Montrose “was right in saying it was dechlorinating. There was no dispute,” Eganhouse said. “But their rates were not realistic. Data didn’t exist at that time to make an accurate prediction.”
Eganhouse's previous analysis of one location on the shelf shows that the breakdown product, called DDMU, is now more prevalent out there than the original DDT/DDE. Virtually nothing is known about the toxicity of that substance or whether it has potential effects on wildlife or human health.
“The general idea exists that the fewer chlorines, the less toxic a substance is, but nothing can replace actual toxicological studies,” Eganhouse said.
Sherwood analyzed about 30 years of DDT data from many locations on the Palos Verdes Shelf and said the rate of decline in the bulk of the sediment was only 1 to 2 percent per year, far slower than anything that could explain the reported loss of 96 tons in five years.
He is particularly suspicious about the accuracy of the new EPA data because it showed most of the PCBs had vanished too, and PCBs do not break down like DDT does.
There also was nothing in Eganhouse's analysis, albeit at only one location, to suggest that the pace of dechlorination has been accelerating. “An analysis of data between 1992 and 2003 and a 2009 core showed the rates haven’t changed out there,” he said.
The upshot, he said, is that “we cannot presently say for sure if the rates have changed in the past or if they will change in the future.”
Eganhouse is now analyzing more locations on the shelf that could provide key answers within six months about what’s happening out there now.
Evolution of microbes?
Bay said perhaps there has been an “evolution or adaptation of the microbes” that break down DDT. Maybe something “revved them up” since 2004, after a decades-long lag, he said.
James Tiedje of Michigan State University said these types of chlorine-gobbling microbes “can have logarithmic growth.” The growth of their colonies starts out slowly, then reaches a point of exponential growth that transforms the entire environment.
“Some can grow up over night. There’s a number of examples where microbes in nature grow up and increase their numbers substantially and increase their rate of dechlorination,” said Tiedje, Distinguished Professor of microbiology and director of the university’s Center for Microbial Ecology.
Changes in carbon, for one, can cause an explosion of growth because it provides more food. “If more carbon is breaking down, especially when anaerobic, the microbes reach fatty acids and hydrogen and that’s what the dechlorinators like,” Tiedje said.
There has been a major change in the conditions of that part of the ocean floor. Los Angeles County switched to full secondary sewage treatment in late 2002, which greatly reduced the volume of organic substances released into the ocean and increased the oxygen in the system. Tiedje, however, thought that might have the opposite effect, slowing the breakdown of the chlorinated chemicals.
Another possibility – one that alarms some experts – is that the chemicals diffused into the water so they are no longer on the ocean floor.
Gully of the county sanitation agency said that scenario is concerning because it means more fish in a wider area could be contaminated directly through the water, not just through the food web. Any cleanup would be virtually impossible. Water testing, however, has not detected any increase, he said.
Fish and birds are the real indicators
When the area’s bottom-dwelling fish were last tested in 2004, they remained highly contaminated, “and that’s the part people shouldn’t lose sight of," said Gold, who was executive director of the environmental group Heal the Bay before joining UCLA last year.