Geotechnical, geological, and selected radionuclide retention characteristics of the radioactive waste disposal site near the farallon islands

Abstract

Abstract A geotechnical and geological investigation of the Farallon Islands low‐level radioactive waste (LLW) disposal area was conducted to qualitatively assess the host sediments’ relative effectiveness as a barrier to radionuclide migration, to estimate the portion of the barrier that is in contact with the waste packages at the three primary disposal sites, and to provide a basic physical description of the sediments. Box cores recovered from within the general disposal area at depths of 500, 1,000, and 1,500 m were subcored to provide samples (≍30 cm in length) for detailed descriptions, textural and mineralogical analyses, and a suite of geotechnical tests (index property, CRS consolidation, and CIU triaxial compression). The sediments are coarse, consisting chiefly of sands and silts, yet are highly plastic and compressible at the two deeper stations. Although they are very soft (vane shear strengths typically ≤ 1 kPa in the top 20 cm), all sediments exhibit overconsolidated behavior. The very low sediment shear strengths, absence of strong currents, and presence of sediment on upward‐facing surfaces of waste drums imply that the contemporary geological environment at the two deepest dumpsites is depositional. If systematic changes within and among the three samples represent areawide trends, then the sediments tend to be finer grained downslope and in the upper few centimeters. This could reflect a diminishing offshore influence of the coarse fraction source area, the Farallon Islands and surrounding shelf, and a recent relative increase in the sedimentation rate of fines, which enter the region primarily from the north. Although the geologic setting favors mass movement, there is no evidence of such an event since the waste canisters were delivered to the seafloor. A simplified provisional analysis suggests that the radionuclide retention potential of the sediments increases in the downslope direction. This trend is based on and reflects the more fundamental apparent trends of downslope increases in (1) the percentage of smectite and illite, which, due to their ion exchange capacity, have the highest retention potential with respect to two of the important radionuclides found in LLW (90Sr and 137Cs), and (2) the amount of predicted canister embedment, which allows greater use of the sediments as a barrier to radionuclide migration. Predictions of initial (dynamic) penetration indicate that a free‐falling, 55‐gallon drum of low‐level waste and matrix material would partially to completely embed itself at the 1,000 m or 1,500 m core sites and, therefore, presumably at the two deeper dumpsites. Subsequent settlement and/or deposition would increase the drum's contact area with the sediment and promote isolation of the LLW. Moreover, an absence of significant bioturbation implies that the effectiveness of the sediment as a barrier to radionuclide migration would not be seriously compromised by the activity of infauna.