Spatial distribution and geochemical baselines of major/trace elements in soils of Medak district, Andhra Pradesh, India

摘要

Regional-scale variations in soil geochemistry were investigated with special reference to differences among soil groups and lithology in an area of 9,699 km~2 in Medak district, Andhra Pradesh, India. The concentrations of 29 elements (major: Si, Al, Fe, Mn, Mg, Ca, Na, K, Ti, P and trace: As, Ba, Cd, Co, Cr, Cu, F, Mo, Ni, Pb, Rb, Se, Sr, Th, U, V, Y, Zn, Zr) in 878 soil samples collected (557-topsoil, 321-subsoil) at a sampling density of 1 site/17 km~2 from 557 sites representative of all the soil types present in studied area were determined, and their elemental composition are discussed. The baseline levels of these elements in soils are determined over different lithological units for the identification of anomalous values relative to these. For the first time, geochemical maps for Medak district are prepared on 1:50,000 scale and the lithogeo-chemical database generated provides information on the lateral and vertical distribution of elements in soil. The spatial variations in the distribution of elements reflect underlying geologic characteristics. Box-plots reveal that the concentration of most of the elements in soils were not strongly dependent on the soil group but the soil-geochemistry abruptly changes with the change in the soil parent materials indicating that the distribution of elements is mostly influenced by the bedrock lithology and other natural processes acting on them. For instance, the concentrations of Co, Cu, Fe, Mn, Ti, V and Zn are high in soils developed on basaltic terrain while the soils developed on granitic and gneissic terrain exhibit high elemental concentrations of K, Pb, Rb, Si, Th and Y. Al-fisols had relatively high contents of elements while enti-sols had lower concentrations of most of the elements. The database can be used in the chemical characterisation of different geological units as well as applications in various environmental and agricultural fields. The results indicate that regional geology is an important determinant of soil geochemical baselines for soil pollution assessment and further emphasizes the importance of determining background levels locally. The defined baselines can be used to establish background values for future soil surveys.