Chlordane,3 DDT,4 PCBs,5 mercury, and lead are present in streambed sediment and aquatic tissues in the Potomac River Basin (fig. 22). (Note: Chlordane concentrations presented are the sum of trans-chlordane, cis-chlordane, cis-nonachlor, trans-nonachlor, and oxychlordane; DDT concentrations presented are the sum of o,p'-DDT, p,p'-DDT, o,p'-DDD, p,p'-DDD, o,p'-DDE, p,p'-DDE; PCB concentrations presented are the sum of all PCB congeners.) Each of these compounds or metals can directly impair aquatic organisms living in or near the streambed and can be detrimental to the health of humans or other animals through the food chain. Lead, mercury, chlordane, and DDT have been designated as "toxics of concern" to the Chesapeake Bay (U.S. Environmental Protection Agency, 1991b,c). The use of chlordane, DDT, and PCBs has been banned or restricted for nearly two decades. The occurrence of these compounds in streambed sediment indicates a persistent potential for toxic effects from regional and point sources of contamination in parts of the Potomac River Basin.
3Chlordane concentrations presented are the sum of trans-chlordane, cis-chlordane, cis-nonachlor, trans-nonachlor, and oxychlordane.
4DDT concentrations presented are the sum of o,p'-DDT, p,p'-DDT, o,p'-DDD, p,p'-DDD, o,p'-DDE, p,p'-DDE.
5PCB concentrations presented are the sum of all PCB congeners.
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Figure 22. Chlordane, DDT, PCBs, lead, and mercury were detected in streambed sediment at many sites throughout the Potomac River Basin. Many concentrations were greater than screening levels, indicating that there is some potential for these contaminants to adversely affect aquatic life.
A three-tiered system is used for screening measured concentrations of potentially toxic compounds in streambed sediment. Concentrations in tier 1 (greater than the upper screening value) have a high probability of causing adverse effects on aquatic life; concentrations in tier 2 (between the upper and lower screening values) have an intermediate probability of causing adverse effects on aquatic life; and concentrations in tier 3 (less than both screening values) have a low probability of causing adverse effects on aquatic life (Gilliom and others, in press). Screening levels for mercury, lead, DDT (Long and others,1995) and chlordane (Long and Morgan, 1990) are the ERL (effects range - low) and ERM (effects range - median) developed for bottom sediments in estuarine waters. Screening levels for PCBs are the PEL (Probable Effect Level) and TEL (Threshold Effect Level) developed by the Florida Department of Environmental Protection (1994). Because these values were developed for screening purposes, measurements within tiers 1 and 2 do not indicate that adverse effects have occurred at particular sampling sites.
Chlorinated organic compounds and trace metals are present in streambed sediment of the Potomac River and its tributaries at concentrations that have some potential to adversely affect aquatic life. Sediment from 14 of 25 sites contained chlordane, DDT, PCBs, lead, or mercury at concentrations that pose an intermediate probability of causing adverse effects on aquatic life; concentrations at six sites indicate a high probability of causing adverse effects on aquatic life.
Chlordane was detected in streambed sediment from 13 of 26 sites in the Potomac River Basin, even though most uses of chlordane were banned in 1978 and a total ban was implemented in 1988 (U.S. Environmental Protection Agency, 1991c, 1992, 1994a). Chlordane is a synthetic organic compound that was primarily applied to soil surrounding building foundations as an insecticide to control termites and ants. For this application, chlordane was developed to adhere to soil particles and to degrade slowly-- offering long-term protection from pests. These chemical properties, however, render chlordane a persistent problem in many streams in the Potomac River Basin.
At least a moderate potential for occasional adverse effects to aquatic organisms exists at all sites where chlordane was detected in stream-bed sediment. Seven of these sites are in the Great Valley (fig. 23). Although the Great Valley is largely agricultural (including row crops, pasture, and orchards), several urban and industrial centers extend through the valley from Waynesboro, Va., to Chambersburg, Pa. Chlordane in this region could be derived from a wide variety of sources and applications.
Concentrations of chlordane in streambed sediment indicate a high probability for adverse effects on aquatic biota at four sites downstream from urban areas (fig. 23). The maximum concentration (66.6 ppb, parts per billion) was measured in a sample from the Anacostia River (site 22) in a tidal area near Washington, D.C. This concentration was more than 10 times the upper screening threshold. The second highest concentration was measured in sediment from the North Branch Potomac River, at a site downstream from a heavily industrialized area of Cumberland, Md.
