Watershed analysis and inverse distance weighted interpolation of stream sediment composition for the geochemical mapping and environmental risk assessment of the Volturno river basin (southern Italy).

Geochemical analysis of stream sediments is used for mining exploration and environmental studies. For this purpose, two main different types of spatial analysis of the analytical results are commonly used to produce geochemical maps. The first method uses geostatistics interpolators based on samples proximity, like the Inverse Distance Weighted interpolator (IDW). The second method is based on the catchments delineation upstream from the sampling points. In this study the two mapping methods are compared, analyzing the geochemistry of stream sediments in the Volturno river basin (5,455 km2). 998 stream sediment samples were collected, prepared and analyzed with ICP-MS for 53 elements. These geochemical data were processed with a statistical analysis. Then, the spatial distribution of the elements concentration was analyzed in a Geographic Information System (GIS), using the two mapping methods mentioned above. The samples have been split into two sets: 900 samples for the geochemical mapping, and 98 samples as tests for accuracy analysis. The geochemical maps based on the watersheds delineation show better accuracy in the mountainous areas, where the topography has a high impact on the spatial distribution and dispersion in the environment of the elements. Instead, IDW, based only on the proximity of the sampling points on a map, which not necessarily imply that they share correlated variables, is less accurate in mountain areas, but shows better performance in the gentle alluvial plains, where elements transport is less influenced by gravity and slope.