Enteric organisms may accumulate in sediments. Virus levels may be 100 times greater in the sediments than in the water column, and sediments have been found to contain 100 to 1,000 times greater levels of indicator and pathogenic bacteria than the overlying waters (Volterra et al. 1985, Van Donsel and Geldreich 1971). Greater than 99 percent of the enteric viruses were found to adsorb to marine sediments, and suspension in sewage effluents did not alter this pattern (LaBelle and Gerba 1979). It has been well documented that survival time is greatly enhanced for enteric organisms associated with sediments or with shellfish.
Nutrient addition to waters, nitrogen, phosphorus, and organics has been related to increased bacterial numbers generally affecting the indigenous microflora. This may indirectly influence the survivability of introduced microorganisms. Algal blooms may protect microorganisms at the surface from photoinactivation. Decreases in dissolved oxygen are also related to increases in survival, and naturally occurring organisms may show an antagonistic effect on pathogens. These effects are tied to the temperature of the water. Biological inactivating factors have been shown to be antiviral and are often associated with particulates and bacteria (Fujioka et al. 1980), but their significance under natural conditions remains unknown. Although natural solar light may affect bacteria and viral particles through direct inactivation, this would only occur at the surface in waters with low turbidity. This phenomenon would have little impact for submerged outfalls or in coastal waters with greater turbidity.
Much more research is needed in order to understand the fate of enteric pathogens introduced into the marine environment. The complexities of the interactions between the factors effecting survival and transport will ultimately determine the public health impact of pathogen-laden discharges to coastal waters.
A number of epidemiological studies have documented the risks of acute gastroenteritis among those bathing in contaminated seawater (Cabelli et al. 1983, Cheung et al. 1990, Balarajan et al. 1991, Fleisher 1991, Alexander et al. 1992, Fewtrell et al. 1992). One study showed the risks to be three times greater for children under the age of two who immerse their heads in water than for adults (Cheung et al. 1990).
It has been estimated that for each swimming event, for those individuals who submerge their heads in the water, the exposure is on the average of 100 milliliters of seawater. This value may represent a child's potential dose rather than an adult's. Recreational exposure generally will come from the contaminants suspended in the water column and via oral ingestion. Although aerosols and inhalation of water may also be potential expo-