WASHINGTON - Government scientists who study pollution in U.S. coastal waters have released a 20-year study showing that overall levels of pesticides and industrial chemicals are generally decreasing.

The Mussel Watch program of the National Oceanographic and Atmospheric Administration examined levels of 140 chemicals from 1986 to 2005 in coastal areas and estuaries of Alaska, Hawaii, Puerto Rico, the East and West coasts, the Gulf of Mexico and the Great Lakes. Mussel Watch is the longest continuous contaminant monitoring program in U.S. coastal waters.

Gunnar Lauenstein, an oceanographer who's the lead scientist of the program, said the levels are continuing to decrease, many years after environmental laws were enacted in the 1970s.

"Different regions have different stories," Lauenstein said, with some contaminants increasing in some regions.

But, Lauenstein said, "when you look at all the numbers and evaluate them statistically, it shows that on a national basis, concentrations are going down."

The pesticide DDT shows significant decreasing trends around the country, even in Southern California, which had the heaviest concentrations, Lauenstein said. Industrial chemicals, such as PCBs, a material used in electrical products, also show declines.

Some problems highlighted in the report include oil-related compounds from motor vehicles and shipping, which continue to flow into the waters. NOAA also is studying flame retardants, known as PBDEs, and plans to release findings this year about their effects on marine and human health.

No overall national trends could be determined for trace metals. High levels of metals and organic contaminants remain near urban and industrial areas.

The report also found that levels of tributyl-tin, a compound that was used to kill marine organisms on boat hulls, were declining. The compound affected not only the organisms it was meant to kill but also other marine and fresh-water life. Tributyl-tin was regulated in the late 1980s, and its use is decreasing nationally.