Coastal Sediment Quality

Contaminated sediments can pose an immediate threat to benthic organisms and an eventual threat to entire coastal ecosystems. Sediments can be resuspended by anthropogenic activities, storms, or other natural events; as a result, organisms in the water column can be exposed to contaminants, which may accumulate through the food web and eventually pose health risks to humans.

There are several ways to measure sediment quality. Sediments can be assessed in terms of their toxicity to specific organisms in bioassays, or in terms of the levels of contaminants that are present. Sediment quality also can be inferred by assessing the condition of benthic communities, which largely reflect the quality of the sediments in which they live (although other stressors may be reflected as well). To generate a more complete picture of sediment quality, scientists frequently use several of these measures together.

This indicator presents data on sediment toxicity and contaminant levels. The data are from probabilistic surveys conducted as part of EPA’s National Coastal Assessment (NCA) and presented in EPA’s fourth National Coastal Condition Report (U.S. EPA, 2012a). The survey was designed to provide a national picture of sediment quality by sampling sites in coastal waters throughout the United States, including Alaska, Hawaii, Puerto Rico, Guam, and the U.S. Virgin Islands. Each site was sampled during the summer months. Different regions were sampled during different years between 1999 and 2006. Data are sufficient to show differences in sediment condition over time for five regions (Northeast Coast, Southeast Coast, Gulf Coast, West Coast, and Hawaii).

Sediment toxicity is typically determined using bioassays that expose test organisms to sediments and evaluate their effects on the organisms’ survival. For this indicator, toxicity was determined using a 10-day static test on the benthic amphipod Ampelisca abdita, which is commonly used as a screening tool to identify sediments that pose sufficient concern to warrant further study. Sediments were classified as “potentially toxic” if the bioassays resulted in greater than 20 percent mortality compared to a control (a reference condition), or “not likely toxic” if the bioassays resulted in 20 percent mortality or less (U.S. EPA, 2012a).

Contaminant concentrations do not directly reflect toxicity because toxicity also depends on contaminants’ bioavailability, which is controlled by pH, particle size and type, organic content, and other factors (e.g., mercury vs. methylmercury). Contaminant concentrations are a useful screening tool for toxicity, however, when compared with concentrations known to cause particular effects on benthic life. For this indicator, sediment samples were homogenized and analyzed for nearly 100 contaminants, including 25 polycyclic aromatic hydrocarbons (PAHs), 22 polychlorinated biphenyls (PCBs), 25 pesticides, and 15 metals, using standard wet chemistry and mass spectroscopy. The observed concentrations were then compared with “effects range median” (ERM) values established through an extensive review of toxicity tests involving benthic organisms, mostly Ampelisca (Long et al., 1995). ERM values were available for 28 contaminants. For each contaminant, the ERM represents the concentration at which there is a 50 percent likelihood of adverse effects to an organism, based on experimental data. For this indicator, a site was rated “potentially toxic” if one or more contaminants exceeded an ERM value. In practice, about 25 percent of samples that exceed one ERM also cause greater than 20 percent mortality in the Ampelisca bioassay (Long, 2000).

Benthic community condition also can be a useful indication of sediment quality, particularly in terms of chronic or community effects that would not be captured in an acute exposure bioassay. The NCA evaluated coastal sites for several aspects of benthic community condition, and these results are presented as a separate ROE indicator (Coastal Benthic Communities).

More than 10 percent of the coastal sediments of the West Coast, the U.S. Virgin Islands, and the Gulf Coast in recent years were rated “potentially toxic” based on the Ampelisca toxicity screening assay during the years sampled (Exhibit 1). The Gulf Coast is the only region with a statistically significant increase in the percent of coastal sediments rated “potentially toxic” over time. The percent of coastal sediments rated “potentially toxic” in the Southeast coast peaked at 17 percent in 2003, but was much lower in all other years sampled. Ninety-nine and 100 percent of the coastal sediments in Southcentral and Southeastern Alaska, respectively, were rated “not likely toxic.”

In terms of the sediment contaminants index, most ecoregions saw the vast majority of their coastal area score “not likely toxic,” especially in recent years (Exhibit 2). The sediment contaminants index was “not likely toxic” in greater than 95 percent of coastal sediments for all years sampled in the Southeast Coast, West Coast, Alaska, Guam, and U.S. Virgin Islands. Contaminants were present at “potentially toxic” levels in more than 10 percent of coastal sediments in the Northeast Coast and the Gulf Coast in 2000 and 2001. These levels have decreased over time to less than 3 percent and zero, respectively. The trend for the Northeast Coast was statistically significant.

• Sediment toxicity has not been assessed in the coastal areas of Hawaii, American Samoa, and Guam. Contaminant concentrations are not available for American Samoa.
• Sample collection is limited to an index period during the summer. It is not likely that contaminant levels vary from season to season, however.
• The Ampelisca bioassay is a single-organism screening tool, and the ERMs are general screening guidelines based largely on toxicity data from Ampelisca. Thus, these measures do not necessarily reflect the extent to which sediments may be toxic to the full range of biota (including microbes and plants) that inhabit a particular sampling location.
• The Ampelisca bioassay tests only for short-term, not long-term, exposure. Both screening tests characterize sediments in terms of their effects on benthic organism mortality. This indicator does not capture other effects of sediment contaminants on benthic organisms, such as disease, stress, and reproductive effects.
• This indicator cannot be compared quantitatively with indicators that use other types of contaminant guidelines. For example, the Pesticides in Agricultural Streams indicator uses thresholds intended to be protective of aquatic life with a margin of safety, instead of thresholds shown to cause biological effects (e.g., ERMs). The ERM approach also is not directly comparable with other sediment contaminant approaches, such as EPA’s equilibrium partitioning (EqP) benchmarks.

This indicator is based on an analysis published in EPA’s fourth National Coastal Condition Report (U.S. EPA, 2012a). Southcentral Alaska data were presented in the third National Coastal Condition Report (U.S. EPA, 2008). The NCA data are housed in the National Coastal Database (U.S. EPA, 2012b) (https://www.epa.gov/national-aquatic-resource-surveys/data-national-aquatic-resource-surveys).