Figure 23. Chlordane and DDT were frequently found in streambed sediment in the Potomac River Basin, even though these insecticides are now banned. The greatest chlordane concentrations were measured near the Washington, D.C., urban areas and Cumberland, Md.; whereas the greatest DDT concentrations were measured in streams in the Great Valley.
DDT was detected in streambed sediment from 23 of 26 sites sampled in 1992 and 1996. DDT is an insecticide used to control mosquitoes and other pests. Its use was banned in 1972 because of its harmful effects on birds and other wildlife and its potential to cause cancer and damage the human nervous system, liver, kidney, and skin (U.S. Environmental Protection Agency, 1992). Its widespread occurrence in the Potomac River Basin two decades since its ban is attributable to both widespread use and chemical stability.
Although DDT was present at a large proportion of sampled sites, concentrations at most sites indicate little potential for adverse effects on aquatic biota. DDT concentrations in sediment at seven sites indicate a moderate potential for adverse effects on aquatic life. Of these, five are in the Great Valley. The application of DDT to orchards has been identified as one potential source of DDT in several streams in the central Great Valley (Gerhart and Blomquist, 1995). As with chlordane, sediment from the Anacostia River near Washington, D.C. (fig. 23) had the highest concentration of DDT (41.9 ppb).
PCB concentrations in sediment from the South Fork Shenandoah River at Front Royal, Va., indicate a high potential for adverse effects on aquatic life. The high levels of PCBs at this site are attributable to a textile plant immediately upstream (Virginia Water Control Board, 1992). This plant has been closed since 1989, when the PCB releases were discovered. Other high levels of PCBs were measured in sediment from the Potomac River at Shepherdstown, W. Va. (site 11), but no local sources have been identified (Gerhart and Blomquist, 1995).
Mercury was detected in all streambed sediment samples from the Potomac River Basin and poses a potential threat to aquatic life at six sites (Gerhart and Blomquist, 1995). Although it occurs naturally in trace amounts in the Earth's crust, mercury may be introduced as a contaminant to the environment as a consequence of human activities. Mercury is used in the manufacture of paints, paper, and vinyl chloride, and it can also be found in batteries and fungicides.
High levels of mercury at three sites are directly attributable to a long-term industrial source of mercury contamination on the South River in Waynesboro, Va. (fig. 24). In 1977, mercury was found in soils at the Waynesboro plant, where it was used in industrial processes until 1950. Mercury contamination may have continued at this site for several decades prior to its discovery (Brooks, 1977). This potentially long-term source has led to measurable mercury contamination as far as 171 miles downstream, near Harpers Ferry, W.Va. (fig. 24). It is possible that the Waynesboro plant is a contributing source of mercury measured as far downstream as Washington, D.C., although more detailed study would be necessary to make this link.
Figure 24. Mercury contamination from an industrial source in Waynesboro, Va. (possibly lasting over a period of decades), has led to widespread contamination of the South Fork Shenandoah and Shenandoah Rivers (to about 171 river miles downstream). It is possible that contamination from this source has spread as far downstream as the Potomac River at Washington, D.C.
Lead was detected in streambed sediment at all 25 sampled sites in the Potomac River Basin. The highest lead concentration (110 ppm, parts per million) was measured in sediment collected from the Anacostia River near Washington, D.C., which also contained the highest concentrations of chlordane and DDT. Sediment from two other sites in the Washington, D.C., area and one in Cumberland, Md., contained lead at concentrations indicating a moderate potential for adverse effects on aquatic life.
Each of the sediment samples containing lead at concentrations potentially harmful to aquatic life were collected in urban and industrial areas. Although lead occurs naturally in trace amounts, many potential sources of lead contamination exist, including batteries, vehicle emissions, solder, and corroding brass, pipes, and plumbing. Other sources include paints, gasoline, and lead shot, although uses of lead for these purposes has been restricted. Sediment in urban areas may accumulate lead from any of these sources.
Contaminants present in stream-bed sediment are bioavailable and have been incorporated into the food chain. Chlordane, DDT, PCBs, mercury, and lead were detected in Asiatic clam (Corbicula fluminea) tissue and fish tissue samples collected in 1992 and 1996 (Zappia, 1996). Organochlorine compounds were detected less frequently in clam tissue than in streambed sediment, as chlordane and DDT were detected in only 3 of 16 sites and PCBs were detected in 4 sites (table 3). Chlordane was generally found in higher concentrations in whole-fish tissue than in clams.
