Caesium-137 soil sampling and inventory variability in reference locations: A literature survey

Soil sampling design, the number of samples collected and the lateral variation of caesium-137 (137Cs) in uneroded reference locations were extracted from previously published work. The focus was on published work which used 137Cs reference inventory (Bq m−2) for qualitative or quantitative estimation of sediment redistribution (SRD) within the landscape. The objective of this study was to address one of the methodological concerns facing the 137Cs technique—that is, the lack of a rigorous statistical treatment of reference locations. The limited attention paid to the reference location is not justified as ‘true’ estimates of SRD are based on the assumption of an unbiased, independent, random probability sample estimate, commonly the arithmetic mean. Results from the literature survey indicated that only 11% of the reference locations sampled for 137Cs expressly stated that a probability sampling design was used (transect or systematic-aligned grid). The remaining locations were generally sampled using a non-probability based design, more commonly known as haphazard sampling. Of the 75 reference study areas identified only 40 provided enough information to determine the dispersion around the mean, and from this the coefficient of variation (CV) was calculated for all available data. The median CV was 19·3%, with 95% confidence limits of 13·0–23.4%, indicating that approximately 11 random, independent samples would generally be necessary to adequately quantify the reference 137Cs area activity with an allowable error of 10% at 90% confidence. Further analysis indicated that only one-third of the studies sampled a sufficient number of 137Cs reference locations. This value would actually be lower as sampling frameworks were based on non-probability sampling procedures. For 137Cs reference locations it is recommended that a probability sampling design be utilized, preferably the systematic-aligned grid method, and as a minimum first-order estimate about 11 samples should be collected for inventory estimates.