ISOTOPIC TRACERS

Author: COLL PUJOL ANNA.
Year: 2004.
University: POLITÉCNICA DE CATALUÑA [www.upc.edu].
Place of defense: INSTITUT DE TÈCNIQUES ENERGÈTIQUES..
Place of preparation: ETSEIB AVDA. DIAGONAL, 647 Campus SUD.

Summary: The primary purpose of this work is the implementation, in soils of the Mediterranean area Catalan techniques that use radioisotopes present in the field for the study of mechanisms for redistribution soil. The growing loss of fertile soil is a global problem because of its impact on the environment and the decline in soil productivity. The use of radioisotopes present in the field to study the redistribution of land to overcome limitations associated with traditional techniques to reduce effort, cost and time to obtain results. This paper analyzes the intrinsic natural radioactivity of the soil (40K, descendants of radioactive chains natural 238U and 234Th), and some radioisotopes incorporated into the ground by deposition as 137Cs (artificial) and 210Pbexceso (natural). The latter are noted for having a half-life sufficiently long and fixed strongly to soil particles, which are useful for evaluating the processes of redistribution through soil models. The areas of study are chosen in Catalonia (northeastern Spain). One of the areas is in the region pre-pirenaica (Mexico-Vallcebre) with two experimental watershed: an old abandoned terraces that give stability to the ground (Cal Parisa) and the other mostly acarcavada with clear erosion phenomena (Carot). The other area is an experimental station in the depressed Vallés-Penedés (Caldes de Montboi) in a basin agricultural subjected to intensive cultivation (Marimon Tower). We have employed three types of sampling: superficial surveys and profiles. We have collected 200 samples in 104 points with an accuracy traceable  ± 5 m has been validated sampling, spatially and temporally. Each of the soil samples have been characterized physicochemistry and radiologically. For the technique of gamma spectrometry, have joined optimizations that facilitates the analysis and reduce uncertainty and also has been validated with participation in intercomparaciones. It has designed a quality program, which ensures the reliability of the results, allowing the laboratory accreditation test for the standard ISO17025. The distribution surface (0-3 cm) 137Cs and 210Pbe, Cal Parisa shows correlation with the topography, and the values of activity are much higher (160 and 300 Bq / kg, respectively) at Tower Marimon (20â70 Bq / kg), which dilutes the intensive cultivation activities on the horizon of cultivation (~ 20 cm). In stable areas of each basin has been given the inventory reference to the 137Cs: 2400 Bq/m2 Torre Marimon and 5300 Bq/m2 in Vallcebre and for 210Pbe: 2700 and 6000 Bq/m2 respectively. We evaluate the depth distribution profiles in the ratio of 137Cs i 210Pbe with the content of organic matter in the ground, the height of point sampling, rainfall and soil particle size in the Similarity behavior of 137Cs and 210Pbe, regarding their retention in the soil, indicating the possibility of using 210Pb as tracers in studies of the land redistribution. The erosion rate average in the agricultural area varies between 15 and 30 T / ha 8 / yr, 4a7 depending on the model used, suggesting that is an area subject to erosion of low to moderate intensity due to tillage. In the stable area, the average rate of redistribution of land is less than  ± 5 T / ha / yr, or not presenting significant erosion or sedimentation and the area acarcavada the erosion rate is higher than 70 T / ha / yr, indicating a regime of intense erosion. It indicates the possibility of incorporating a correction factor to the erosion model for soil grown depending on the content of organic matter.