Streambed sediment apparently serves as a source of chlordane to the food chain, because chlordane concentration in fish tissue is correlated with concentration in sediment (fig. 25).
Figure 25. Chlordane concentrations in whole-fish samples of bottom-feeding fish are correlated with concentrations in streambed sediment (Gerhart and Blomquist, 1995). These sediments apparently serve as a source of chlordane in the food chain. One sample of shorthead redhorse (a carp-like fish) shows chlordane contamination near Frederick, Md., where none exists in the streambed sample. This contamination may be from exposure to contamination in other locations or from historical exposure.
Chlordane concentrations in fish tissue pose a threat to fish-eating wildlife at two sites in the Potomac River Basin. Whole-fish tissues from two streams in Virginia, Bull Run (site 25) and Accotink Creek (site 24), contained chlordane at concentrations exceeding the National Academy of Sciences, National Academy of Engineering (NAS/NAE) (1973) recommended maximum concentration for the protection of fish-eating wildlife. These sites are in the heavily urbanized Washington, D.C., area. A human health advisory including chlordane currently exists for the consumption of fish caught in Washington, D.C., waters, "due to PCBs and other compounds" (Hamid Karimi, District of Columbia Department of Consumer and Regulatory Affairs, written commun., 1994). Possible effects of chlordane on human health include cancer, dizziness, headache, fatigue, and convulsions. No Asiatic clam samples contained chlordane at concentrations threatening to humans or wildlife.
No Asiatic-clam or whole-fish samples contained PCBs at concentrations threatening to human health or wildlife, although human health advisories have been issued for some streams. Asiatic clams from the South Fork Shenandoah River at Front Royal, Va. (site 15), contained the highest measured concentration of PCBs in the basin. Human-health advisories for PCBs in fish exist in this part of the South Fork Shenandoah River. Human-health advisories also exist for other reaches of the North Fork, South Fork, and mainstem Shenandoah River and for Washington, D.C., waters (Emily Jones, Commonwealth of Virginia, Department of Game and Inland Fisheries, written commun., 1996; Hamid Karimi, District of Columbia Department of Consumer and Regulatory Affairs, written commun., 1994).
Mercury was detected in tissue samples from nine sites, including four sites in the Shenandoah River watershed downstream from the contamination source in Waynesboro, Va. Mercury was detected in Asiatic clams from seven sites, but no concentrations exceeded the FDA action level for the protection of human health in edible-shellfish tissue. The highest measured concentration of mercury in Asiatic clams (0.71 ppm) was detected at site 14, downstream from Waynesboro, Va. Mercury was also detected at five sites (8, 13, 16, 19, and 20) in fish livers, although FDA action levels and NAS/NAE criteria are not applicable to fish livers.
The Virginia Department of Health has established warnings to restrict consumption of fish from the South River and the South Fork of the Shenandoah River because of concentrations of mercury in fish tissue (Vickie Odell, Virginia Department of Health, written and oral commun., 1997).
Table 3. Summary of chlordane, DDT, PCB, lead and mercury concentrations in Asiatic clam tissues and relation to standards for the protection of human health
[ppb, parts per billion; also equivalent to micrograms per kilogram]
U.S. Food and Drug Administration action level1 (ppb)
1 U.S. Food and Drug Administration, 1992.
All samples of Asiatic clams (Corbicula fluminea) contained chlordane, DDT, and PCB at concentrations well below the U.S. Food and Drug Administration action levels for protection of human health. Mercury concentrations approached the FDA action level in a sample from the South River near Waynesboro, where the greatest concentration was measured in streambed sediment .
Table 4. Summary of chlordane, DDT, and PCB concentrations in whole-fish tissue and relation to criteria for the protection of fish-eating wildlife
[ppb, parts per billion, also equivalent to micrograms per kilogram]
NAS/NAE recommended maximum concentration for the protection of fish-eating wildlife1 (ppb)
1 National Academy of Sciences, National Academy of Engineering, 1973.
Two samples of bottom-feeding fish contained chlordane at concentrations greater than the NAS/NAE recommended maximum concentration for the protection of fish-eating wildlife. These samples, collected in 1992, show a persistence in the food chain because chlordane use was restricted in 1978 and banned in 1988.