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.

Behavior of Iodine in a Forest Plot, an Upland Field and a Paddy Field in the Upland Area of Tsukuba, Japan. Iodine Concentration in Precipitation, Irrigation Water, Ponding Water and Soil Water to a Depth of 2.5 m

“…For instance, in Andosols, the dissolution ratio of iodine was the lowest, at pH 4.0-5.5, while above or below these pH values, in particular, above 6.0, the ratio increased by 1-2 orders of magnitude (Yuita et al 1991). The pH of soil at a depth of 0.2 m in the upland field was 6.7, a value higher than the 5.7 and 5.9 range recorded at 0-0.06 m and 0.2 m depths in the forest plot, respectively, and it was higher than the 6.0 and 6.1 range recorded at 0.5 and 1.0 m depths in the upland field, respectively (Yuita and Kihou 2005). On the other hand, on the average, 6.4% of the naturally occumng iodine in 23 soils in the U.K. was desorbed with a diluted (0.016 M) KH,PO, solution (Whitehead 1973).…”

“…The study sites (a forest plot, an upland field and a paddy field) were located in the southern part of a diluvial upland area (river terrace) commonly referred to as the Tsukuba uplands, at the campus of the National Institute for Agro-Environmental Sciences (NIAES). The topography, land use, climate and vegetation of the study sites were previously reported (Yuita and Kihou 2005). Moreover, the soil profile, soil characteristics, geological stratigraphy and groundwater level of the study sites were also reported (Yuita and Kihou 2005).…”

“…The topography, land use, climate and vegetation of the study sites were previously reported (Yuita and Kihou 2005). Moreover, the soil profile, soil characteristics, geological stratigraphy and groundwater level of the study sites were also reported (Yuita and Kihou 2005).…”

“…In a previous paper (Yuita and Kihou 2005), the vertical distribution of iodine in the soil horizons/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 controlled by the oxidation-reduction conditions and the groundwater level of the soil horizonsflayers.…”

Biogeochemical Considerations Related To The Remediation Of I-129 Plumes

“…In one soil type, the soil solution concentration of organo-iodine increased 2.5 fold under flooded conditions. A very extensive sampling of iodine soil water from a forest, upland field, and rice paddy field in Japan revealed a strong negative relationship between soil Eh and soil water iodine concentrations (Yuita and Kihou, 2005). In particular, soil water iodine concentrations increase with decreasing Eh values below 200 mV.…”