Toxic chemicals are typically generated inland and are carried by air and/or fresh water to estuaries where they accumulate in tributaries. Like other Chesapeake tributaries, the Severn has long been the recipient of various toxic wastes. Although recent environmental regulations have eliminated most of the classic industrial release of toxics, the cumulative contributions of smaller sources throughout the watershed continues. Increasing combustion of fossil fuels has created a major source of loading of toxic compounds from the atmosphere via rainfall and runoff.
Toxic compounds arriving in fresh water generally tend to be deposited in sediments soon after they reach estuaries, and those that break down slowly can accumulate there and build up to levels which impact organisms. Although the Severn watershed has not been the site of heavily polluting industries, the build-up of long-lived toxic compounds is a current threat to life in the river. Given the continued population growth and development of the watershed, the greatest long-term threat from toxics in the Severn undoubtedly comes from the continued untreated release of multiple chemicals in stormwater runoff.
Are man-made toxic compounds a current threat to life in the Severn River? Toxic compounds can be defined as those having a negative effect on living organisms, with negative effects including not only death and disease, but also diminished reproduction. Because many species live in an area like the Severn River, and their health and reproduction is affected by factors like food availability, salinity, eutrophication, cycles of predators and pathogenic organisms, it is often hard to know if man-made chemicals have compromised living organisms. One approach is to use chemical analysis of water, sediment, or biological tissue samples to detect compounds known to be toxic, e.g., dioxin or DDT. Another approach is to study the species composition of an area to see if it is depleted of species which are sensitive to potential pollutants, and enriched in species which are resistant to pollutants. A third approach is to remove a sample of water or sediment, place it in an aquarium with pollution- sensitive organisms, and compare the health of the test organisms with standard samples known to be clean and others known to be contaminated. The Severn has been studied by all three of these approaches, which were the basis for its recent characterization as an "area of emphasis" by Chesapeake Bay Program scientists. These studies and their further analysis by Severn River Commission scientists are described in more detail elsewhere.
Response: While the Severn River Commission is concerned about some of the toxics data suggesting possible negative impacts of toxics in the sediment, clear evidence for significant impacts is lacking. The Commission also deplores the lack of a monitoring program to assess whether the Severn's toxics problems may be worsening with time, but has not identified any means to implement such a program. It is also unclear if funds will become available to provide additional studies on toxics in fish and shellfish tissue, which have not been carried out for the Severn. Overall, however, the Commission believes that toxics represent less of an immediate threat to life in the Severn than do excess nutrients and sediment.