137Cs inter-plant concentration ratios provide a predictive tool for coral atolls with distinct benefits over transfer factors

Inter-plant concentration ratios (IPCR) [Bq g−1 137Cs in coral atoll tree food crops/Bq g−1 137Cs in leaves of native plant species whose roots share a common soil volume] can replace transfer factors (TF) to predict 137Cs concentration in tree food crops in a contaminated area with an aged source term. The IPCR strategy has significant benefits relative to TF strategy for such purposes in the atoll ecosystem. IPCR strategy applied to specific assessments takes advantage of the fact that tree roots naturally integrate 137Cs over large volumes of soil. Root absorption of 137Cs replaces large-scale, expensive soil sampling schemes to reduce variability in 137Cs concentration due to inhomogeneous radionuclide distribution. IPCR [drinking-coconut meat (DCM)/Scaevola (SCA) and Tournefortia (TOU) leaves (native trees growing on all atoll islands)] are log-normally distributed (LND) with geometric standard deviation (GSD) = 1.85. TF for DCM from Enewetak, Eneu, Rongelap and Bikini Atolls are LND with GSDs of 3.5, 3.0, 2.7, and 2.1, respectively. TF GSD for Rongelap copra coconut meat is 2.5. IPCR of Pandanus fruit to SCA and TOU leaves are LND with GSD = 1.7 while TF GSD is 2.1. Because IPCR variability is much lower than TF variability, relative sampling error of an IPCR field sample mean is up 6- to 10-fold lower than that of a TF sample mean if sample sizes are small (10–20). Other IPCR advantages are that plant leaf samples are collected and processed in far less time with much less effort and cost than soil samples.