A study of 137Cs in soil profiles from the Marshall Islands

In late 1989, the Republic of the Marshall Islands commissioned an independent radiological monitoring program to determine the degree of deposition and the geographical extent of atomic weapons test fallout over its islands. The sampling and measurement of 137Cs in vertical soil profiles has been an important part of that monitoring program. Over 200 profiles were collected in 5-cm increments to a depth of 30 cm. The sampling sites included locations at all 29 of the nationʹs atolls and five separate reef islands. The nation is unique in that a rainfall gradient exists from south to north, the northern islands receiving approximately one-third the annual precipitation of the southern islands. Annual rainfall and soil particle size distribution was studied to determine the inter-relationships with cesium permeation and areal inventory (Bq/m2). Relaxation lengths of cesium in the soil column were calculated from measurements of 137Cs activity in profile increments. Using a common model for a depth-dependent concentration gradient (C(d) = Co e−αd), the inverse of the rate of change of concentration (i.e. 1/α) has been defined as the relaxation length of the profile. In this study, the relaxation length was calculated from the concentration data of the first three sample increments (i.e. 0–5, 5–10, 10–15 cm) and from all six increments to a total depth of 30 cm. Relaxation lengths with r2 values ≥0.90 were used for data analysis. Typical values for relaxation lengths in the drier northern atolls are 7–12 cm, though our observations included values between −800 and 3500 cm. Marshall Islands soils are primarily calcium carbonate, and with the absence of clay material, 137Cs permeation was expected to increase with increasing rainfall and increasing particle size. Our findings showed that 137Cs permeation was highly variable with particle size, though relaxation length was significantly correlated with annual rainfall. Areal inventory was determined to be significantly correlated with the 0–5 cm 137Cs soil concentration. The 0–30 cm fitted relaxation length was also determined to give a good estimate of the areal inventory of 137Cs in the soil to a depth of 30 cm regardless of the rate of change in concentration with depth.