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An expedition led by UC San Diego’s Scripps Institution of Oceanography mapped more than 36,000 acres of seafloor between Santa Catalina Island and the Los Angeles coast in a region previously found to contain high levels of the toxic chemical DDT in sediments and the ecosystem. The survey on Research Vessel (R/V) Sally Ride identified an excess of 27,000 targets with high confidence to be classified as a barrel, and an excess of 100,000 total debris objects on the seafloor. 

“Unfortunately, the basin offshore Los Angeles had been a dumping ground for industrial waste for several decades, beginning in the 1930s. We found an extensive debris field in the wide area survey,” said Eric Terrill, chief scientist of the expedition and director of the Marine Physical Laboratory at Scripps Institution of Oceanography. “Now that we’ve mapped this area at very high resolution, we are hopeful the data will inform the development of strategies to address potential impacts from the dumping.”

The expedition that ran March 10-24, was developed in collaboration with NOAA’s Office of Marine and Aviation Operations and the National Oceanographic Partnership Program. The project, part of ongoing collaboration with NOAA’s Uncrewed Systems Operations Center, tested autonomous underwater vehicle (AUV) technology to map the seafloor. As marine robotic technology continues to advance, NOAA is collaborating with Scripps to transition ocean robotics from research to operational uses.   

Two AUVs, the REMUS 6000 (right), capable of working up to depths of 6,000 meters (19,600 feet), and Bluefin (left), capable of depths up to 1,500 meters (4,900 feet), were deployed to work in tandem to map the seabed at a high resolution. Credit: Scripps Institution of Oceanography at UC San Diego.

In 2011 and 2013, UC Santa Barbara professor David Valentine discovered concentrated accumulations of DDT in the sediments in the same region, and visually confirmed 60 barrels on the seafloor. Scientists are also finding high levels of DDT in marine mammals including dolphins and sea lions, with exposure to PCBs and DDT linked to the development of cancer in sea lionsReporting on this issue by the Los Angeles Times noted that shipping logs from a disposal company supporting Montrose Chemical Corp. of California, a DDT-producing company, show that 2,000 barrels of DDT-laced sludge could have potentially been dumped each month from 1947 to 1961 into a designated dumpsite. In addition to Montrose, logs from other entities show that many other industrial companies in Southern California used this basin as a dumping ground until 1972, when the Marine Protection, Research and Sanctuaries Act, also known as the Ocean Dumping Act, was enacted.

Barrels and targets of interest were found in nearly all areas of the 36,000 acres surveyed and extended beyond dumpsite limits, which is roughly 12 miles offshore Los Angeles, and eight miles from Catalina Island. The 25,000 targets identified with confidence to be barrels had stronger brightness in their acoustic signal and distinct geometry in the shape of the image. The other objects identified also showed these signals, however not as bright or distinct which could be due to how deep they were deposited in the sediments, or deterioration of the material. There were also patterns that indicate how the barrels were dumped. 

“There are several distinct track-line patterns in the surveyed area, suggesting that the dumping was repeatedly done from an underway platform such as a moving ship or barge. Some of those lines are as long as 11 miles and approach state waters,” said Terrill. “While our mapping sonars cannot measure the contents inside the barrels, the target locations are consistent with the previously identified dumpsite and extend much further than we expected.”

Barrels and targets of interest were found in nearly all areas of the 36,000 acres surveyed and extended beyond dumpsite limits, which is roughly 12 miles offshore Los Angeles, and eight miles from Catalina Island. Credit: Scripps Institution of Oceanography at UC San Diego.

The expedition included a team of 31 scientists, engineers, and crew conducting 24-hour, around-the-clock operations to deploy two AUVs used for the expedition from R/V Sally Ride. The research vessel is one of the most technologically-advanced vessels in the U.S. Academic Research Fleet, and is owned by the Office of Naval Research and operated by Scripps on behalf of the U.S. research community. The search entailed work at depths up to 900 meters (3,000 feet), in what is considered a semi-abyssal, steep seafloor between Catalina and Los Angeles. The two AUVs, the REMUS 6000, capable of working up to depths of 6,000 meters (19,600 feet), and Bluefin, capable of depths up to 1,500 meters (4,900 feet), were deployed to work in tandem to map the seabed at a high resolution. 

