Suitability of ^239+240Pu and ¹³⁷Cs as tracers for soil erosion assessment in mountain grasslands

Highlights

• Plutonium deposition in the Swiss Alps is mainly from nuclear bomb fallout.

• The distribution of ^239+240Pu in soils was more homogenous as for ¹³⁷Cs.

• Pu isotopes are suitable tracers for soil erosion assessment in Alpine grasslands.

• Erosive processes have a high dynamic and spatial heterogeneity.

Abstract

Anthropogenic radionuclides have been distributed globally due to nuclear weapons testing, nuclear accidents, nuclear weapons fabrication, and nuclear fuel reprocessing. While the negative consequences of this radioactive contamination are self-evident, the ubiquitous fallout radionuclides (FRNs) distribution form the basis for the use as tracers in ecological studies, namely for soil erosion assessment. Soil erosion is a major threat to mountain ecosystems worldwide. We compare the suitability of the anthropogenic FRNs, ¹³⁷Cs and ^239+240Pu as soil erosion tracers in two alpine valleys of Switzerland (Urseren Valley, Canton Uri, Central Swiss Alps and Val Piora, Ticino, Southern Alps). We sampled reference and potentially erosive sites in transects along both valleys. ¹³⁷Cs measurements of soil samples were performed with a Li-drifted Germanium detector and ^239+240Pu with ICP-MS. Our data indicates a heterogeneous deposition of the ¹³⁷Cs, since most of the fallout origins from the Chernobyl April/May 1986 accident, when large parts of the European Alps were still snow-covered. In contrast, ^239+240Pu fallout originated mainly from 1950s to 1960s atmospheric nuclear weapons tests, resulting in a more homogenous distribution and thus seems to be a more suitable tracer in mountainous grasslands.

Soil erosion assessment using ^239+240Pu as a tracer pointed to a huge dynamic and high heterogeneity of erosive processes (between sedimentation of 1.9 and 7 t ha⁻¹ yr⁻¹ and erosion of 0.2–16.4 t ha⁻¹ yr⁻¹ in the Urseren Valley and sedimentation of 0.4–20.3 t ha⁻¹ yr⁻¹ and erosion of 0.1–16.4 t ha⁻¹ yr⁻¹ at Val Piora). Our study represents a novel and successful application of ^239+240Pu as a tracer of soil erosion in a mountain environment.