Study: The atmospheric transport of iodine-129 from Fukushima to British Columbia, Canada and its deposition and transport into groundwater

“…dilution and dispersion have reduced 129I contamination as further inputs have ceased.”

Where are the authors getting their information on Fukushima?

From American Geophysical Union
Water Resources Research  Journal

First published: 17 December 2015
DOI: 10.1002/2015WR017325


The Fukushima-Daiichi nuclear accident (FDNA) released iodine-129 (15.7 million year half-life) and other fission product radionuclides into the environment in the spring and summer of 2011. 129I is recognized as a useful tracer for the short-lived radiohazard 131I, which has a mobile geochemical behavior with potential to contaminate water resources. To trace 129I released by the FDNA reaching Canada, pre-accident and post-accident rain samples collected in Vancouver, on Saturna Island and from the National Atmospheric Deposition Program in Washington State were measured. Groundwater from the Abbotsford-Sumas Aquifer was sampled to determine the fate of 129I that infiltrates below the root zone. Modeling of vadose zone transport was performed to constrain the travel time and retardation of 129I. The mean pre-accident 129I concentration in rain was 31 × 106 atoms/L (n = 4). Immediately following the FDNA, 129I values increased to 211 × 106 atoms/L and quickly returned to near-background levels. However, pulses of elevated 129I continued for several months. The increases in 129I concentrations from both Vancouver and Saturna Island were synchronized, and occurred directly after the initial release from the FDNA. The 129I in shallow (3H/3He age <1.4 years) Wassenaar et al. (2006) groundwater showed measurable variability through March 2013 with an average of 3.2 × 106 atoms/L (n = 32) that was coincident with modeled travel times for Fukushima 129I. The groundwater response and the modeling results suggest that 129I was partially attenuated in soil, which is consistent with its geochemical behavior; however, we conclude that the measured variability may be due to Fukushima 129I entering groundwater.

1 Introduction

On 11 March 2011, a magnitude 9.0 earthquake occurred near Tōhoku, Japan, causing a tsunami that damaged the Fukushima-Daiichi Nuclear Power Plant (FDNPP) and released radionuclides to the ocean and atmosphere. The primary radionuclides released were 131I (half-life: 8 days), 134Cs (half-life: 2 years) and 137Cs (half-life: 30 years). However, numerous other fissionogenic isotopes were also released and are measurable in the environment including iodine-129 (half-life: 15.7 × 106 years) [Steinhauser, 2014]. The total quantity of 131I released to the atmosphere has been estimated between 120 and 200 PBq and using this number the quantity of 129I can be calculated from the 129I/131I fissionogenic ratio, which was estimated to be 31.6 [Miyake et al., 2012]. This calculation yields a range of atmospheric 129I released from 0.81 to 1.4 kg [Tokyo Electric Power Company, 2012]. This quantity, while significant, is far less than the annual gaseous releases from nuclear fuel reprocessing, which have been estimated at 6–12 kg/yr [Moran et al., 1999]. However, the nature of the releases from the Fukushima-Daiichi Nuclear accident (FDNA), which were isolated in time and space, provide a rare opportunity to study the transport of 129I and its environmental cycling and distinguish them from other releases of 129I from nuclear fuel reprocessing and past bomb testing.

Radionuclides released from the FDNA have been detected across the globe [Bikit et al., 2012; Evrard et al., 2012; Landis et al., 2012; Macmullin et al., 2012; Melgunov et al., 2012; Piñero García and Ferro García, 2012; Wetherbee et al., 2012]. These radionuclides were transported primarily in the atmosphere and deposited by both dry deposition and washout [Wetherbee et al., 2012]. Atmospheric transport and deposition via precipitation of 129I has been well studied in the context of releases from nuclear fuel reprocessing sites such as La Hague and Sellafield, and Paul et al. investigated the dispersion and wet deposition of 129I from the Chernobyl accident [Paul et al., 1987; Persson et al., 2007; Aldahan et al., 2009]. Iodine has been shown to have an atmospheric residence time of 10–14 days, which allows releases of 129I and 131I to travel great distances from their point source [Jabbar et al., 2013]. It has been assumed that the fate of 129I deposited on the ground surface is to mix and distribute throughout the environment and a large amount of research has focused on the terrestrial cycling of iodine in surficial environments such as soil, rivers, and plants [Bamba et al., 2014; Muramatsu et al., 2015]. However, a few studies have investigated the infiltration of 129I into groundwater. Modern recharge waters in the nearby Milk River Aquifer, Alberta, showed meteoric values for 129I concentration and the 129I/127I ratio of 8.6 × 105 atoms/L and 1.1 × 10−11, respectively [Fabryka-Martin et al., 1991]. Young groundwater from the Orange County Aquifer in southern California had 129I concentrations that ranged from 12–53 × 106 atoms/L and 129I/127I ratios of 0.9 to 4.5 × 10−10 in samples with groundwater ages of less than 2 years [Schwehr et al., 2005].

