When it rains or snows, the soil particles and organic matter in runoff water pick up oils, metals, pesticides, and other contaminants. The contaminants adhere to the soil or organic matter rather than mixing readily into water. Once in the bay, organic contaminants may float to the surface to avoid water, forming a sea-surface microlayer. Other contaminants adhere to sediments and particles of organic detritus. Initially, sediment and particles settle to the sea bottom near where they enter marine waters, though over time they may get dispersed by tides, currents, storms, or dredging. Water that lies over contaminated sediments may even test as clean because the toxics tend not to be water-soluble.
Atmospheric deposition is a natural process by which pollutants are transferred from air to soil, surface water, sediment, and groundwater and potentially to living organisms. Wet and dry deposition processes (e.g., rain out, wash out, impaction, adsorption and absorption) remove particulate and gaseous pollutants from the atmosphere and deposit them on the surface of water bodies, vegetation, buildings and structures, and soil.
The most common toxic pollutants in Casco Bay are Polyaromatic Hydrocarbons (PAHs), a class of organic compounds primarily found in fossil fuels such as oil or coal. Most PAHs found in the sediments of the bay come from combustion sources (i.e., car and truck exhausts, and industrial and residential chimneys). PAHs enter the bay through combined sewer overflows and storm drains (particularly those that drain roads and parking lots), licensed discharges, old industrial sites or dumps, spills, deposition of atmospheric pollution from urban sources in and upwind of Maine, and highly developed residential and industrial areas in the bay watershed.
Metals in Casco Bay are concentrated in and around Portland Harbor. Sources are numerous, including vehicle emissions, licensed discharges, air deposition, and historic industrial sites. Evidence of these early industries remains in the bay. At the site of a former coal gas works plant, which operated in Portland for almost a century, coal tar can still be seen oozing into the Fore River estuary.
Bottom-Dwelling Animals: There is evidence of damage. Animals that would be expected to occur in the flats of Back Cove are missing, potentially due to such factors as oil-related contaminants, heavy metals, combined sewer overflow discharges, sedimentary disturbances, or a combination of factors. Benthic life in the inner Fore River has been dramatically impaired.
Fisheries: Sediment contamination can have serious ramifications for fisheries and marine life in Casco Bay. Fish and crustaceans can absorb toxics directly by exposure to contaminants in the water, and indirectly by eating contaminated food--particularly bottom-dwelling organisms that live and feed among the "modern mud" sediments on the bay's bottom. Blue mussels sampled in the outer Fore River had elevated levels of lead in their tissues, while those in the Presumpscot River had elevated levels of mercury. Mussels are used nationally as an indicator species of toxic pollution.
Wildlife: Mammals and birds that feed on benthic organisms or fish may absorb concentrated amounts of contaminants. Some of the tidal mudflats that represent the most important feeding areas for shorebirds, waterfowl, and wading birds--the Fore River, Back Cove, and Presumpscot River--also have the highest concentration of contaminated sediments in the bay.
Human Health: Various toxic pollutants (e.g., PCBs, DDT, some PAHs, and dioxin) concentrate in the liver, fat, and tissue of animals and can cause significant human health impacts. Bioaccumulative toxic chemicals can cause cancer, adverse reproductive effects, birth and developmental effects, organ damage, and deleterious impacts on the nervous, immune, and endocrine systems. With the exception of testing for dioxin in lobsters and clams, and the testing of toxics in lobster and mussel tissue, there has been no risk assessment of potential health hazards from eating seafood in Casco Bay.
