The Challenge of Toxic Contaminants

Chapter II Marine Pollution in the United States 

Pew Oceans Commission 2001

Prepared for the Pew Oceans Commission by 
Donald F Boesch ^§, Richard H Burroughs *, Joel E Baker ^§, Robert P Mason ^§, Christopher L Rowe^§, Ronald L Siefert ^§ 

^§ University of Maryland Center for Environmental Science
* University of Rhode Island

Toxic pollutants include trace metals (e.g., cadmium, copper, lead, and mercury), a variety of biocides (e.g., DDT, tributyl tin) and their by-products, industrial organic chemicals (e.g., PCBs and tetrachlorobenzene), and by-products of industrial processes and combustion (e.g., polycyclic aromatic hydrocarbons, or PAHs, and dioxins). Those pollutants meriting greatest attention are widespread and persistent in the environment, have a propensity to accumulate in biological tissues, or induce biological effects at extremely low concentrations.

The historic use of some compounds no longer manufactured or used in the United States-like DDT, PCBs, and lead additives in gasoline-has left a legacy of contamination. Generally, legacy contaminants in U.S. coastal environments have declined. However, these compounds are still in use in other countries and they continue to run off the land. For example, it has been estimated that less than 10 percent of the total lead deposited from the atmosphere onto the Sacramento and San Joaquin river basins has yet been delivered to San Francisco Bay ¹. As the concentrations of some heavy metals and organochlorine compounds decrease in the marine environment, other contaminants are still being released and do not show a clear downward trend. Some may even be increasing. For example, analyses of lake and reservoir sediments show increasing levels of PAHs associated with suburban development ². PAHs come from multiple sources, including petroleum and the combustion of fossil fuels and biomass, some of which have been reduced (e.g., coal coking) and some of which continue (e.g., urban runoff and atmospheric deposition of combustion by-products).

Humankind will be dealing with legacy contaminants of the marine environment well into the future. Repositories of persistent contaminants in marine sediments can be sources of long-term exposure to marine life well after the inputs of these contaminants have largely ceased. Examples of this include DDT in the Southern California Bight (Box 1) and PCBs in San Francisco Bay ³. The deep sea may be the final sink for some persistent organic pollutants ⁴.

Toxic effects, both lethal and sublethal, have been extensively documented in laboratory experiments, but concrete examples of contaminant effects on populations of marine organisms are limited ⁵. Key issues considered here include the potential for bioaccumulation of toxicants by marine life; the effects of disruptions of organisms' immune, endocrine, and reproductive systems on their populations; and the effects on marine communities of chronic exposure to the high concentrations of contaminants found in coastal sediments.

Organisms may accumulate contaminants from water, sediments, or food in their tissues. This can result in concentrations of the contaminant many times higher than those found in the environment. The degree of bioaccumulation depends on the level of exposure and the mechanisms by which the organism expels, stores, or metabolically breaks down the contaminant. Compounds such as organochlorine pesticides and PCBs tend to accumulate in fatty tissues (lipophilic compounds), where they may remain for long periods of time. Animals in the upper levels of the food web may accumulate these compounds from prey until lipid storage sites are saturated. Their metabolism is then challenged to degrade and excrete the contaminants or their metabolic byproducts, some of which are much more toxic than the original form. In this way, highly persistent and bioaccumulative compounds can magnify through the food web, having little noticeable toxic effect except at the highest trophic levels. Trace metals are also subject to bioaccumulation, but except for metal-containing organic compounds (e.g., methyl mercury) do not biomagnify in marine organisms.

Bioconcentration and biomagnification of toxicants pose particular risks to predators of fish, including birds, marine mammals, and humans. High concentrations of toxicants, such as PCBs and mercury, necessitate health advisories for frequent consumers of fish in some regions ⁶. Perhaps the most widely recognized effect of persistent contaminants on marine populations is the decline of populations of bald eagles and brown pelicans during the 1960s and 1970s. DDT and its breakdown products accumulated in adult birds from their prey, leading to changes in calcium metabolism in breeding females. The birds produced abnormally thin eggshells and ultimately experienced reproductive failures ⁷.

Extensive evidence demonstrates that toxicants can disrupt the metabolic, regulatory, or disease defense systems of an organism, eventually compromising its survival or reproduction. For example, genetic damage, malformations, and reduced growth and mobility were observed in Pacific herring embryos exposed to PAH (from weathered oil) levels as low as 0.7 ppb ⁸. Mollusks exposed to PCBs in New Bedford Harbor, Massachusetts, experienced both a loss of reproductive output and increased susceptibility to disease ⁹. Accumulation of PCBs and PAHs in Puget Sound rock sole has been correlated with reductions in spawning success ¹⁰. Bioconcentration of PCBs has also been linked with impaired immune defenses that lead to disease and death in marine mammals, including seals and dolphins ¹¹.

Particular attention is currently being devoted to the disruption of endocrine systems by toxic contaminants. Some organochlorine pesticides, PCBs, dioxins, and other compounds functionally mimic or alter the production of hormones ¹². Tributyl tin (TBT), a biocide used in antifouling paints, has been shown to disrupt hormones controlling sexual development in mollusks exposed to concentrations as low as 10 parts per trillion, leading to irreversible reproductive abnormalities (e.g., females developing male sex organs) and reproductive failures ¹². Significant declines in marine snail populations have been documented in regions of North America and Europe where use of TBT was intense ¹³. Most uses of TBT paints in the U.S. were discontinued as a result of these findings. Feminization of males due to exposure to estrogen mimics and masculinization of females exposed to estrogen blockers have been observed in various animals, including mollusks, fish, reptiles, birds, and mammals ¹⁴. For example, endocrine-disrupting chemicals have been implicated in the incidence of hermaphroditism in Norwegian polar bears and St. Lawrence beluga whales ¹⁵.

