The River Arno catchment, northern Tuscany: chemistry of waters and sediments from the River Elsa and River Era sub-basins, and sulphur and oxygen isotopes of aqueous sulphate

摘要

Within the hydrologic balance of the River Arno catchment (northern Tuscany), the Rivers Elsa and Era are important tributaries entering the main river from the left bank in the lower part of the watershed. Waters and bed sediments were sampled in June 2000 during low discharges in the Rivers Elsa and Era, as well as in major tributary streams. Water samples were analysed for major chemistry and sulphur isotope composition of sulphate, and sediment samples were analysed for major composition and selected trace elements of environmental concern (Zn, Cu, Pb, Cr and Ni). The main results for the waters are: (1) Na and Cl in solution show consistent downstream positive trends in the main rivers, thus supporting progressive contributions of anthropogenic salts; the highest concentration values are observed in tributaries; (2) as shown by sulphur isotopes, sulphate in solution is mainly controlled by dissolution of evaporites (Elsa basin) or oxidation of reduced organic/biogenic sulphur (Era basin), with anthropogenic contributions in most streams not higher than 10% in both the basins. A δ~(34)S signature in the range -2 to +3 per thousand is estimated for pollutant sulphate in the basins studied. The main results for the sediments are: (1) major chemistry is essentially controlled by the lithotypes drained by the waters; (2) pollution by heavy metals does not reach high levels; (3) compared with local fine-grained rocks, copper is more frequently anomalous, whereas lead and zinc show only occasional anomalies; (4) local high concentrations of chromium and nickel can be attributed to upstream occurrences of ophiolites.