Molybdenum Isotopic Fingerprints in Taiwan Rivers: Natural versus Anthropogenic Sources

Abstract

Two rivers from Taiwan with different source inputs, the Danshuei (polluted) and Liwu (non-polluted), were selected to study the behavior of molybdenum (Mo) isotopes during weathering and riverine transport. In the Danshuei River, δ98/95Mo ranges from 0.83‰ to 1.50‰ (wet season) and 0.54‰ to 1.25‰ (dry season). With a few exceptions, δ98/95Mo in the Danshuei River is lighter during the wet season, while, in contrast, heavier in the dry season. In the Liwu River, δ98/95Mo varies from 0.54‰ to 1.30‰ and gets heavier along the mainstream. Using the MixSIAR model, three Mo sources are identified in the Danshuei River: seawater intrusion, rock–water interaction, and anthropogenic inputs. Seawater intrusion can explain the heavy δ98/95Mo downstream signal during the wet season contributing 14–39% from the MixSIAR model. However, the lighter δ98/95Mo signal during the dry season is most likely due to anthropogenic inputs in the middle and lower reaches of the Danshuei River contributing 75–98%. In the Liwu River, dissolved Mo isotopes correlate with SO42−/Na and (Sr/Na) ×1000 ratio, suggesting that pyrite oxidation coupled with carbonate weathering governs the heavy δ98/95Mo signature, with sequestration of light δ98/95Mo into secondary mineral phases in bedload sediments. Furthermore, these results have important implications for riverine Mo sources to the ocean, controlled by anthropogenic activity and weathering processes.

Iron and Molybdenum Isotope Application for Tracing Industrial Contamination in a Highly Polluted River

“…The variations in Mo isotopes in different natural reservoirs due to anthropogenic pollution have also been observed in the literature. The Mo isotopic fractionation can be utilized to discern whether the natural water reservoirs, such as rivers and oceans, have been affected by anthropogenic pollution [34][35][36]. Lastly, Mo isotopic differentiations could also offer the further investigation of anthropogenic contamination in various nature environments such as sediments, soils, and aerosols [37][38][39].…”