Plutonium isotopes (239+240Pu) in volcanic soils of Iceland – Depth patterns, behavior and applications for soil redistribution tracing

Plutonium isotopes represent a relatively new tool for quantifying soil erosion and denudation rates and have not yet been applied in the dynamic environmental settings of Iceland. Though Iceland is widely believed to exhibit extremely high erosion rates, but empirical investigations are very rare. Therefore, an investigation was conducted into eleven soil profiles from southwestern (Reykjanes – Rey1, Rey2, Rey3, Rey4), southeastern (Kotafjall – K3, K4, K6, K7) and northwestern (Blönduos – B1, B2, B3) Iceland were investigated. The studied sites exhibited mean 240 Pu/ 239 Pu ratios between 0.17 and 0.22, consistent with the global fallout from atmospheric nuclear weapons testing. Spatial inventories of 239+240 Pu varied among the study areas, ranging from 18.8 Bq m −2 in northwestern Iceland to 150 Bq m −2 in the southeast. Calculated soil erosion and deposition rates differed depending on the conversion model applied, although all three models (PDM, IM and MODERN) produced consistent interpretations of dominant processes that were mutually consistent for each site. The estimates obtained using MODERN model were considered the most reliable, as this model does not rely on assumptions regarding the distribution of 239+240 Pu within soil profile. At the Blönduos, redistribution rates (from −3.08 to +3.03 t ha −1 yr) were likely overestimated due to the difficulty in finding a representative, undisturbed reference site. At the Kotafjall, the deposition of plutonium-free ash from a recent eruption (Grímsvötn volcano) disrupted the inventories used for calculations, and thus only high erosion rates (up to −16.01 t ha −1 yr) were obtained. Therefore, the redeposition rates from both sites should be treated as tentative. In contrast, soil redeposition rates (from +0.61 to +2.67 t ha −1 yr) determined at the Reykjanes location are reliable in reflecting the Icelandic landscape. These findings highlight the need for further research to refine a plutonium-based assessment of soil redistribution in Iceland's highly dynamic landscape.