Comprehensive multimetal isotopic model for regional-scale water management in a mining city: Tracing contamination sources, pathways, and geochemical processes using Cu, Mo, S-O, and Zn isotopes

Major mining cities worldwide have been suffering from diverse contamination sources such as tailings, mine drainages, geology enriched with toxic metals, and other industrial and domestic sources. This study established a multi-isotopic comprehensive model for elucidating water contamination sources and geochemical reactions in the city of Erdenet in Mongolia. The Erdenet city was contaminated with As, Cu, Mo, Zn, and SO₄^{2 −}, and we used isotopes of Cu, Mo, Zn, and S–O in SO₄²⁻. Contamination sources for groundwater and surface water were differentiated as tailings dump, excavated ore, heap leaching, ash pond of a power plant, and argillic alteration zone. Groundwater in the residential area was influenced by the argillic alteration zone, as indicated by low δ³⁴S_SO4 and high δ⁶⁵Cu values, while δ⁶⁶Zn fingerprints may have been masked by adsorption and mixing. Additionally, δ⁹⁸Mo fingerprints from two major Mo contamination sources (the ash pond and tailings dump) were evident in the stream. The tailings dump substantially impacted δ⁶⁵Cu, δ⁹⁸Mo, and δ⁶⁶Zn values in the stream, with isotopic fractionation occurring through oxidative dissolution and adsorption. Furthermore, to assess oxidative dissolution of sulfides and adsorption, δ⁶⁵Cu and δ⁹⁸Mo+ δ⁶⁶Zn were found to be particularly useful, respectively. This study highlights the effectiveness of multimetal isotopic ratios in tracing contamination sources and geochemical processes in regions with diverse contaminants, presenting a robust spatial model for isotopic fingerprinting.