Radioisotopic and physicochemical background indicators to assess soil degradation affecting olive orchards in southern Spain

Soil degradation is a major agrienvironmental issue under Mediterranean climatic conditions. To assess soil erosion magnitude under orchard plantation, soils in an undisturbed area – located within an archaeological protected site in southern Spain – were analysed to establish its physicochemical status, the initial 137Cs fallout and the natural level of radioactivity taking into account the content of naturally occurring radionuclides (NOR). The vertical profiles of NOR mass activities confirmed its non-disturbance. 90% of the 137Cs content was concentrated in the top 20 cm and the physicochemical parameters confirmed as well the undisturbed status of the site. The base-line level of 137Cs was established at 1925 ± 250 Bq m−2 with a coefficient of variation of 23% and an allowable error of 11%. This 137Cs background was used to assess soil erosion magnitude in a close orchard field using the 137Cs method. The maximum erosion rates reached 19 t−1 ha−1 yr−1 and a sediment delivery ratio of 29% was evaluated, both values confirming an unsustainable soil loss magnitude due to the combination of water and tillage erosion processes since the 1950s. The radium equivalent activity and the absorbed dose rate results highlighted a difference between eroded and deposition sectors in the cultivated field confirming that these parameters could be used to some extend to assess pedologic processes.

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► Radioisotopic and physicochemical background was established in an undisturbed Spanish archaeological site.
► Using the 137Cs method, erosion rates reaching 19 t ha−1 yr−1 were evaluated in a nearby olive orchard field.
► Naturally occurring radionuclides (i.e. 40K, 226Ra and 232Th) and gamma dose rate varied according to soil redistribution processes.