Second Seafloor Survey of Dumpsite Off Coast of Southern California Completed

Researchers mapped 135 square miles and found many discarded military munitions.

As part of ongoing efforts to understand the scale of the environmental impact of industrial waste dumping off the coast of Southern California, researchers from UC San Diego’s Scripps Institution of Oceanography revisited two industrial undersea dumpsites in April 2023 to identify objects discarded on the seafloor.

Led by Scripps oceanographers Sophia Merrifield and Eric Terrill, the 2023 survey used a deep water autonomous underwater vehicle (AUV) with state-of-art synthetic aperture sonar and a remotely-operated vehicle (ROV) with a high definition video camera, both capable of working up to full ocean depth of 6,000 meters (19,600 feet). The expedition took place with support from the U.S. Navy’s Supervisor of Salvage and the Office of Naval Research.

Between the 1930s and 1970s, the site was a known location for industrial dumping, including byproducts from the manufacturing of the pesticide DDT. It was initially surveyed using robotic vehicles by the same team in April 2021. The second survey aimed to extend seafloor maps using higher resolution acoustic sonar imaging techniques, applying video imaging systems to classify objects in a previously mapped debris field and collecting observations of deep-sea ocean currents. The 2023 survey mapped 350 square kilometers (135 square miles) and recorded more than 300 hours of video footage.

The 2021 survey, published in Environmental Science and Technology, documented thousands of barrel-sized objects organized in lines across the basin. Imagery collected in 2023 along debris lines found most objects to be multiple types of discarded military munitions and pyrotechnics. Barrels discarded due to industrial dumping and several old fishing vessels also were found. The barrels on the seafloor disposal were concentrated in two locations, not spread across the dump site.

“The resolution of the sonar provided by the U.S. Navy Supervisor of Salvage provides us an unprecedented map of the seabed which will take some time to fully appreciate and analyze,” said Terrill.

The U.S. Navy has received the 2021 and 2023 survey findings. According to a statement from the Navy, “These munitions are likely a result of World War II-era disposal practices. While disposal of munitions at sea at this location was approved to ensure safe disposal when naval vessels returned to U.S. ports, the Navy follows Department of Defense guidance for the appropriate disposal of munitions that aligns with state and federal rules and regulations.”

The Navy also will review the findings to determine the best path forward to ensure that the risk to human health and the environment is managed appropriately and within applicable federal and state laws and regulations.

Additionally, scientists mapped whale falls, which are sunken whale carcasses. Seven whale falls were confirmed with video imagery, but the sonar data suggests more than 60 may exist in the footprint of the survey data collected by the AUV. A whale fall is a term used to describe the ecological event that occurs when the carcass of a whale or other large marine mammal falls to the ocean floor. When a whale dies, its body eventually sinks to the seabed, providing a localized, concentrated source of organic matter in the deep ocean. Whale falls play a significant ecological role by supporting unique and specialized communities of organisms.

“The number of whale falls seems quite high relative to previous models of how many may occur on the seafloor off California,” said Scripps Oceanography marine biologist Greg Rouse, who has studied the ecosystems around whale falls. “However, the skeletons were mainly in very low oxygen water that likely slowed decomposition markedly and the burial rate by sediment may also be very slow there. This would mean the whale falls may have accumulated over many decades.”

The science team also deployed a seabed mooring at a location known as Dumpsite No. 2, outfitted with a deep ocean current meter and oxygen sensor. This mooring will measure seabed currents to help scientists better understand transport mechanisms that might impact the contaminated sediments.

“Our survey provides an opportunity to develop and apply analytical techniques to acoustic and optical imagery over wide areas,” said Merrifield, an observational physical oceanographer who specializes in ocean robotics. “We anticipate these datasets will inform additional studies addressing impacts of dumping activities on the marine food web.”

The survey was funded as part of a community project supported by two U.S. senators, the late Dianne Feinstein and Alex Padilla, and two represenatives, Mike Levin and Alan Lowenthal.

The National Ocean and Atmospheric Administration (NOAA) awarded $5.6 million to the project in 2022 to further characterize, monitor and research potential ecosystem impacts of the DDT dumpsite.  An additional $6 million, directed by Feinstein and Padilla, was awarded through NOAA in September to fully assess contamination from DDT and other pollutants along with bioremediation mitigation strategies.

The survey data follow ongoing studies by UC Santa Barbara’s David Valentine, who discovered concentrated DDT in the sediments in 2011 and 2013 and visually confirmed 60 barrels on the seafloor. Valentine is currently mapping DDT in sediments collected across the San Pedro Basin as part of the same project as the seafloor survey.

“The preliminary findings of our analysis of sediments are showing that bulk dumping of DDT acid waste was the norm, that DDT immediately entered the environment and was likely not in barrels,” said Valentine, who, in a 2019 study, characterized the disposal of DDT waste as inherently sloppy. “Once dumped, DDT spreads at the seafloor, expanding its footprint to at least the base of the Catalina slope. We are finding that original DDT remains abundant in the seafloor today, in absolute and relative terms.”

The Environmental Protection Agency (EPA) also has stated that most DDT found at offshore dumpsites was likely deposited through bulk-dumping rather than containerized barrels. The Merrifield and Terrill survey confirms the agency’s previous archive investigation based on historical shipping manifests and aerial photographs.

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