Quantitative Source Apportionment and Origin Analysis of Sulfide in the Middle and Lower Reaches Groundwater of Chaobai-Wenyu Alluvial-proluvial Fan, Beijing

• • APCS-MLR quantified that water desulphidation (F3) contributes 21.32% of groundwater sulfide, while indigenous sources (AVS dissolution, ES disproportionation, geothermal, well materials) contribute 63.66%, indicating natural dominance. • • δ34S and δ18O isotope analysis confirmed sulfate mainly from evaporite dissolution, with desulphidation-derived sulfides classified as geological background reduction, not anthropogenic contamination. • • High-sulfide groundwater is predominantly in Na-type water, concentrated in Shunyi and the first three aquifers, with no significant seasonal variation (wet vs. dry), suggesting stable geological release. • • PCA identified four principal components (F1–F4) explaining sulfide variability; F3 (desulphidation) had the highest factor loading (0.418), while other components had negative loadings, underscoring desulphidation as the primary process.