Surficial redistribution of fallout 131iodine in a small temperate catchment Joshua D. Landisa,1, Nathan T. Hamma, Carl E. Renshawa, W. Brian Dadea, Francis J. Magilliganb, and John D. Gartnera aDepartment of Earth Science, bDepartment of Geography, Dartmouth College, Hanover, NH 03755 Edited by Mark H Thiemens, University of California San Diego, La Jolla, CA, and approved January 19, 2012 (received for review November 14, 2011) Isotopes of iodine play significant environmental roles, including a 54 d) is a natural, cosmogenic tracer commonly used in particle- limiting micronutrient (127I), an acute radiotoxin (131I), and a geo- and contaminant-transport studies, is also atmospherically de- chemical tracer (129I). But the cycling of iodine through terrestrial rived, and is known to be highly surface-reactive, to accumulate ecosystems is poorly understood, due to its complex environmental efficiently in surface vegetation and topsoil, and to associate chemistry and low natural abundance. To better understand iodine irreversibly with fine particles in fluvial transport (15). Cesium- transport and fate in a terrestrial ecosystem, we traced fallout 137 is another fission product released to the atmosphere in large 131iodine throughout a small temperate catchment following quantities by the FDNPP disaster (14). The relatively long half- contamination by the 11 March 2011 failure of the Fukushima life of 137Cs (30.3 a) has permitted detailed study of its fate in Daiichi nuclear power facility. We find that radioiodine fallout is terrestrial ecosystems following historical nuclear events (16). actively and efficiently scavenged by the soil system, where it is Here, we use gamma spectroscopic measurements of 131I con- continuously focused to surface soils over a period of weeks tamination, in conjunction with measurements of 7Be and 137Cs, following deposition. Mobilization of historic (pre-Fukushima) to develop a broad perspective of iodine transport and fate in a 137 cesium observed concurrently in these soils suggests that the terrestrial ecosystem. Our observations of 131I, summarized in focusing of iodine to surface soils may be biologically mediated. Fig. 1 and elaborated in the following sections, yield a catchment- Atmospherically deposited iodine is subsequently redistributed scale view of iodine behavior that both complements and validates from the soil system via fluvial processes in a manner analogous current interpretations of iodine environmental chemistry. to that of the particle-reactive tracer 7beryllium, a consequence of the radionuclides’ shared sorption affinity for fine, particulate EARTH, ATMOSPHERIC, Results and Discussion AND PLANETARY SCIENCES organic matter. These processes of surficial redistribution create Experimental Setting and Sample Collection. At the onset of the iodine hotspots in the terrestrial environment where fine, particu- FDNPP crisis we began a weekly regime of collection and analysis late organic matter accumulates, and in this manner regulate the of precipitation, terrestrial, and fluvial-sediment samples in the delivery of iodine nutrients and toxins alike from small catchments Mink Brook catchment of Hanover, New Hampshire, USA to larger river systems, lakes and estuaries. (43°42′18′′ N, 72°17′08′′ W; Fig. S1). Elevation of the Mink Brook catchment ranges from ca. 500 m above mean sea level SCIENCES beryllium-7 ∣ fluvial transport ∣ iodine cycling ∣ particle tracer ∣ radioiodine in the headwaters to 120 m at its confluence with the Connecticut ENVIRONMENTAL 131 River. At the start of our sampling central New Hampshire was he iodine isotope I (half-life of 8.02 d) is a by-product of entering spring, which is a period of peak stream flow. Deci- Tnuclear fission, is highly radioactive and acutely toxic, and meters of snowpack remained at elevations above ca. 400 m, presents a health risk (1) upon its release to the environment while terrain below ca. 200 m was generally snow free. Frequent from fuel reprocessing or industrial accidents. But assessments rainfall amounted to 70 mm during the period March 11—April of radioiodine exposure incorporate large uncertainties (2, 3) 11 and to 120 mm for the period April 11—May 11. On April 8, because iodine environmental behavior is not well understood near the observed peak in local 131I deposition and in the lee of a (4, 5, 6). In the Earth near-surface, iodine exists in multiple oxidation states ranging from −1 to þ5, which form inorganic subsiding flood event, we sampled fluvial sediments at eight sites − − along the 15-km length of Mink Brook. (e.g., iodide, I , and iodate, IO ) and organic (e.g., CH3I) spe- 3 Stream sampling locations at each site included, in close jux- cies in gaseous (e.g., iodine, I2), particle-reactive or