LOS ANGELES - Off Southern California's shore, purity is an illusion that lies only a few feet deep.
The trouble's not with the water; it's what lies beneath it.
From Santa Catalina Island to New York Harbor, the mud and silt that line the bottom of rivers, bays and lakes contain chemicals deemed potent enough to kill aquatic animals and endanger the health of people who consume marine life. Dangerous compounds such as mercury, arsenic, lead, PCBs and DDT - the residue of years of pollution - are hidden below the surface.
The underwater legacy of sediment contamination is one of the country's most extensive and intractable, yet overlooked, pollution problems.
"For the last 20 years, we've focused on the water and there are appreciable changes for the better," said Jim Keating, who is heading up an unprecedented study of the problem for the U.S. Environmental Protection Agency. "But there has not been a lot of focus on the sediment. And sediments are the ultimate sink for water pollutants."
Nearly 5,200 bodies of water - three out of every four targeted for testing - contain sediment likely to injure marine life or human health, according to the EPA's National Sediment Quality Survey.
People who eat fish, mussels or other aquatic life from 2,300 sites face a significantly heightened chance of cancer or birth defects, the EPA data shows.
Individual problem areas have long been recognized, such as Puget Sound, Cape Cod and Chesapeake Bay. But the sheer number discovered to pose a high risk has astonished the EPA research team.
There is so much "hot sediment" in so many places that there is little hope of a quick or easy cure.
In the meantime, the buildup of silt is also wreaking economic havoc. Where sediment is contaminated, routine dredging often is halted, creating "mud lock" that blocks ships at many of the busiest ports and marinas.
Soft, muddy sediments are like sponges that slowly soak up the world's most dangerous and persistent chemicals, including some now banned because of their toxicity.
Poisons are spread throughout the food web from fish to bird to mammal, starting with the variety of creatures that feed and spawn in the silt and sand.
Particles embedded in the mud are ingested by small burrowing animals, such as worms and crabs. Crustaceans and other organisms can die from poisoning, and fish can grow cancerous tumors and cataracts. Once-thriving shellfish harvests have been shut down on both coasts, including much of the Gulf of Mexico and Chesapeake Bay. If a creature survives, its body can build up a toxic load over its lifetime that passes to whatever consumes it.
While never touching the sediment itself, fish-eating birds such as eagles and pelicans can perish from poisoning, or produce unhatchable eggs or chicks with deadly birth defects. Seals, dolphins and other water-reliant animals may grow tumors or lose their ability to fight off disease.
People are not immune. In the water itself, the pollution is often barely detectable, so swimming above the sediment is safe. But eating the tainted fish can cause cancer or birth defects.
Some places are so severely damaged by sediment that they are virtually void of life.
"There's no question that some systems are highly stressed by toxics," said Raymond Alden, director of the Applied Marine Research Laboratory at Old Dominion University in Virginia, who has studied sediment along the Eastern Seaboard for almost 20 years. "We see certain species disappearing, and eventually everything starts disappearing. Diversity goes down and that's a good measure of how healthy a community is."
Still, scientists in the relatively new field of sediment toxicology question how serious the ecological risk is in the thousands of places where the injury to animals is less obvious. If a type of worm, or brittle star, is killed in one spot, what, if anything, does that mean to a marine ecosystem as a whole? No one at this point has an answer.
For decades, sediment has been a case of out of sight, out of mind.
Some of the contamination dates to the chemical boom just after World War II. Until the late 1960s, disposal offshore was considered safe because the chemical doses were too low to be considered poisonous. It came as a harsh surprise when many of the compounds, insoluble in water, worsened over the years by accumulating in animals' bodies.
Today, much of the waste dumping has stopped under laws protecting water quality. However, toxic chemicals still flow from modern sewage plants, urban streets, farm fields and industrial sites. Some fall from the air, such as mercury spewed by coal-burning power plants.
Some sites are getting worse, some better, but the vast majority have stayed the same despite the array of pollution laws, the National Oceanic and Atmospheric Administration recently concluded.
In a report to be unveiled today, a committee of the National Academy of Sciences will identify sediment contamination as an immense problem that warrants more attention. The panel of experts will recommend policies aimed at finding effective yet reasonable solutions.
Getting rid of tainted sediments - or at least ensuring they are entombed - poses a monumental engineering challenge.
Does digging them up make matters worse by stirring them up? And once removed, what do you do with tons of contaminated material? Where, especially in congested urban areas is there room on land to dump hundreds of truckloads? And when left in offshore waters, do tomb-like pits covered with sand really keep the material sealed permanently?
Most sediments are not bad enough to be declared hazardous waste. Instead they are half-jokingly called "chemically challenged" - although perilous in waters as they build up in animals, they are fairly safe on land.
Biologists and chemists disagree about the magnitude of the threat to underwater life, some questioning whether the EPA used too stringent criteria in highlighting 5,200 sites.
Robert Risebrough, who discovered in the 1960s that DDT-tainted sediment off California was inflicting severe ecological damage, says most of today's lingering problems are nowhere near as serious as they were 30 years ago. At most sites yesterday, he says there is no proof of serious injury to birds and mammals, so expensive cleanups are unwarranted.
"I don't believe there is any hazard to most of these sediments in the real world," said Risebrough, a researcher at the nonprofit Bodega Bay Institute in Berkeley. In the laboratory, "you put a tiny amphipod in the mud, and if it doesn't like it, then the sediments are considered toxic. You can't predict anything from those laboratory tests."
EPA officials acknowledge that many questions remain, and testing of many waterways remains sparse or outdated. Such uncertainty is one reason why they haven't ordered cleanups, or told anglers to avoid eating fish at most of the thousands of sites identified as a risk to humans. Only a few are posted with health warnings - including the Palos Verdes area near the Los Angeles Harbor and parts of the Great Lakes. The EPA's Keating said the goal of the new analysis is to highlight troublesome areas that warrant more thorough looks by local authorities.
Alden said the uncertainty comes in "quantifying how bad is bad" when it comes to the threat chemicals pose to underwater life and the people who feed on them.
"It's a political issue as much as a scientific one," Alden said. "Do you try to get a more realistic answer about certain chemicals or do you err on the side of protecting the environment and human beings?"