1 Cesium radioisotope content of wild edible fungi, mineral soil, and surface litter in western 2 North America after the Fukushima nuclear accident. 3 4 Matthew J. Trappea, Leah D. Mincb, Kimberly S. Kittredgec, Jeremias W. Pinkd 5 6 a Department of Forest Ecosystems and Society, 321 Richardson Hall, Oregon State University, 7 Corvallis, Oregon, USA, 97331, [email protected], corresponding author, Tel. 01- 8 541-737-6072 9 10 b Radiation Center, 100 Radiation Center, Oregon State University, Corvallis, Oregon, USA, 11 97331, [email protected] 12 13 c Northwest Mycological Consultants, 702 NW 4th Street, Corvallis, Oregon, USA, 97330, 14 [email protected] 15 16 d Department of Anthropology, 238 Waldo Hall, Oregon State University, Corvallis, Oregon, 17 USA, 97331, [email protected] 1 18 ABSTRACT 19 We measured activity levels of cesium radioisotopes 134Cs and 137Cs in wild edible fungi, 20 mineral soil, and surface litter of the west coast of North America from southern California to 21 northern Vancouver Island after the Fukushima nuclear accident. All activity measurements 22 were below governmental limits for human health. 137Cs activity increased to the north in 23 mineral soils and fungal samples, while 134Cs activity increased to the south in surface litter 24 samples. Chanterelles did not significantly bioconcentrate either radioisotope, but chanterelle 25 activity levels were correlated with those of mineral soil. Activity levels demonstrated a high 26 degree of variability, even in samples from the same site. In most cases the level of 137Cs 27 activity was substantially higher than that of 134Cs, suggesting that 137Cs was present in the 28 environment prior to the Fukushima release. 29 30 Keywords: bioaccumulation, cesium, chanterelle, Fukushima, fungi, mushroom, radiation 31 32 INTRODUCTION 33 The disaster at the Fukushima Daiichi nuclear power plant after the March 11, 2011 earthquake 34 and tsunami released an estimated 35.8 (±16.5) PBq (3.66 x 1016 Bq) each of the radioisotopes 35 cesium–137 (137Cs) and cesium–134 (134Cs) between March 11 and April 18, 2011. About 19% 36 of the airborne radioactive particles were deposited on Japan, 79% in the Pacific Ocean, and 2% 37 on land areas other than Japan (Stohl et al. 2012). 38 39 The purpose of this study was to measure and report the levels of cesium radionuclides in wild 40 edible fungi and their substrates on the North American west coast after the Fukushima accident. 2 41 Fungi are of particular interest because they can bioaccumulate heavy metals (Campos et al. 42 2009) including radionuclides (Vinichuk et al. 2010) and are consumed by humans (Pilz and 43 Molina 2002). This bioaccumulation is often referred to as a “Transfer Factor” (Ehlken and 44 Kirchner 2002); the radioisotope level detected in the mushroom tissue divided by that of its 45 substrate. A Transfer Factor of 1 indicates that activity levels in fungal tissue are the same as 46 their substrate, above 1 indicates bioaccumulation above substrate levels, and below 1 indicates 47 that activity levels in fungal tissue are below those of their substrate. 48 49 We analyzed samples of edible fungi and associated surface litter and mineral soil on a 50 latitudinal gradient from northern Vancouver Island, British Columbia to Los Angeles, 51 California for activity of 134Cs and 137Cs radioisotopes. The cesium isotopes were selected 52 because the relatively short half-life of 134Cs (2.1 yr) makes it a good indicator of recent events, 53 and the longer half-life of 137Cs (30.2 yr) makes it a persistent environmental contaminant. 54 55 Different taxa of fungi have been shown to bioaccumulate radionuclides at different rates 56 (Vinichuk and Dolhilevyech 2005). One factor that may affect fungal bioaccumulation is their 57 trophic status (Gillett and Crout 2000). Mycorrhizal fungi form symbioses with roots of plants, 58 and are efficient at absorption and transport of minerals and nutrients from soil to roots (Leake et 59 al. 2004). Saprobic fungi enzymatically decompose organic material, absorbing not only the 60 resultant carbohydrates but also other compounds present in the substrate (Bazala et al. 2008). 61 The hyphae of saprobic fungi dominate the surface litter layer, while the hyphae of mycorrhizal 62 fungi dominate deeper soil horizons (Lindahl et al. 2007). Some fungi contain pigments that 63 chelate and retain cesium (Garaudée et al. 2002). Many abiotic site factors including clay 3 64 content (Staunton and Levacic 1999), soil pH (Kruyts and Delvaux 2002), precipitation and 65 runoff (Parsons and Foster 2011), and microsite conditions (Bunzl et al. 1997) can also affect 66 bioaccumulation. 