Boron sources and transport mechanisms in river waters collected from southwestern Taiwan: Isotopic evidence

Abstract

Boron (B) isotopes are sensitive tracers for constraining water–rock interactions and pollution sources in river systems. In this study, we analyzed δ11B using multi-collector inductive coupled plasma mass spectrometry, in addition to major and trace elements, to delineate B sources and transport mechanisms in two river catchments, the largest Kao-ping River and the most polluted Erren River in southwestern Taiwan.Dissolved δ11B values vary more than 35‰ in Kao-ping River, but keep relatively constant in Erren River, 12.2–26.7‰. Four major B sources identified in these river waters are cyclic seasalt, water/rock interaction, geothermal water and anthropogenic inputs. All specimens collected near the estuaries were influenced strongly by the seawater, B=4.5mg/L and δ11B=39.5‰. The Erren River results provide unique information on B and δ11B compositions in anthropogenic sources, in particular for fertilizer and hog manure, characterized with high B (up to 3.43mg/L) and intermediate δ11B (average 13‰). The low δ11B values in 2006 Kao-ping River are attributed to contributions from local geothermal inputs, average 2.5ppm and 2‰, prior to massive landslides caused by 2009 Morakot typhoon. Mass balance calculations suggest that major contribution of B in river waters was derived from silicate weathering, characterized by low B and intermediate δ11B, contributed approximately 49–97% in 2010 Kao-ping River. The detected changes of δ11B (Δw-rock) reaches a maximum variation of 18‰ during chemical weathering in catchment and highlights the importance of water/rock interaction enhancement after major climatic events.