Land Use and Runoff Influence 137Cs Variability Along Mediterranean Toposequences: Evidence from Proximal LaBr3

In subhumid Mediterranean mountains, rainfed agriculture coexists with Mediterranean forest and scrub and with abandoned agricultural land afforested with pine. These contrasting land uses and management histories create heterogeneous hillslopes affected by complex soil redistribution processes. Proximal gamma ray spectrometry (PGRS) provides an opportunity to examine spatial variation in fallout 137Cs directly in the field. This study evaluated the capability of a portable LaBr3(Ce) scintillation detector to identify spatial variations in 137Cs along two contrasting hillslope toposequences. Measurements were carried out for 1500 s at 26 locations distributed along a predominantly cultivated toposequence and another covered by pine afforestation and Mediterranean forest and scrub. Soil properties were also analysed at 17 locations, while laboratory HPGe measurements from bulk samples and selected sectioned profiles provided complementary information on 137Cs distribution. The LaBr3 measurements showed marked spatial variability along both toposequences. Detection of 137Cs differed among land use and terrain domains. 137Cs was detected at all Mediterranean forest and scrub sites, at five of the eight pine afforested sites and at only four of the fourteen cultivated sites, indicating a lower frequency of detection in cropland. Detectable 137Cs was associated with specific local hillslope configuration. Across both toposequences, detectable and undetectable signals alternated over short distances and were associated with field boundaries, slope discontinuities, terraced surfaces, machinery tracks and local runoff pathways, highlighting the importance of local terrain configuration and previous land management for interpreting the spatial pattern. The results show that in situ LaBr3 measurements can identify spatial patterns of 137Cs at hillslope scale. Their interpretation requires consideration of land use, flow paths and soil properties, together with the local organisation of runoff and sediment redistribution.