129I has shown promise as a conservative, long lived, tracer of groundwater [Schwehr et al., 2005], although its attenuation during recharge through the soil must be considered. A wide variety of Kd values for iodine have been measured in many different soil types and due to the biophilic nature of iodine, soils that have high organic matter content tend to be very efficient at adsorbing iodine at ambient concentrations with Kd values as high 1800 L/kg in peat [Sheppard et al., 1996; Zhang et al., 2011]. However, in soils with low organic matter iodine has been observed to behave semi-conservatively and have Kd values as low as 0.1 L/kg in the I state [Alvarado-Quiroz et al., 2002; Schwehr et al., 2005]. Typically, Kd values for iodine fall in the range of 0.2–35 L/kg for sandy soils [Söderlund et al., 2011]. Iodide, I, has lower Kd values than the iodate inline imageIO3− [Fukui et al., 1996] and the Kd decreases for both iodide and iodate at neutral to alkaline pH [Fukui et al., 1996]. Studies of 129I migration and retardation in groundwater have been conducted at the 129I contaminated Hanford and Savannah River nuclear sites. These nuclear fuel reprocessing plants, which are now closed, were point sources of 129I in the region in the past. Results have shown that the mobility of 129I contamination in the subsurface is dependent on the iodine concentration, with high concentrations having lower Kd‘s as well as other factors such as organic content, speciation and pH. The most important of these is the organic content [Hu et al., 2005, 2012; Zhang et al., 2011]. Studies of groundwater from the Idaho Falls nuclear site have also shown that 129I can be transported vertically and laterally great distances within the aquifer. However, dilution and dispersion have reduced 129I contamination as further inputs have ceased [Bartholomay, 2013].

The objective of this study was to trace the fate of the FDNA 129I release through precipitation and groundwater recharge in a well-characterized sandy aquifer on the west coast of Canada. Archived pre-FDNA precipitation samples were obtained and weekly precipitation samples were collected for 1 year immediately following the FDNA. We then investigated the fate of the 129I deposited and its attenuation during infiltration into the shallow, sandy aquifer. Sampling of two different well sites with different recharge times was conducted on a monthly to bi-monthly basis over the course of a year. The recharge time of the wells was established by 3H/3He dating to be 11 and 14 months, respectively. This aspect of the work simulates an aquifer scale tracer experiment and provides insight into the long term environmental behavior of 129I in the hydrosphere and contaminant travel times in the vadose zone of the aquifer.

4 Summary and Conclusions

The atmospheric transport of 129I from the Fukushima-Daichii Nuclear Accident and its deposition via precipitation has been quantified for a year following the accident using weekly precipitation sampling. The transport of 129I has also been traced into local groundwater to elucidate the long term fate of the 129I. The data show that FDNA 129I was rapidly transported to the west coast of North America and was first detected on the week of 17 March 2011 within 6–8 days of the accident. This pulse is corroborated by samples taken from three sampling locations: Vancouver, Saturna Island, and the NADP site WA-19. The peak magnitude of the FDNA pulse, 211 × 106 atoms 129I/L, was approximately 7 times above pre-accident background of 31 × 106 atoms/L. However, 129I concentrations returned to background levels within a few days suggesting relatively little mixing of 129I between air masses during trans-Pacific transport. Several precipitation events with 129I concentrations substantially higher than background were also observed throughout the year including one in July with a concentration even higher than that measured directly following the FDNA. These peaks occurred after most atmospheric radionuclide monitoring for FDNA fission products has ceased and their cause is likely due to a combination of natural processes such as resuspension from the ground surface, ocean volatilization and atmospheric concentration due to drought periods as well as releases from nuclear fuel reprocessing.

The peak 129I mass flux was observed immediately following the FDNA and in the same sampling period that contained the peak 129I concentration. The 129I concentration is the principle factor determining washout. The correlation coefficient between the 129I concentration and washout is greater than that for the precipitation quantity.

Groundwater 129I concentrations in two nearby wells showed minor anomalies over the sampling period which could be due to rapid infiltration of the FDNA atmospheric 129I signal. Vadose zone modeling shows that it was possible for a component of the 129I deposited by the FDNA to be conducted rapidly from the ground surface to the water table along preferential flow paths with little retardation such that anomalies within the input signal can be observed at the water table. The balance of the input 129I, in contrast, traveled through the upper soil with a Kd value of ∼15 L/kg where variable input concentrations were attenuated.

We conclude that it is possible that a fraction of 129I from the FDNA is transported conservatively in this aquifer via preferential flow paths to the water table while the remaining 129I in the system interacts with soil and its transport is retarded within the vadose zone. The result is that attenuated 129I anomalies of atmospheric origin can be observed in the groundwater. Further, the modeled lag times of these attenuated atmospheric anomalies are consistent with 3H/3He groundwater ages previously measured in two separate wells. This finding helps constrain contaminant and/or tracer transport from the ground surface to the water table in the ASA.

This paper provides, for the first time, insight into the transport of 129I from an atmospheric release and its fate during wet deposition and recharge to groundwater. While 129I is largely retained on organics in the soil of the upper vadose zone the long term fate of 129I from the FDNA includes recharge into the Abbotsford-Sumas aquifer groundwaters. Continuing work on the fate of 129I and its transport should address the influence of local redox conditions in detail and the inorganic and organic speciation of 129I and 127I in precipitation, the vadose zone, and groundwater.

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