Evidence for incorporation of H₂S in groundwater fulvic acids from stable isotope ratios and sulfur K-edge X-ray absorption near edge structure spectroscopy

Groundwater samples collected in a shallow oxic and reduced deep groundwater system revealed the influence of dissolved sulfide on the chemical and isotopic composition of fulvic acid associated sulfur. Stable isotope compositions of groundwater sulfate and fulvic acid sulfur and sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy data were used to determine the sources and processes affecting fulvic acid sulfur in the aquifer. A δ³⁴S value of 2.2 ‰ for the shallow groundwater sulfate and a δ³⁴S value of fulvic acids of 4.9 ‰ accompanied by a contribution of up to 49% of the most oxidized sulfur species (S₊₆) documented that fulvic acid sulfur is mainly derived from soil S compounds such as ester sulfates, with δ³⁴S values similar to those of atmospheric sulfate deposition. In contrast, in the deep groundwater system with elevated δ³⁴S values in groundwater sulfate of up to 20‰ due to bacterial sulfate reduction, δ³⁴S values in fulvic acid sulfur were negative and were up to 22‰ lower compared to those of groundwater sulfate. Furthermore, reduced sulfur compounds constituted a significantly higher proportion of total fulvic acid sulfur in the deep groundwater compared to fulvic acids in shallow groundwater, supporting the hypothesis that fulvic acids act as a sink for dissolved hydrogen sulfide in the deep aquifer. Our results suggest that the combination of sulfur K edge XANES spectroscopy and stable isotope analysis on fulvic acids represents a powerful tool to elucidate the role of fulvic acids in the sulfur cycle in groundwater.