Abstract
Early observations of the tritium (3H) activity in precipitation from Switzerland started in 1967 in Bern and a monitoring program with improved analytical techniques was carried out between 1971 and 2009. Between 2012 and 2015, we performed tritium analyses on daily precipitation samples from north-western Switzerland to better constrain the tritium variability. We also collected waters dripping inside Milandre Cave (Jura Mountains) with a 4–6 months’ resolution in order to estimate the age of the drip water, which is mandatory to interpret the high-resolution speleothem (cave carbonate deposits) records. Over the monitoring period, the mean tritium concentration in the daily precipitation samples was approximately 8.7 ± 4.7 TU with distinct seasonality showing higher values in spring and summer (from April–May to August–September). The similarity in trends with the other cosmogenic radionuclide beryllium-7 (7Be) suggests that, for the study site, tritium in precipitation essentially originates from stratospheric input in spring. In winter, precipitation dynamics with increasing moisture originating from the Atlantic and diluted during transport contribute to low values close to the Atlantic background. In cave drip water, the depleted activity level of tritium indicates a relatively long percolation time from the surface to the cave of several years. A residual superimposed short tritium variability provides evidence for the occurrence of fast flowing water paths. The contribution from both components were quantified based on a two-component lumped parameter-mixing model. Finally, we show that tritium concentrations in cave drip water are linked to the moisture origin and atmospheric transport pathways.
Highlights
Observations of the tritium (3H) activity in precipitation from Switzerland started in 1967 in Bern and a monitoring program with improved analytical techniques was carried out between 1971 and 2009
In the atmosphere and in the hydrosphere, tritium concentrations vary mainly due to atmospheric transport (Cauquoin et al, 2015) and/or regional anthropogenic sources such as nuclear facilities, chemical in dustries, wood-based heating systems or industries producing tritium light sources, which emit tritium leading to local contamination (Schürch et al, 2003; Rozanski et al, 1991)
The monitoring of the Climate and Environmental Physics (CEP) stations provided monthly mean values and was used to document the long-term decrease of the continental atmospheric load occurring since the atomic bomb testing in the 1960’s (Schürch et al, 2003; FOPH, 2018)
Summary
Observations of the tritium (3H) activity in precipitation from Switzerland started in 1967 in Bern and a monitoring program with improved analytical techniques was carried out between 1971 and 2009. For the Journal of Environmental Radioactivity 222 (2020) 106346 estimation of the percolation time of cave waters, we use the residual (seasonal) variability and/or shifts due to radioactive decay of tritium in precipitation and cave drip waters. It was used as a tracer in Bunker Cave in Germany (Kluge et al, 2010a), Soreq Cave in Isra€el (Kaufman et al, 2003), and more recently in Villars Cave in France (Jean-Baptiste et al, 2019).
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