RICHMOND (CBS SF/AP) — A half-century after California officials found that large amounts of the pesticide DDT were discharged into a San Francisco Bay canal, the chemical is still poisoning fish and posing a threat to human health there despite numerous cleanup attempts.

DDT was dumped into the Lauritzen shipping channel near the city of Richmond by the pesticide processing company United Heckathorn from the 1940s through 1966.

READ MORE: UPDATE: Wind-Whipped Big Sur Wildfire Grows To 1,500 Acres, Blaze Erupts Near Geysers Peak In Sonoma

In 1990 the U.S. Environmental Protection Agency listed the former plant and the canal as one of the most polluted places in the nation, and began trying to clean it.

READ MORE: Napa Valley Looks to Restaurant Week to Help Jump-Start Economic Comeback

EPA records show that the last cleanup project, which cost about $12 million, made no progress in the canal.

Recent fish tests found DDT levels at the same or higher levels than tests in 1994.

MORE NEWS: Wind-Whipped Wildfire in Big Sur Shuts Hwy 1, Forces Evacuations

(Copyright 2012 by CBS San Francisco. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)