Radioactive contamination of tropical rainforest soils in Southern Costa Rica

Radionuclide content in soils from four locations in a tropical rainforest near Golfito in Southern Costa Rica was investigated. For comparison, two n earby locations in open grassland were also studied. From each site 5 soil cores down to a depth of 15 cm were taken. The median contamination with Cs -137 was 584 Bq m(-2) (reference date 1 January 1996) and the coefficient o f variation (CV) was 50%. This contamination can be attributed to global fa llout from atmospheric nuclear weapon tests between 1945 and 1980. The mean contamination is slightly lower than the value expected for the latitude ( 8 degrees 42': 700 Bq m(-2)), which may be explained by migration of radioc aesium to subsoil below 15 cm or by uptake into the living biomass. Out of the total variability of 50%, around 20% can be attributed to the sampling and measuring process uncertainties, thus leaving a 45% contribution of spa tial variability. A significant difference between forest and meadow sites could be detected: the meadow sites showed lower radiocaesium soil inventor ies (median: 291 Bq m(-2)) than the forest sites (643 Bq m(-2)). This may b e explained by the agricultural activities carried out on meadow sites whic h lead to an increased redistribution of caesium in the soil profile and th erefore a larger fraction of the total Cs-137 lying below 15 cm. Another re ason for higher contamination levels under forest can be attributed to the high interception potential of dense tree canopies for dry deposition. Extr apolating the Cs-137 concentration below the sampling horizon, i,e. account ing for the cut-off of the profiles by the sampling technique, results in a n estimated mean of 710 Bq m(-2) for the forest sites, which is very close to the expected figure. The mainly mineral part of the forest soil profiles was analysed for the Cs-137 transport parameters, apparent convection velo city (v = 0.14 +/- 0.09 cm a(-1)) and apparent diffusion constant (D = 0.79 +/- 0.49 cm(2) a(-1)). The maximum concentration can be found at 5.3 +/- 2 .9 cm depth, the half-value depth being 7.4 +/- 1.3 cm. The mean K-40 activity concentration was 175 Bq kg(-1) dry matter (CV = 69% ) and Ra-226 and Ra-228 concentrations of 9.90 Bq kg(-1) (CV = 23%) and 7.9 3 Bq kg(-1) (CV = 20%) have been found, respectively. (C) 2001 Elsevier Sci ence Ltd. Air rights reserved.