Toxic substances in sediments appear to have localized effects in U.S. bays and estuaries and in certain offshore regions that received wastes, such as the New York and Southern California Bights. In the past decade, EPA's Environmental Monitoring and Assessment Program (EMAP) and National Sediment Quality Survey and NOAA's National Status and Trends Program have extensively measured the concentrations of contaminants in bottom sediments in the nation's bays and estuaries, collected collateral data on the communities of benthic organisms living in those sediments, and assayed toxicity of sediments to sensitive amphipod crustaceans. Using these three components-contaminant concentrations (and their probable effects based on an extensive database), the health of the communities living in the sediments, and experimental toxicity-Long ¹⁶ concluded that biologically significant chemical contamination and toxic responses occurred throughout the nation's coastal waters, especially in the most urbanized and industrialized regions. Chemical concentrations exceeding guidelines for probable effects occurred in 26 percent of samples, representing 7.5 percent of the bays and estuaries surveyed. Generally, sediments proved toxic to the crustaceans where contaminant concentrations were high and benthic communities degraded.

This three-pronged approach involving field studies does not fully resolve which contaminants and other factors are actually responsible for the toxicity and community degradation. The synergistic, additive, or antagonistic interactions among contaminants are poorly understood and challenging to assess, thus making it difficult to predict biological responses simply based on knowledge of the types and concentrations of contaminants present in a given area ¹⁷.

The most effective way to reduce the harmful impacts of toxic contaminants on marine ecosystems is to eliminate or restrict their use or production. The experiences with lead additives in gasoline, DDT, and PCBs show that in the long term this approach can reduce environmental concentrations and exposure for marine organisms. In addition to discontinuing the use or production of these substances, source controls, recycling and reuse, and other forms of "pollution prevention" provide the first line of defense ¹⁷. Treatment and removal of pollutants from effluents and atmospheric emissions provide a second line of defense. Improved knowledge of the fate and effects of various classes of compounds and screening processes for new chemical products have reduced, but not totally eliminated, the risk of "surprises" such as DDT, PCBs, and TBT.

Legacy contaminants must be managed for decades to centuries into the future. Options include control of losses from waste sites and contaminated soils on land, treatment of urban stormwater, and remediation of contaminated sediments. Contaminated sediments exist in many ports, where they pose a risk of reintroduction of toxicants into the water column by physical disturbance of sediments or transferal through the food chain. Options for managing contaminated sediments include: leaving them in place to allow recovery to proceed through degradation and burial, capping them with clean sediments, treating them in place, and removing them for containment or treatment ¹⁹.

In the case of the pesticide kepone in the James River estuary, Virginia, the decision was to leave the contaminated sediments in place, and subsequent reductions of contaminants levels in the ecosystem and organisms were observed ¹⁹. However, when contaminant levels are high and the risks of reintroduction are great, capping may speed recovery of the ecosystem. The EPA has proposed placing clean sediments atop portions of the DDT deposits off Palos Verdes, California, in order to test the feasibility and effectiveness of this remediation method. Representatives of the DDT manufacturer have criticized this method because DDT concentrations in surface sediments have been declining and the process may expose heavily contaminated sediment below the surface ²⁰. A similar controversy surrounds proposals to cap the dredged sediment disposal site in the apex of the New York Bight. These cases exemplify the dilemma faced in making decisions regarding remediation of contaminated sediments.

Box 1 - Southern California Bight Ocean Discharges

Wastes from the nation's largest metropolitan center (17 million people) are discharged into a bight of the Pacific Ocean via deepwater (about 200 feet) outfalls. Pollution from publicly owned treatment works (POTWs) has been reduced significantly since the 1970s even though the population served and wastewater volumes grew steadily ²¹ (Figure 1). This reduction was accomplished through source control, pretreatment of industrial wastes, reclamation, and treatment-plant upgrades, including secondary or other advanced treatment (concentrating on chemical removal of suspended solids). Capital improvements to POTWs throughout the Southern California Bight cost more than five billion dollars.

Discharges from POTWs of most pollutants into the bight have decreased: 50 percent for suspended solids and biological oxygen demand, 90 percent for combined trace metals, and more than 99 percent for chlorinated hydrocarbons. Bight sediments show a record of decreasing contamination. Concentrations of contaminants in fish and marine mammals have declined. Kelp beds near the POTWs have returned.

The extent of degraded bottom communities has contracted by about two-thirds; and the incidence of tumors and other maladies in bottom fish has returned to background levels.

A unique problem for the bight is the fact that large quantities of the pesticide DDT were previously discharged, particularly through the Los Angeles County's POTW. This facility received wastes from the world's largest DDT manufacturer. In 1971 an estimated 440,000 pounds of DDT were discharged via an outfall off Palos Verdes. Today, only 3 pounds of DDT are discharged from all Southern California POTWs combined (Schiff et al., 2000). Concentrations of DDT and its degradation products have declined greatly in fish and marine mammals. Populations of brown pelicans, which were decimated by the eggshell thinning induced by DDT contamination, have rebounded. However, brown pelicans, bald eagles, and peregrine falcons are still being affected by the residual DDT contamination in the bottom sediments of the bight. Although this "legacy" contamination is slowly being buried, some DDT is still remobilized into the food chain.

References

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