Environmental radionuclides ( 40 K, 137 Cs, 210 Pb, 226 Ra) were determined in soils under semi-arid stands, a transition area from thorn to deciduous forest in El Angolo, and heath stands in the Alto Mayo in Northern Peru in order to elucidate their activity level in soil, and to trace underground environment in remote areas of little anthropogenic intervention. Anthropogenic 137 Cs was only found in the uppermost portion of the soils in small amount, whereas the 210 Pb activity was exceptionally high at the soil surfaces in a tropical stand of Alto Mayo. The results suggest a fallout nuclide, 210 Pb, having been transported from local source(s) not to be specified yet.

Depth profiles potassium and its isotope ratio ( 40 K/K) were investigated in several forest soils from different geological and climatic backgrounds. The ratio was found not to be constant (0.0117 %) but varied appreciably from 0.0085 to 0.0167 % among different soils.Possible factors affecting variability of 40 K/K values were considered by a multivariate factor analysis which suggested that aqueous parameters like humidity and pH might be a controlling factor in the surface soils. The amount (and probably chemical forms) of iron oxide may be another one influencing 40 K/K values in mineral components at deeper portions.

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