67 68 Although we welcomed all samples of wild edible fungi, for consistency in data analysis we 69 focused on obtaining chanterelles (Cantharellus spp.) across a latitudinal gradient. Cantharellus 70 species are among the more widely distributed and frequently consumed edible taxa (Pilz et al. 71 2003). No single species of Cantharellus extends throughout the range of this project; in the 72 Pacific Northwest (PNW) we sampled Cantharellus cascadensis, C. formosus, and C. subalbidus 73 (associated with conifers), and in California, Cantharellus californicus (associated with oaks). 74 75 Our first hypothesis was that activity levels of cesium isotopes in wild edible mushrooms are 76 below the FDA Derived Intervention Limits (FDA 1998) of 1200 Bq/kg. Many studies have 77 documented radioisotope uptake by mushrooms in Europe after the Chernobyl accident of 1986, 78 but we found little quantitative data on the safety of edible wild mushrooms in western North 79 America. Given the relatively small proportion of the release that reached North America, we 80 expected that activity levels would be less than 1200 kg/Bq. 81 82 Our second hypothesis was that cesium activity would be higher in samples farther north due to 83 jet stream influenced precipitation patterns. Most deposition of radioisotopes occurs during rain 84 events (Clark and Smith 1988) and typical jet stream behavior brings more winter precipitation 85 to the PNW than California. 86 4 87 Our third hypothesis was that chanterelles would bioaccumulate cesium isotopes at levels above 88 those of their substrates (Transfer Factor > 1.0). Bioaccumulation of radioisotope and Transfer 89 Factors are well documented in closely related European fungal species, but we know of no such 90 studies in North America. The last known survey of radiation levels in Pacific Northwest forest 91 ecosystems was performed by Eberhardt et al. (1969). 92 93 MATERIALS AND METHODS 94 Samples of wild edible fungi, mineral soil, and surface litter were collected by a network of 95 volunteers on the west coast of North America from Los Angeles, California, to northern 96 Vancouver Island, British Columbia in the fall/winter season of 2011-2012, ca 6-10 mo after the 97 Fukushima accident. From each location we requested samples of 100 g dry weight of each 98 material type, however some samples were smaller. Because our priority was to include a large 99 sampling area, many samples were unreplicated from a given location. “Surface litter” was 100 undecomposed material from the O horizon collected from ca 1 m around a fungal sporocarp 101 sample. “Mineral soil” was blended humic-mineral A horizon material collected beneath the 102 stem of the sampled mushroom ca 5-10 cm deep. We also analyzed 8 fungal and 2 substrate 103 samples that had been collected and preserved (dried) before the Fukushima accident. 104 105 Fungal samples were cleaned of most adhering soil and debris, air dried at 60°C for 12–24 h, and 106 ground to a uniform powder with a Wiley mill. Each sample was then weighed, placed in a 107 polyethylene container (either 120 cc “urine cups” for smaller samples and 450 cc “cottage 108 cheese” containers for larger samples), and the volume determined to the nearest 10 ml. Each 109 sample was analyzed at the Oregon State University Radiation Center for ~24 h, and gamma 5 110 activity recorded at the 795 keV line for Cs-134 and at 662 keV for Cs-137 on one of three high 111 purity germanium detectors. All detectors utilized are ORTEC coaxial HPGe with certified 112 relative efficiencies of 28%, 32% and 34%, and resolutions of 1.72, 1.70, and 1.74 (FWHM at 113 1333 keV), respectively. The detectors are oriented in a vertical configuration within graded 114 shielding (Pb-Al-Cu) and located within a low-background counting facility. 115 116 In order to accurately quantify activity for the diversity of sample volumes represented, 117 calibration curves were developed for each detector for a range of sample geometries in both 118 container types using a mixed isotope certified calibration source (Analytics 69477-717). The 119 liquid source was first transferred to a urine cup containing 40 ml of lab-grade cellulose powder 120 and allowed to dry; loss of activity due to transfer was determined by counting the original 121 source container immediately before and after transfer on the same detector. Additional 122 cellulose powder was added to the urine cup and mixed with the source to simulate the various 123 volumes of fungal samples (40, 60, 90, and 130 ml), and the detector efficiency determined for 124 each volume with the urine cup placed on the detector face. The cellulose power was then 125 transferred to a cottage-cheese container, and additional powder was added to simulate sample 126 volumes of 200, 300, 350, 400, 450, and 500 ml. Results for the final (500 ml) cellulose powder 127 source were compared against a certified 500 cc calibration source (Eckert and Ziegler 1555-12- 128 2), and measured activities found to be within 6% of certified activities for the 662 and 1333 keV 129 peaks at the same geometry.