Behavior of Iodine in a Forest Plot, an Upland Field, and a Paddy Field in the Upland Area of Tsukuba, Japan: Vertical Distribution of Iodine in Soil Horizons and Layers to a Depth of 50 m

Abstract

In the course of a series of studies conducted to investigate the long-term behavior of 129I (which has a half-life of 16 million years) in the environment, the vertical distribution of stable iodine (127I) in the soil horizons and layers to a depth of 50 m was determined in a forest plot, an upland field, and a paddy field in the upland area of Tsukuba, Japan. The iodine content was strongly controlled by the groundwater level and by the oxidation-reduction conditions of the soil horizons/layers. In the forest plot and the upland field, the iodine content was the highest in the oxidative surface horizons (0-0.3 m), ranging from 42 to 71 mg kg-1. The iodine content decreased with increasing depth, falling sharply at depths below the mean water table (depth of about 2.5 m) to a low value of around 0.1 mg kg-1 at the lowest depth of the Joso Clay layer (first aquiclude) (depth of about 5.0 m). In the paddy field, the surface Apg horizon (0-0.18 m) showed a low iodine content of 2.8 mg kg-1, with eluviation of iodine under strongly reductive conditions. The mildly oxidative 2Bw horizon (0.60-0.89 m) showed the highest iodine content (12.0 mg kg-1) among the horizons in the paddy field, with illuviation of iodine. At depths below the mean water table during the non-irrigation period (depth of about 1.25 m), the iodine content decreased with increasing depth to a low value of 0.09 mg kg-1 at the lowest depth of the Joso Clay layer (depth of about 4.3 m), as in the forest plot and the upland field. Below the Joso Clay layer, the iodine content at all the sites was, without exception, lower than that in the corresponding vadose oxidative layers. The upper layers of the Narita Formation (second aquiclude) at all the sites showed the highest iodine content (0.5-5.0 mg kg-1) below the Joso Clay layer, but these values were almost the same or lower than those of the reductive surface horizons (0-0.6 m) in the paddy field. In another layers, the iodine content ranged from 0.01 to 1.0 mg kg-1, the lowest content recorded to a depth of 50 m at all the sites.