Application of in-situ measurement to determine 137Cs in the Swiss Alps

Establishment of (137)Cs inventories is often used to gain information on soil stability. The latter is crucial in mountain systems, where ecosystem stability is tightly connected to soil stability. In-situ measurements of (137)Cs in steep alpine environments are scarce. Most studies have been carried out in arable lands and with Germanium (Ge) detectors. Sodium Iodide (NaI) detector system is an inexpensive and easy to handle field instrument, but its validity on steep alpine environments has not been tested yet. In this study, a comparison of laboratory measurements with GeLi detector and in-situ measurements with NaI detector of (137)Cs gamma soil radiation has been done in an alpine catchment with high (137)Cs concentration (Urseren Valley, Switzerland). The aim of this study was to calibrate the in-situ NaI detector system for application on steep alpine slopes. Replicate samples from an altitudinal transect through the Urseren Valley, measured in the laboratory with a GeLi detector, showed a large variability in (137)Cs activities at a meter scale. This small-scale heterogeneity determined with the GeLi detector is smoothed out by uncollimated in-situ measurements with the NaI detector, which provides integrated estimates of (137)Cs within the field of view (3.1 m(2)) of each measurement. There was no dependency of (137)Cs on pH, clay content and carbon content, but a close relationship was determined between measured (137)Cs activities and soil moisture. Thus, in-situ data must be corrected for soil moisture. Close correlation (R(2) = 0.86, p < 0.0001) was found for (137)Cs activities (in Bq kg(-1)) estimated with in-situ (NaI detector) and laboratory (GeLi detector) methods. We thus concluded that the NaI detector system is a suitable tool for in-situ measurements in alpine environments. This paper describes the calibration of the NaI detector system for field application under elevated (137)Cs activities originating from Chernobyl fallout.