Sediment provides important habitat for many organisms that are essential links in the food chain. Mudflats, for example, composed of sediments and the organisms that live in them, are some of the most productive -- if least glamorous -- habitats in the Bay-Delta estuary. But when they become contaminated, the effects travel up the food chain, to birds, seals, and other animals, and eventually to humans. They also intensify. For example, a fish might eat contaminated mud-dwelling clams, and in turn be eaten by a seal, which eats many such contaminated fish. Each step of the way, the concentration of chemicals becomes greater, in a process called bioaccumulation. Today, many bay fish are contaminated at levels that pose health risks to wildlife and people who eat them.
Chemicals in Sediment
Chemicals contaminating the Bay-Delta estuary's sediment -- and through it, the rest of the food chain -- pose a range of threats. One group of chemicals, for instance, called polycyclic aromatic hydrocarbons, can impair organisms' growth, metabolism, reproduction, immune function, and photosynthesis. These chemicals, which are produced by combustion (of gasoline and other fuels, for example) are also carcinogenic to humans. Mercury, which is found in large sediment deposits at the bottom of the bay, can harm fish, birds, and other wildlife; it can also cause neurological and developmental damage in humans. Mercury bioaccumulates and is likely to decrease in concentration only very gradually.
Other chemicals of particular concern are nickel, zinc, and copper (because their concentrations are particularly high) and DDT, chlordane, dieldrin, and dioxins (because they are highly toxic).
These chemicals come from myriad sources, and they enter the estuary in different ways: from air pollution that settles into the water, from direct discharges, and from runoff (the flow of rainwater over pavement and other surfaces, during which it picks up contaminants). Metals are some of the most frequent contaminants; they come from innumerable products, from brake linings (copper) to thermometers (mercury), as well as manufacturing and other industrial activities.
Some sediment contamination is the result of past activities -- in many cases, activities that ceased long ago. For instance, the pesticides DDT and chlordane were banned in the 1970s, but they remain in the Bay Area's sediment, their levels decreasing but detectable. Many other chemicals that contaminate bay sediment remain in use. And some, like the byproducts of combustion (polycyclic aromatic hydrocarbons) are created every day and continue to enter the estuary in a variety of ways.
When they are embedded in sediment, many chemicals do not break down readily. In addition, sediment moves through the estuary -- because of dredging, as well as natural processes like wind and waves -- so the contamination moves with it, sometimes for decades. By comparison, contaminants suspended in water are diluted by floods and tidal action, and some contaminants decay faster in water.
Levels of sediment contamination are likely to remain high for many years. But with a combination of wise policy decisions and the passage of time, the condition of the estuary's sediment can improve. The bans on DDT and chlordane, for example, have helped lower the concentrations of these chemicals from their peak levels in the 1970s. Levels of most other contaminants are also below their peaks. It will take more of these aggressive policy measures -- and the individual actions of Bay Area residents -- to further reduce sediment contamination.