Results of 1999 Spectral Gamma-Ray and Neutron Moisture Monitoring of Boreholes at Specific Retention Facilities in the 200 East Area, Hanford Site

Twenty-eight wells and boreholes in the 200 East Are% Hanford Site, Washington were monitored in 1999. The monitored facilities were past-practice liquid waste disposal facilities and consisted of six cribs and nineteen ''specific retention'' cribs and trenches. Monitoring consisted of spectral gamma-ray and neutron moisture logging. All data are included in Appendix B. The isotopes 137Cs, 60Co, 235U, 238U, and 154Eu were identified on spectral gamma logs from boreholes monitoring the PUREX specific retention facilities; the isotopes 137Cs, 60Co, 125Sb, and 154Eu were identified on the logs from boreholes at the BC Controlled Area cribs and trenches; and 137Cs, 60Co, and 125Sb were, identified on the logs from boreholes at the BX specific retention trenches. Three boreholes in the BC Controlled Area and one at the BX trenches had previous spectral gamma logs available for comparison with 1999 logs. Two of those logs showed that changes in the subsurface distribution of 137CS and/or 60Co had occurred since 1992. Although the changes are not great, they do point to continued movement of contaminants in the vadose zone. The logs obtained in 1999 create a larger baseline for comparison with future logs. Numerous historical gross gamma logs exist from most of the boreholes logged. Qualitative comparison of those logs with the 1999 logs show many substantial changes, most of which reflect the decay of deeper short-lived isotopes, such as 106Ru and 125Sb, and the much slower decay of shallower and longer-lived isotopes such as 137Cs. The radionuclides 137Cs and 60Co have moved in two boreholes since 1992. Given the amount of movement and the half-lives of the isotopes, it is expected that they will decay to insignificant amounts before reaching groundwater. However, gamma ray logging cannot detect many of the contaminants of interest such as 99Tc, NO3, or 129I, all of which can be highly mobile in the vadose zone and, for the radionuclides, have long half-lives