The waters and wetlands of the San Francisco Bay-Delta estuary provide habitat for a great wealth of fish and wildlife. Pesticides, industrial discharges, treated sewage, and other forms of pollution have long sullied the estuary; yet surprisingly little monitoring of its water quality is performed. As a result, data on the severity of this contamination are lacking. But what information has been gathered -- including tests showing that the water is sometimes toxic enough to kill shrimp and small fish -- suggests that contamination threatens the estuary's health.
NRDC analysts examined results from a series of water-quality tests conducted from 1997 to 2000 and published by the San Francisco Estuary Institute, a research organization. Testers evaluated the water's toxicity by observing its effect on juvenile opossum shrimp and inland silverside fish. They took regular samples from a site at the head of the estuary, where water enters from the Sacramento and San Joaquin rivers, as well as from various points in San Francisco Bay. After rainstorms, they also sampled areas of the bay subject to urban runoff (rainwater that washes across pavement and other surfaces, carrying pollutants with it).
Most animals survived the tests. But in several cases animals died, sometimes within 24 hours, indicating that bay water is prone to episodes of acute toxicity. The toxic episodes were as short as a day or as long as a week and could extend across tremendous distances. One episode affected nearly 30 miles of the bay, spanning the area from Vallejo to Antioch. Scientists believe that runoff and pesticides (from agricultural discharges) are the major sources of the toxicity found in these tests.
In 1997 and 1998, testing revealed two major episodes of shrimp toxicity. The first occurred several days after a major rainstorm, suggesting that its cause was urban runoff. But the second episode did not follow rain, which suggests that it resulted from other causes, perhaps from agricultural discharges.
Other notable findings include cases of toxicity along the lower Napa River. In September 1999 and August and September 2000, water taken from the river killed inland silverside fish within 48 hours. Additional research is needed to determine the cause of this toxicity.