Gianni Cortecci

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

Abstract The δ 34 S isotope signature of aqueous SO 4 2− 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 km 2 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 Ca 2+ and HCO 3 − , but the proportions of Na + , Cl − and SO 4 2− increase downstream, in keeping with the basically anthropogenic origin of the latter. The δ 34 S(SO 4 2− ) 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 δ 34 S, Na + , Cl − and SO 4 2− downstream. The δ 34 S of SO 4 2− 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 δ 34 S of anthropogenic SO 4 2− should be variable within the range 1–8‰.

Chemistry and sulfur isotopic composition of precipitation at Bologna, Italy

Individual and monthly precipitation samples from the polluted atmosphere of Bologna (Emilia-Romagna province) were collected during March 1996 to May 1997 and analyzed for major ions in solution and S isotopes in dissolved SO4. Weighted mean enrichment factors relative to seawater are found to be 1.0 for Na, 15.2 for K, 105 for Ca, 3.3 for Mg, 17.3 for SO4 and 663 for HCO−3. Very good positive correlations are observed for the Ca2+–Mg2+–HCO−3–SO2−4–NO−3 system, indicating that dissolution of Ca (±Mg)-carbonate particles by H2SO4 and HNO3 from combustion of oil and gas is a major process controlling the chemical composition of rain and snow. Na+ and Cl− in monthly precipitation derive essentially from sea spray, but the contribution of Na+ from continental sources is appreciable in a number of individual rains. NH+4 appears to be on average more abundant in spring and summer precipitation, its main sources being microbial activity in soils and application of fertilizers. K+ is probably of continental origin from soil dust. The S isotopic composition of SO4 is systematically positive, with mean δ34S values of +3.2±1.6‰ (n=40) in individual precipitation and +2.8±1.4‰ (n=12) in monthly precipitation. These isotopic compositions are interpreted in terms of a dominant contribution of S from anthropogenic emissions and subordinate contributions from biogenic and marine sources. Pollutant SO4 is estimated to have a δ34S value in the range +2.5 to +4.5‰, whereas a distinctive δ34S of −4.5‰ or lower indicates SO4 from oxidation of biogenic gases. The isotopic and chemical compositions of SO4 do not depend on wind direction, thus testifying to a mostly local source for pollutant S in the Bologna atmosphere.

Chemical and isotopic compositions of water and dissolved sulfate from shallow wells on Vulcano Island, Aeolian Archipelago, Italy

Abstract Twenty-two cold and thermal waters from shallow wells sampled in June 1995 in the Vulcano Porto area, Vulcano Island, were analyzed for major and minor chemical constituents, oxygen and hydrogen isotopes and tritium contents, and sulfur isotopes in the dissolved sulfate. The sulfur isotopic composition of the dissolved sulfate ranges between +0.6 and +6.5‰ (mean +3.7±1.7‰), and is interpreted as deriving mainly from fumarolic SO 2 undergoing oxidation in deep and shallow aquifers, with possible minor contributions from oxidation of H 2 S. Dissolution of secondary anhydrite may have been a minor source of the isotopically heavy aqueous sulfate in the cold groundwaters. The chemical and isotopic features of the waters support previous interpretative hydrologic models of Vulcano Porto, which comprise a number of aquifers fed basically by two major end-members, i.e. meteoric water and crater-type fumarolic inputs, the latter in the form of absorbed emissions or condensate. These data, along with the sulfur isotopes of aqueous sulfate, exclude involvement of seawater in the recharge of the groundwater system of the island.