Assessment of Estimated Bedrock and Stream Sediment Geochemical Backgrounds in Catchment Basin Analysis

Abstract

The weighted mean and the multiple regression techniques are two methods that are employed to estimate elemental background concentration of lithologies upstream of stream sediment samples and the background value of elements at catchment basin outlets due to the lithology. However, these techniques have not previously been validated against real background values by sampling from lithological units. A geochemical survey was completed in central Iran by collecting samples from stream sediments along with rock samples from upstream lithologies. The accuracy and the precision of both techniques were examined graphically and statistically. The results demonstrated the advantage of multiple regressions over the weighted mean method. However, estimated background values from both techniques show high deviations from the analyzed values for some elements including Cr, Ni, Pb and Zn. While the weighted mean method is not as sensitive as the multiple regression approach to small areas of lithological units (independent variables), the estimated background values emerging from the weighted mean technique follow narrow ranges among different lithological units and cannot highlight the differences between elemental concentrations in various lithologies. However, multiple regression analysis is more successful than the weighted mean method in estimating expected background values of sediments at the catchment basin outlet. The multiple regression method uses the areas of all lithologies to estimate the background value of a particular lithology, whereas in the weighted mean method, only the area of the lithology is involved in the background estimation of that lithology. The existing equations essentially take into account only mechanical dispersion but overlook hydromorphic dispersion of elements in fluvial systems. Moreover, these equations only use area of rock units as the weighting factor or independent variables. Several parameters such as rate of weathering and erodibility, transportation mode, scavenging effects, the sediment delivery ratio of a catchment basin and each lithological unit control sediment chemistry at the basin outlet and should also be taken into account.