INTRODUCTION

Between 1946 and 1970, approximately 47,800 55-gallon drums, concrete blocks, and other containers of low-level radioactive waste were reportedly dumped offshore of San Francisco Bay. Three sites on the continental shelf and slope adjacent to the Farallon Islands were designated for the disposal of the waste (Joseph et al., 1971; NOAA, 1990; Noshkin et al., 1978; Waldichuk, 1960). According to the records, approximately 150 drums were deposited in a water depth of about 90 m, 3,600 in a water depth of about 900 m, and 44,000 in a nominal water depth of 1,800 m. These sites are referred to here as the 90-m, 900-m, and 1,800-m sites, although the actual water depths at and around each site vary from the nominal values.

In reality, many of the drums were probably not disposed of at the specific sites. It is more likely that they litter a 1,400-km² area of sea floor, the Farallon Island Radioactive Waste Dump (Noshkin et al., 1978), defined by the irregular polygon on figure. 1.

Excluding tritium, an estimated total of some 540 TBq (14,500 Ci) of thorium, uranium, transuranic, and other activation products, and mixed fission products were disposed of at the three locations (Joseph et al., 1971; Waldichuk, 1960). The precise amounts of individual radionuclides are not known. In addition to radioactive wastes, other wastes, including phenols, cyanides, mercury, beryllium and other heavy metals, dredge spoils, explosives, and garbage were also dumped in and around the area (NOAA, 1990).

Much of the Farallon Island Radioactive Waste Dump lies within the boundaries of the Gulf of the Farallones National Marine Sanctuary, which was designated in 1981 (fig. 1). In 1990, the U.S. Geological Survey (USGS) and the Gulf of the Farallones National Marine Sanctuary jointly surveyed part of the Farallon Island Radioactive Waste Dump with a sidescan-sonar system. The result of this survey (Karl et al., 1994), following subsequent computer enhancement of the data to assist in the identification of barrels, was the mapping of barrel locations over an area of 125 km². The identification of the barrels was verified during the initial survey using an underwater camera/video and subsequently using the manned Navy submersible Sea Cliff and the unmanned Advanced Tethered Vehicle. Visual observations showed the barrels to be in all states of preservation, ranging from completely intact to completely disintegrated.

The Gulf of the Farallones and adjacent areas support a major commercial fishery. The area is also used extensively for sport fishing (NOAA, 1990). Fears of radioactive contamination have previously had an adverse impact on the fishery (E. Ueber, personal communication).

Discussions between the USGS and the British Geological Survey (BGS) led to a proposal to carry out a radioactivity survey of parts of the Farallon Islands Radioactive Waste Dump Site (fig. 1). This was done using the proven BGS towed seabed spectrometer (EEL) system (e.g., Jones, 1994; Miller et al., 1977), modified to operate in deeper water (Deep Tow EEL). The main aim was to obtain regional-scale information on sea floor radioactivity levels for parts of the site, particularly with reference to USGS mapping of barrel locations using sidescan sonar data (Karl et al., 1994). Previous radioactivity measurements had been restricted to the analyses of samples of water, sediments and biota (Dyer, 1976; Noshkin et al., 1978; PneumoDynamics Corporation, 1961; Schell and Sugai, 1980; Suchanek et al., 1996). In the case of the sediments, only a relatively small number of sites had been sampled, although some samples were collected from areas adjacent to barrels using submersibles (Dyer, 1976; Schell and Sugai, 1980).

The survey was carried out in April-May 1998 on the NOAA ship McArthur and involved interagency collaboration between the BGS, USGS, U.S. Environmental Protection Agency, National Oceanographic and Atmospheric Administration, and Gulf of the Farallones National Marine Sanctuary. Work was concentrated on the shallower parts of the site where commercial fishing occurs and where locations of barrels had been mapped.