Geologically-oriented geochemical maps: a new frontier for geochemical mapping?

摘要

Geologically-oriented geochemical maps represent a more efficient alternative approach to conventional geochemical mapping based upon geostatistical methods. These maps provide a significant contribution to the understanding of the physical processes that control natural metal distribution in alluvial and coastal sediments. A pilot study from the northern Adriatic coastal region, close to the Po Delta, shows that spatial distribution of environmentally important heavy metals, such as Cr, is strongly controlled by differences in drainage-basin composition and by the prevailing sediment dynamics, which are the expression of the complex interplay of fluvial and coastal processes. Sediment provenance (Apenninic versus Po River sources) primarily controls the major changes in geochemical composition of sediments. Grain size plays a subordinate, but significant role, and patterns of heavy-metal concentrations are observed to vary in a consistent manner with superficial geology. Within a single depositional system, geometry and lateral extent of sedimentary bodies represent the major factors controlling the spatial distribution of potentially toxic metals. The effects of hydraulic sorting are assessed for several elements, reflecting the different hydrodynamic behaviour of suspended versus bedload sediment. Particularly, metal concentration in alluvial and deltaic sediments appears to be related with the percentage of clay, fine-grained (floodplain and delta-plain) deposits showing invariably higher values than their coarsegrained (crevasse/levee and delta-front) counterparts. This study shows that detailed sedimentological analysis constitutes a priority for geochemical mapping of Quaternary deposits, and that precise identification of a geochemical signature for the different sedimentary facies in a given area is essential to obtain reliable information on the natural backgrounds. An appropriate sampling strategy, including characterization of all outcropping facies associations, is recommended to adequately represent the remarkable variability in terms of sediment provenance and grain size. Geologically-oriented geochemical maps, i.e. maps where boundaries of geochemical units are drawn on the basis of elements of geological significance, can result in a more reliable spatial distribution of natural background values of potentially toxic metals than conventional geochemical maps. The new type of geochemical map presented in this paper constitutes a powerful tool to assess the anthropogenic impact on alluvial plains and coastal plain areas, and can be of use for legislative purposes and for planning strategies for environmental protection.