Source Identification and Apportionment of Abnormal High Boron Concentrations in Yellow River Basin, China

Despite the well-documented anomalously high dissolved boron (B) concentrations in the Yellow River (YR), the precise sources and processes driving this enrichment remain unclear. To quantify and apportion B sources, we systematically analysed B concentrations across riverine, precipitation and wastewater samples within the YR basin. Complementary leaching experiments were performed on loess and suspended particulate matter (SPM) to decipher loess erosion impacts on river B dynamics. Forward mixing model results demonstrate that riverine B predominantly derives from silicate weathering (49.7%–81.3%) and evaporite dissolution (17.8%–42.3%), while contributions from carbonate weathering, rainfall and anthropogenic activities are negligible. Leaching experiments further confirmed that evaporite dissolution during loess runoff processes contributes 93.6% of the total evaporite-derived B flux in YR water. Meanwhile, we found that 89.2% of B in loess exists in silicate phases. Although the proportion of silicate weathering in loess is only 8.9%, the massive erosion of SPM (3.33 × 10⁸ t/year) elevates silicate weathering to the primary source of B in the YR. Furthermore, the YR's characteristically low discharge (28.3 km³/year) amplifies B concentrations through hydrological concentration effects. In 2012, the YR exported 2.21% of the global riverine B flux to the ocean, underscoring its disproportionate contribution to the global B cycle. This study clarifies the critical role of silicate weathering amplified by massive loess erosion in reshaping understanding of particulate-phase geochemical processes in global riverine B cycling.