soluble phases, all with widely divergent chemical behaviors (7) and facile taposition, active channel sediments as well as surficial channel interconversions. The most important iodine species in terrestrial margin sediments which were deposited at the falling waterline of B systems are “organoiodine” (5, 8), thought to be incorporated in a recent high-water event (Fig. S1 ). Suspended sediments were aromatic moieties (9) but otherwise poorly characterized as to collected by filtration of 20 L stream water samples. Terrestrial origin, reactivity, or fate. Coupled with the very low natural abun- samples, including vegetation, litter, soil, snowpack (where pre- dance of iodine (10), the complexity of iodine chemistry has sent), were collected concurrently at lawn, forest, and agricultural historically challenged analytical capabilities (10, 7, 11) and con- sites in the Mink Brook catchment. Multiday cumulative precipi- tinues to limit our understanding of diverse problems including tation samples were collected at the Dartmouth College Obser- the true hazards of nuclear energy production or amelioration of vatory in an open collector. For all samples we measured, using 131 134;137 human iodine deficiency (12). gamma spectroscopy, fallout radionuclides I and Cs, and 7 Following the Mw 9.0 Tōhoku earthquake of 11 March 2011 naturally occurring Be. (13) and subsequent failure of the Fukushima Daiichi nuclear power plant (FDNPP), sufficient radioactive 131I was released Author contributions: J.D.L. and N.T.H. designed research; J.D.L., N.T.H., and J.D.G. into the global atmosphere (14) to generate measurable fallout performed research; J.D.L., N.T.H., F.J.M., and J.D.G. contributed new reagents/analytic even at the great transport distance required to reach the north- tools; J.D.L., C.E.R., W.B.D., and F.J.M. analyzed data; and J.D.L., C.E.R., and W.B.D. wrote eastern United States. While regrettable, this event created an the paper. opportunity to observe the cycling of iodine across a natural land- The authors declare no conflict of interest. 131 scape, as the radioactivity of I, when measured by gamma spec- This article is a PNAS Direct Submission. troscopy, provides a highly sensitive iodine tracer. Two additional 1To whom correspondence should be addressed. E-mail: [email protected]. environmental radionuclides, if measured concurrently, provide a This article contains supporting information online at www.pnas.org/lookup/suppl/ context for comparison of iodine behavior. Beryllium-7 (half-life doi:10.1073/pnas.1118665109/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1118665109 PNAS Early Edition ∣ 1of6 Downloaded by guest on September 29, 2021 A B Fig. 1. Cartoon depiction of 131I transport pathways following deposition from the atmosphere. (1) Retention of ca. 50% of 131I in surface soil. (2) Percolation of ca. 50% of 131I to deeper soil. (3) Return of 131I to surface soil or litter via root or mycchorhizal uptake. (4) Return of 131I to surface soil, 131 vegetation, or litter via volatilization of I2. (5) Erosion of I-bearing particles from soil surface, transport in streams as suspended material. (6) Penetration of 131I-bearing particles into stream channel bed; they may be resuspended. (7) Deposition of 131I-bearing particles at channel margin during flood events. Atmospheric Deposition. Fallout from the Fukushima event was first detected at the Mink Brook research site during the week 131 131 Fig. 2. Daily I inventory in precipitation of the northeastern US following March 18—March 25 (Fig. 2A), when I activity in precipitation 131 the 11 March 2011 Fukushima Daiichi reactor failure. (A) Running daily I exceeded 2.5 Bq L −1. A secondary peak in 131I deposition during 7 inventory was estimated from composite sample collections according to early April coincided with a spike in natural Be deposition daily rainfall (see Methods); confidence band (in red) includes propagated 7 (Table S1). Given that the cosmogenic production of Be in the analytical uncertainty (Æ2σ). Cumulative precipitation is shown in gray. atmosphere is approximately constant (15), we attribute the sec- (B) comparison of precipitation inventory to collected inventories from ond 131I peak to more efficient removal of radionuclides from the terrestrial settings (shown as diamonds) of the Mink Brook catchment atmosphere rather than to a secondary pulse of 131I from Japan. (New Hampshire, USA). In this manner, the 131I∶7Be ratio (Table S1) at our location pro- vides a metric that is consistent with the history of the FDNPP pancy between amounts deposited on, and residing in, terrestrial plume approaching North America (17). Progressive decline of inventories to the penetration of unreactive iodine species (7) 131I in precipitation at our