Natural and anthropogenic SO4 sources in the Arno river catchment, northern Tuscany, Italy: a chemical and isotopic reconnaissance Pages 79-92

The δ34S isotope signature of aqueous SO42− in the main river Arno and principal tributaries was investigated along with major ion chemistry in order to check the pollution effects in the water network of a heavily industrialized and densely urbanized catchment extending in Tuscany over an area of 8228 km2 from the Apennine Ridge to the Tyrrhenian Sea. Major towns include Florence and Pisa. Analyses are presented for 29 water samples from the Arno river and 36 water samples from the tributaries, all collected in 1996–1997 during low flow hydrologic conditions. Samples from the Arno are distributed from source to mouth and those from tributaries upstream from the confluence points. The Arno river solutes consist mainly of Ca2+ and HCO3−, but the proportions of Na+, Cl− and SO42− increase downstream, in keeping with the basically anthropogenic origin of the latter. The δ34S(SO42−) signature also increases downstream, with the most depleted values in the source zone. The seawater ingression into the river is documented by distinctly higher chemical and isotopic parameters at about 13 km from the mouth. In general, the tributaries behave similarly to the Arno for both their chemical and isotopic compositions, with increasing δ34S, Na+, Cl− and SO42− downstream. The δ34S of SO42− in natural inputs is estimated to range approximately between −15 and +4‰ from oxidation of pyrite disseminated in country rocks, and between +15 and +23‰ from dissolution of evaporites, whereas the δ34S of anthropogenic SO42− should be variable within the range 1–8‰.