Presenza di radionuclidi naturali ed artificiali nell’area Bosco Negri - 1: Caratteristiche Chimico-Fisiche del suolo

Siro Negri forest, extends for 11 ha on the Ticino river right side at 10 km from Pavia (North of Italy), it is the best example of the ancient alluvial forest of the Po plain. It was declared a Fully Protected Nature Reserve in the 1967. In the fi rst part of this research have been studied the chemical and physical characteristics of soils collected in this site, for evaluating the relation between soils properties and distribution of natural and artifi cial radionuclides in pedological profi le. This work is the fi rst step in investigating of environmental radioactivity in this natural area. For this purpose samples of soil was collected in three different points and at different depth, inside this area, to investigate the vertical and horizontal migration of natural (the two series 238U, 232Th and 40K) and artificial (137Cs) radioisotopes in the soils profile. Uranium and Thorium are found in the upper layer of the earth. They exist only as unstable, radioactive isotopes which undergo a long chain of radioactive decays to end fi nally with stable isotopes of the Pb (lead). Due to their short half – lives, these decay products are present in the environment only in small quantities but their activity concentration can modify the environmental radioactivity. In addition, these radioactive products of decay of the two series 238U, 232Th and the radioisotopes 40K, 137Cs are carried from river and deposited into the forest soils. In the fi rst area, located in the North East part of the forest (named station A), have been collected seven soil profi les at 10, 25, 40, 55, 70, 85 and 110 cm depth; in the second station, located in the central part of the forest (named station B), have been collected eight samples at 10, 25, 40, 55, 70, 85, 110 and 125 cm depth; in the third station, located in the South West part of the forest (named station C), have been collected eight samples at 15, 30,45, 60, 75, 90, 105 and 125 cm depth. In this part of the work we show the results related to texture, density, humidity, pH, cationic exchange capacity, organic Carbon and soils organic matter. For every analytical determination the soil samples were dried at 40°C and sieved at 2 mm. In the texture determination soils are sieved in different sieves with automatic sieve (2 mm, 200 mm, 50 mm and 20 mm) and with Laser Diffraction System (particle size comprised between 0,2 and 44 mm). The texture of the soil describe the contents of sand, silt and clay in pedological profi le. In this area there is sandy soils with greater percentage at different soil depth in the three stations (station A: 55 cm depth – 88% sand; station B: 70 cm depth – 96% sand; station C: 45cm depth – 94% sand); this depth have been named layer S. Density are typical of inorganic components of the soil (2,4 and 2,9 g/cm3). Soil water content (Humidity percentage) is very small, greater in superfi cial layer (station A: 1,6% at 10 cm depth; station B: 1,2% at 10 cm depth; station C: 1,4% at 15 cm depth) but small in profound pedological profi le. Minimum for this property correspond at layer S (station A: 0,4%; station B: 0,4%; station C: 0,2%). Soil’s pH was determined potentiometrically in water and KCl 1M. Every investigated soil is acid in superfi cial layer (pH units: 4,4÷5) and moderately acid or neutral, at greater depth (pH units: 6÷7). The Cation Exchange Capacity (CEC or CSC) is a very important parameter because the adsorption of some radionuclides on soils and on clays particles is a cation exchange phenomenon. Greater CEC have been determined in superfi cial layer of the station A (27,3 cmol kg-1) more small, but the same order of magnitude, for the others two stations (station B: 23,7 cmol kg-1; station C: 22,6 cmol kg-1). At the layer S depth the CEC is very low but it increased at greater depth (CEC of layer S – station A: 7,8 cmol kg-1; station B: 4,8 cmol kg-1; station C: 5,1 cmol kg-1). The organic matter may make a signifi cant contribution to the CEC of the soil and it can have an indirect effect on radioisotopes adsorption on clay minerals. The organic Carbon were measured using Walkley-Black method and the conversion factor used were calculated like if 58% of the elemental composition of organic matter is constituted by organic Carbon. The organic matter amount in the pedological profile of the soil is elevated in the superficial part greater in station B but the same order of magnitude for station A and C (about 15 g kg-1 of organic Carbon and about 25 g kg-1of organic matter). Also, in all three stations, minimum value for this parameter correspond to layer S.