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

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, seasonal variation in the concentration of stable iodine (127I) in precipitation 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, were determined. Iodine concentration in precipitation tended to increase during the summer (high air temperature) season and low‐rainfall period, and a positive high correlation was observed between annual rainfall and the annual amount of iodine supplied by precipitation. No seasonal variations in iodine concentration in soil water were observed at any depth in the forest plot and upland field unlike at shallow depths (0.2 and 0.5 m) in the paddy field. In the paddy field, from the beginning of summer irrigation, under flooding conditions, iodine concentration in soil water at shallow depths (0.2 and 0.5 m) continuously increased, and immediately before mid‐summer (intermittent) drainage and drainage, the maximum iodine concentration (approximately 50 µg L−1) and lowest Eh values (approximately −150 to −200 mV) were recorded. These high iodine concentration levels and low Eh values were ascribed to high air temperature (approximately > 25°C on average every 10 days) and the continuation of the groundwater level above the ground surface. As for the temporary winter irrigation period (mean daily air temperature 2−4°C), the iodine concentration was low (1.7–3.7 µg L−1) at all depths, as was the case in the non‐irrigation period. After mid‐summer drainage, and drainage, the iodine concentration in soil water at depths of 0.2 and 0.5 m decreased drastically as the groundwater level decreased. The mean annual amount of iodine accumulated in the surface soil horizons (0–0.67 m) in the forest plot was estimated to be approximately 2.9 mg m−2 (7.5 µg kg−1 dry soil), which coincided with the mean annual amount of iodine supplied to the earth surface by precipitation. A mildly oxidative subsurface 2Bw horizon (0.60–0.89 m) in the paddy field was estimated to illuviate approximately 3.1 mg m−2 (20 µg kg−1 dry soil) of iodine annually by retaining iodine in the soil water percolated to this horizon.

The role of exogenous humic and fulvic acids in iodine biofortification in spinach (Spinacia oleracea L.)

“…This might suggest that the introduction of iodine into soil with growing plants promotes the formation of iodine compounds that are soluble in 0.03 M acetic acid. Yuita et al (2006) reported an increase in the iodine content in soil during a spring season that was followed by an increase of air temperature and soil humidity after winter.…”

Feasibility of 129I groundwater dating calibrated by both 81Kr and 4He for the assessment of deep geological repositories in Japan

“…The iodine in the groundwater in the Yuchi Formation has three origins: (1) seawater and lagoon water from when the coastal sediment was deposited, (2) infiltrated meteoric water from before 0.04 Ma (corresponding to the 81 Kr age in groundwater at the Upper Yuchi Formation in Table 1 ), and (3) iodine released from organic matter in the sediment. The concentrations of 127 I supplied from meteoric water and seawater are only several µg L -1 (Yuita et al 28 ) and ~ 58 µg L -1 (Elderfield and Truesdale 29 ), respectively. Iodine is easily adsorbed on sediments 30 , 31 and tends to accumulate in organic matter 32 , cells of marine diatoms 33 and humic substances 34 .…”

Transfers of Iodine in the Soil–Plant–Air System

“…For example, Johnson (1980) analyzed 183 soil samples from the United Kingdom and found that water-extractable values could be  0.1 mg · kg 1 , representing  0.1% of total soil iodine. Yuita et al ( , 2006 determined concentrations of iodine naturally present in soil solutions of forest, upland, and paddy field soils of Japan. Similarly, Hou et al (2003) found that readily available (the most environmentally mobile) 129 I (water-soluble and exchangeable fractions) accounted for around 20% of total 129 I in Chernobyl soil and around 10% of total 129 I in Irish Sea sediment ( Figure 11.2).…”

“…Yuita et al (2006) found that drainage of saturated, anoxic paddy field soil led to reduction in iodine concentration of surface soil due to leaching of soluble iodine with the drainage waters. Yuita et al (2006) found that drainage of saturated, anoxic paddy field soil led to reduction in iodine concentration of surface soil due to leaching of soluble iodine with the drainage waters.…”