Tracing Fe Sources in Suspended Particulate Matter (SPM) in the Mun River: Application of Fe-Stable Isotopes Based on a Binary Mixing Model

The Fe isotopic composition of the riverine suspended particulate matter (SPM) in the Mun River was found to improve the knowledge of the Fe isotope behaviors in the surface environment. The δ56Fe varies from −0.25 to 0.29‰ with an average value of 0.02‰. Most Fe (on average 76.1%) exists in suspended phases rather than dissolved phases in the Mun River, highlighting the significance of investigating the Fe isotopic compositions of riverine SPM. The close relationship between the chemical index of alteration (CIA) value and the δ56Fe value (r = 0.56, P < 0.01), as well as the increasing trend of the δ56Fe value along the weathering trend line (A–CN–K diagram), confirm that the types and intensities of weathering are essential for regulating Fe isotopic fractionation. Evidence from the high enrichment factor (EF, 0.62–3.97) and the significant correlation between (Fe/Al)SPM and (Zn/Al)SPM indicate the important role of the anthropogenic Fe contribution. To decipher the origin of Fe in SPM, the natural and anthropogenic end-members were investigated further. The Fe contents display a clear correlation with Na, Mg, Al, K, and rare earth elements (REE), indicating that clay and heavy minerals are important containers for riverine particulate Fe. A binary mixing model was adopted and the results showed that the relative anthropogenic Fe contribution varies from 5.79 to 97.70% (average 48.46%) for most SPM (excluding high-Mn samples). The spatial distribution characteristics of relative anthropogenic contribution suggest agricultural activities as a likely source of anthropogenic particulate Fe in SPM.