The robots adjust to changes in the topography flying at a constant 20 meters (65 feet) above the seafloor, using high frequency side-scan sonar to send signals 150 meters (490 feet) on each side of the vehicle. The continuous echo-location of these signals reflecting from the seafloor creates images of the bottom and the objects resting there. Scanning the seabed at a rate of 0.75 square kilometers per hour – roughly the size of 140 football fields – the sonar data can be used to detect objects, characterize seafloor habitat, or map hazards. Sonar settings for this expedition were tuned to detect objects as small as a coffee cup.

The ability to operate in deep waters for long duration, and survey large areas at very high resolution is what enabled a wide area survey of this magnitude. Underwater acoustics were also used to broadcast GPS signals from the research vessel to the AUVs so that they could be tracked with high precision through each deployment

Research Vessel Sally Ride oversaw the underwater survey, continuously broadcasting underwater GPS signals to the autonomous underwater vehicles so that the vehicles and their sonar mapping data were highly accurate on the seabed. Crews remained in communication with shore using satellite data links, and were able to share data with scientists who remained on shore. Illustration credit: Scripps Institution of Oceanography at UC San Diego.

Topside, the science team would recharge the instruments, and offload sonar imagery to analyze data. More than 100 gigabytes of sonar data were captured during the expedition. 

Since the expedition, researchers have been analyzing the acoustic imaging data of this complex site. Typically, manually counting the targets is the approach taken with side-scan sonar processing, but this approach was not feasible given the size and extent of the survey area. An automated process was used, perhaps the first time automated approaches have been done at this scale, said Terrill. The 60 barrels confirmed by Valentine in 2011 and 2013 served as a reference point for validating detection algorithms that were developed to find barrels. 

“The data from the Valentine expedition were used to ground-truth our algorithms,” said Sophia Merrifield, a researcher at Scripps who has been leading the data analytics after R/V Sally Ride returned to shore. “Location, size, and acoustic brightness are tracked for each target detected and used to characterize patterns and densities of the debris field.”

The line represents the path of the autonomous underwater vehicle, and the small white targets represent acoustic data identified as barrels and debris field objects. There were several distinct track-line patterns in the surveyed area, suggesting that the dumping was repeatedly done from an underway platform such as a moving ship or barge. Credit: Scripps Institution of Oceanography at UC San Diego.
A heat map showing concentrations of targets detected in the San Pedro Basin. There are several distinct track-line patterns in the surveyed area, suggesting that the dumping was repeatedly done from an underway platform such as a moving ship or barge. Credit: Scripps Institution of Oceanography at UC San Diego.

Terrill’s team is now working to finalize the release of the sonar data, which they hope will serve as a catalyst for an action plan and additional research endeavors to understand environmental impacts.

There is a lot to be understood towards how DDT is impacting our environment and marine food webs, according to Scripps chemical oceanographer and professor of geosciences Lihini Aluwihare, who in 2015 co-authored a study that found high abundance of DDT and other man-made chemicals in the blubber of Bottlenose Dolphins that died of natural causes. 

“The uniquely high body burden of DDT in top predators feeding in Southern California waters has been known for some time. The extent of the dumping ground helps to explain some of these previous observations,” said Aluwihare, who was not part of the survey expedition. “These results also raise questions about the continued exposure and potential impacts on marine mammal health, especially in light of how DDT has been shown to have multi-generational impacts in humans. How this vast quantity of DDT in sediments has been transformed by seafloor communities over time, and the pathways by which DDT and its degraded products enter the water column food web are questions that remain to be explored.”