Sediment -- the "muck" at the bottom of rivers and other bodies of water, which is composed of materials transported and then deposited by water or wind -- is a surprisingly rich and productive environment. The organisms that live in it form the base of a food chain that stretches all the way up to humans. As a result, the condition of sediment in the San Francisco Bay-Delta is an important gauge of the health of the ecosystem. Overall, much of the estuary's sediment is contaminated, and contamination levels are likely to remain high for many years. Year to year, however, the levels of contaminants change.
Using information published by the San Francisco Estuary Institute, NRDC researchers reviewed toxic contamination of sediment in the San Francisco Bay-Delta estuary from 1993 to 1999. All of the sediment samples monitored by SFEI showed the presence of at least two chemicals, nickel and dieldrin; DDT, mercury, chromium, copper, arsenic, and chlordane were also found frequently. Over the 7-year period, 65 percent of the sediment samples tested were toxic to at least one of two test organisms (meaning the organisms were harmed or killed). From year to year, however, the percentage of samples that were toxic varied widely (from 38 percent to 79 percent), with no clear overall trend.