Distribution and Fate of Contaminated Sea-floor Sediment on the Shelf Offshore Los Angeles

Historical background

One of the world's largest producers of DDT was connected to the Los Angeles County sewer system during the 1950's and 1960's. A significant quantity of DDT passed through the sewer system and out the Whites Point outfall into the ocean off the Palos Verdes Peninsula. Several other industries allowed PCBs to pass into the same sewer system, and these substances were also discharged into the marine environment near Palos Verdes.

These contaminants, DDT and PCBs, became associated with organic matter and other solids in the outfall, as well as ordinary sediment particles, to form a contaminated sediment deposit on the continental shelf and slope. These contaminants seriously impacted, and continue to impact, sediment dwelling organisms, fish, and birds.

The largest quantites of DDT discharged to the sewer system by the DDT producer occurred up until the early 1970's, and sediment deposited subsequently is less contaminated with DDT. However, biological and physical mixing processes continue to transfer older, highly contaminated sediment to the sea-floor surface, maintaining a risk to the environment.

Overview

Between 1992 and 1994 the USGS conducted surveys off the southern coast of the Palos Verdes Peninsula. On the Palos Verdes margin, sediment and sediment transport processes are heavily impacted by the Whites Point Outfall of the Los Angeles County Sanitation Districts, which discharges sewage at a water depth of 60 m on the continental shelf. A sediment deposit, derived in part from sewage effluent, has accumulated on the shelf and slope and is contaminated with a number of pollutants, including DDT and PCBs. The purpose of the USGS work was to map the areal and subbottom distribution of DDT and PCBs and to predict the future distribution of these contaminants if no remediation efforts were initiated. A multidisciplinary, multiorganizational project was developed.

Sidescan sonar, high resolution subbottom profiling, and ultra high resolution (chirp) profiling was conducted over a large area in the vicinity of the Palos Verde Peninsula. The area surveyed includes a complete sediment transport cell, extending from near the outfall pipes in the vicinity of Point Fermin, past Point Vicinte, into Redondo Canyon and out onto Redondo Fan within the San Pedro Basin.

In addition to the surveys mentioned previously a seafloor camera-sled survey was conducted on parts of the Palos Verdes continental margin to image the surficial character of effluent-affected sediment deposited on the seafloor near the White's Point outfall.

In addition to the previously mentioned surveys, seventeen 6-meter long vibracores were collected on the shelf between Point Fermin and Redondo Canyon. The cores have been logged for physical properties, split and described. Box cores and gravity cores were taken at 70 sites on the shelf, slope and basin. These cores, which mostly sample contaminated sediment, were logged and tested for contaminants, organic carbon, and texture. Many samples were also taken onshore from beaches on the peninsula.

Prior to this study, there was only a limited amount of information about the likely pathways for transport of the contaminated sediment because historic measurements of environmental influences were limited.

Long-term measurements of currents, wind stress and the associated wave climate were needed so that reliable models that predict the movement, dispersal and eventual fate of contaminated sediment could be developed. Hence, four moorings and an associated benthic tripod were deployed on the shelf and upper slope to measure the vertical and horizontal structure of currents, temperature, salinity, water clarity and wave climate for periods from several months to a year. Predictions of the fate of polluted sediment were developed from numerical models that used these data.

Some of our results:

• Acoustic profiling clearly showed a 20- to 60-cm thick low-density sediment layer extending over much of the 3 km by 10 km continental shelf.
• Physical property testing of cores confirmed the presence of a 20- to 60-cm thick low-density sediment layer on the shelf and also showed that the layer extended well down the 3 km by 10 km continental slope.
• Chemical analyses confirmed that virtually the entire sediment is contaminated with DDT and PCBs.
• Results indicate that at least 100 tons of DDT are present on the Palos Verdes margin, and that significant amounts of DDT and PCBs will remain near the sediment surface and thus remain biologically available well into the next century.
• The effluent-affected deposit covers an area greater than 40 square kilometers. The volume of the contaminated sediment exceeds 9 million cubic meters.

Other institutions involved:

• University of Virginia
• Woods Hole Oceanographic Institution
• University of Southern California
• Battelle Organization
• Arthur D. Little Company

Funding for this study was provided by the National Oceanic and Atmospheric Administration (NOAA).

Several publications are available as a result of this study.

The research vessel Farnella was one of the many vessels used for this project.

For additional information, contact Homa Lee