Sources of sulfur in rainwater as indicated by isotopic δ³⁴S data and chemical composition, Israel

Seventeen rainwater samples from 11 different locations in Israel have been analyzed for their chemical composition and δ³⁴S values. The δ³⁴S range between 0.0 and 15.3% and fall within the δ³⁴S variations in rainwater in nature (- 2.5-19.4%). This range is characteristic for the last 40-50 years over various parts in the world. Higher δ³⁴S values were found in Mediterranean coast samples with relatively high Cl SO₄ ratios, while lower values were recorded inland, especially in the Negev desert and along the Dead Sea Rift Valley. The Na Cl ratios demonstrate a clear marine origin for these ions in the rainwater, while for other solutes, especially for SO₄, a strong non-seaspray (NSS) contribution is observed. In 11 out of 17 samples (i.e. 65%), seaspray sulfate constitutes a minor fraction (< 0.5). A mixing model was formulated to apply the composition of rainwater (Cl and SO₄) and its isotopic δ³⁴S values for the estimation of the SO₄ sources. Four major end-members were considered: (a) seaspray, (b) marine gypsum and anhydrite, (c) marine dimethylsulfide (DMS) and (d) anthropogenic sources. The uniform isotopic composition, around 0%, of the NSS sulfate indicate minor contribution of end-member (b) and (c), and support on end-member (d) as the major non-seaspray donor. This constant δ³⁴S value is in disaccord with the wide δ³⁴S range of anthropogenic sulfur, and can be explained by efficient mixing of anthropogenic sulfates resulting in a homogeneous "hemispherical" reservoir.