations of DDT, an insecticide banned in the U.S. in 1972, are relatively low and gradually decreasing (Figure 13.1). Inputs of mercury, a heavy metal, have been reduced since the 1990s due to regulatory action taken on coal-fired power plants, medical waste and municipal incinerators, but mercury continues to be deposited through wet and dry atmospheric deposition.48 At most sites, including Clark’s Cove in Portsmouth Harbor, mercury levels in shellfish have been fairly stable since 2003 (Figure 13.2), these levels are similar to those seen in other estuaries located close to urban centers.49 PAHs, which mostly come from oils spills, the burning of fossil fuels and some driveway sealants, have been stable across all stations, including Hampton-Seabrook. Only one value was above the national median level of 250 ug/kg (Figure 13.3). Other data collected at that time indicate a possible fuel spill.50 Trend lines are not shown as there were no statistically significant results.

PCBs, DDT and mercury at these three stations—Dover Point, Clark’s Cove and Hampton-Seabrook—are generally representative of the trends in the more comprehensive dataset, which includes over 120 different specific contaminants. Focusing only on these three contaminants, however, does not provide a comprehensive picture of the level of toxic contamination in our estuaries. Many new contaminants have been introduced to the estuary, such as pharmaceuticals, perfluorinated compounds and brominated flame retardants, and they are not being consistently monitored.

Figure 13.1 Concentrations of DDT in mussel tissue at Dover Point. The most recent national median for the Mussel Watch program was 30ug/kg.51 The 85th percentile was 130ug/kg.

Figure 13.2 Concentrations of mercury in mussel tissue at Clark’s Cove, Portsmouth Harbor. The most recent national median for the Mussel Watch program was 0.7mg/kg51. The 85th percentile was 0.13mg/kg.

Figure 13.3 Concentration of PAHs at Hampton-Seabrook Harbor. In 2008, the national median for the Mussel Watch program was 250 ug/kg.51 The 85th percentile was 1250 ug/kg.