Use of stream sediment chemistry to predict trace element chemistry of groundwater. A case study from the Bisagno valley (Genoa, Italy)

Irreversible water–rock mass transfer accompanying the generation of the Bisagno valley groundwaters, starting from local rainwaters, was simulated through reaction path modeling in reaction progress mode. The simulations assumed bulk dissolution of local stream sediments (i.e. without any constraint on their mineralogy and on the dissolution rates of primary solid phases) and precipitation of chalcedony, illite, a smectite solid mixture, a hydroxide solid mixture, a trigonal- and an orthorhombic-carbonate solid mixtures. Concentrations of major chemical elements and several trace elements in groundwaters were satisfactorily reproduced. Thus, in the system studied, stream sediment data can be used as an initial condition in reaction progress calculations to predict the chemistry of groundwaters. If this conclusion would be confirmed through other applications, stream sediment data could be used to assess natural background levels in groundwaters and to identify pollution phenomena. Calculations similar to those carried out for the Bisagno valley could be applied in many other areas, given the large number of stream sediment data available throughout the world and the present efforts to generate worldwide geochemical databases. A global geochemical database for environment and resource management. (Recommendations for international geochemical mapping. Final report of IGCP Project 259. Earth Sciences 19 UNESCO, Paris (1995)).