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

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 concentration of stable iodine (127I) in precipitation, irrigation water and soil water to a depth of 2.5 m in a forest plot, an upland field and a paddy field in the upland area of Tsukuba, Japan, was determined. In the forest plot, the mean iodine concentrations in soil water at all the depths ranged from 0.13 to 0.21 μg L−1, about one‐tenth of the values recorded in precipitation (weighted mean 2.1 μg L−1). This finding suggests that the major part of iodine in precipitation was sorbed onto the surface soil horizon under oxidative conditions. In the upland field, the mean iodine concentration in soil water was 2.2 μg L−1 at a depth of 0.2 m and it decreased to 0.34–0.44 μg L−1 at a depth of 0.5 m or more; these concentrations were about one‐fifth of that in precipitation. This suggested that the major part of the iodine derived from precipitation was sorbed onto the subsurface soil horizon (at depths between 0.2 and 0.5 m). In the paddy field, during the non‐irrigation period, the mean iodine concentrations in soil water at all the depths ranged from 1.8 to 4.8 μg L−1, almost the same values as those recorded in precipitation. During the irrigation period, the mean iodine concentrations at depths of 0.2 and 0.5 m were 18.8 and 16.7 μg L−1, values higher than the 10.9 μg L−1 value recorded in irrigation water and the 11.8 μg L−1 value recorded in ponding water. However, at a depth of 1.0 m or more, the mean iodine concentrations in soil water rapidly decreased from 7.3 to 1.8 μg L−1. These data suggested that a significant amount of iodine flowed out from the paddy field by surface runoff and a considerable amount of iodine that leached to a depth of 0.5 m was retained onto the mildly oxidative soil horizon (2Bw) that lay at depths between 0.5 and 1.0 m. At a depth of 2.5 m in the paddy field, the mean iodine concentration in soil water decreased to 1.8 μg L−1, but this level was much higher than those in the forest plot and upland field at the same depth, which suggested that a significant amount of iodine had leached into the groundwater‐bearing layer. There was a negative correlation (r=‐0.889) between the Eh of soil and the iodine concentration in soil water (0.2 m depth) of the paddy field. Particularly, when the Eh of soil fell below approximately 150 mV, the iodine concentration rapidly increased to above 10μg L−1. As for the chemical forms of iodine in precipitation, irrigation water, ponding water and soil water during the winter irrigation period in the paddy field with oxidative conditions, 58–82% of iodine consisted of IO−3 and 17–42% of iodine consisted of I−. In the soil water during the summer irrigation period in the paddy field under reductive conditions, 52–58% of iodine consisted of I−, and 42–47% consisted of IO−3.

Behavior of iodine in a forest plot, an upland field and a paddy field in the upland area of Tsukuba, Japan: Seasonal variations in iodine concentration in precipitation and soil water and estimation of the annual iodine accumulative amount in soil horizons

“…The topography, land use, climate, vegetation, soil profile, soil characteristics, geologic stratigraphy and groundwater levels of the study sites were previously reported (Yuita and Kihou 2005). The crop cultivation and irrigation methods in the paddy field and upland field have been recorded previously (Yuita et al. 2005).…”

“…2005). The methods for the collection and preparation of the precipitation, irrigation water, ponding water, soil water and underdrainage water samples were also reported previously (Yuita et al. 2005).…”

“…The total iodine concentration in water samples was determined using inductively coupled plasma mass spectrometry (ICP‐MS) (Yuita et al. 2005).…”

“…The methods for the measurement of the redox potential (E h ) and groundwater level in the paddy field were previously reported (Yuita and Kihou 2005; Yuita et al. 2005).…”

“…The present study is one in a series of studies conducted to examine the long‐term behavior of 129 I, which has a half‐life of 16 million years, in the environment. In a previous study (Yuita et al. 2005) we determined the mean concentration of stable iodine in precipitation, irrigation water, ponding water and soil water to a depth of 2.5 m in a forest plot, an upland field and a paddy field in the upland area of Tsukuba.…”