Effective Half-Life of Caesium-137 in Various Environmental Media at the Savannah River Site

During the operational history of the Savannah River Site (SRS), many different radionuclides have been released from site facilities into the SRS environment. However, only a relatively small number of pathways, most importantly ¹³⁷Cs in fish and deer, have contributed significantly to doses and risks to the public. The "effective" half-lives (T_e) of ¹³⁷Cs (which include both physical decay and environmental dispersion) in Savannah River floodplain soil and vegetation and in fish and white-tailed deer from the SRS were estimated using long-term monitoring data. For 1974-2011, the T_es of ¹³⁷Cs in Savannah River floodplain soil and vegetation were 17.0 years (95% CI=14.2-19.9) and 13.4 years (95% CI=10.8-16.0), respectively. These T_es were greater than in a previous study that used data collected only through 2005 as a likely result of changes in the flood regime of the Savannah River. Field analyses of ¹³⁷Cs concentrations in deer collected during yearly controlled hunts at the SRS indicated an overall T_e of 15.9 years (95% CI=12.3-19.6) for 1965-2011; however, the T_e for 1990-2011 was significantly shorter (11.8 years, 95% CI=4.8-18.8) due to an increase in the rate of ¹³⁷Cs removal. The shortest T_es were for fish in SRS streams and the Savannah River (3.5-9.0 years), where dilution and dispersal resulted in rapid ¹³⁷Cs removal. Long-term data show that T_es are significantly shorter than the physical half-life of ¹³⁷Cs in the SRS environment but that they can change over time. Therefore, it is desirable have a long period of record for calculating T_es and risky to extrapolate T_es beyond this period unless the processes governing ¹³⁷Cs removal are clearly understood.