Environmental indexes as a tool to identify contamination sources in alluvial aquifer systems

The increased use of chemicals in many anthropogenic activities and their dispersion into the environment can damage irreversibly the natural resources essential for human life. Among these, groundwater resources are, undoubtedly, strategic for the existence of human communities, primarily in those areas where population density is high and the land human use is intensive, so that a relevant groundwater resource vulnerability exists. The identification and characterization of contamination sources for groundwater can be more difficult than for other environmental pathways; in fact the relationship between environmental pressures and the complex structure of groundwater flow systems involves many processes, which affect the behaviour and transport of pollutants. With regard to the wide variety of structural, volcanic and depositional processes which characterize the alluvial aquifer systems in the Campania Region (South Italy), this paper describes a multidisciplinary approach to delineate aquifer/aquitard unit and recognizes scale-dependent heterogeneity by means of geological, hydrogeological, geochemical and geostatistical techniques. In such complex hydrogeological structures special attention has to be paid to recharge mechanism of confined or semiconfined aquifers, as well as of bedrock karst aquifers underlying heterogeneous deposits such as pyroclastic-alluvial successions. Moreover, heterogeneity affects leakage in complex aquifer systems and increases hydrodispersion parameter uncertainty. Decisions on groundwater protection policy, according to European directives, may be based on the improvement of the concept of “aquifer analogue” to basin fills. It is useful to identify heterogeneity properties, which control the permeability distribution. So hydro-stratigraphic architecture derives from equivalent parameters variability, which describes homogeneous media or hydrogeological units. In this line, the aquifer analogy can be associated to a set of indicators representative of the groundwater composition. Each indicator describes a specific aspect of groundwater quality and leads in combination with others to the definition of groundwater environmental index, due to natural and anthropogenic phenomena. The results of multivariate statistical analyses provide the identification of a number of homogeneous zones even in complex alluvial aquifer systems. At the same time, in wide regions this approach is able to point out the more hazardous sites where contamination sources are likely to be localized, and provides the working scale for planning technical and procedural solutions. The experience carried out in the Solofrana River Valley (South Italy) characterized by a high human impact and by geological and structural complexity of the territory is discussed in detail. Relevant geological and hydrogeological features and some interesting aspects related to pollutant flow and transport are presented.