Source-oriented health risk assessment of heavy metals in a soil-river continuum in northwest China

Heavy metals can easily accumulate in soil and river sediment due to their non-degradability, which seriously endangers the balance of the ecosystem and human health. Thus, research on heavy metals in soil-river composite ecosystems has great significance for risk mitigation. As a seasonal river in a semi-arid region (Yuzhong county) in northwest China, the Wanchuan River supports nearly 500,000 people, but has been polluted by the heavy metals from industrial wastes and sewage discharges. To protect precious water resources and soil ecosystems, it is urgent to do heavy metal research in the Wanchuan River basin. Towards this end, the current study collected samples of soil and river sediment in this basin, and analyzed the distribution characteristics, health risks, and pollution sources of heavy metals, based on the pollution index evaluation method, health risk assessment, a geostatistical model, and the positive matrix factorization (PMF) model. The research results show that the contents of heavy metals in soil and river sediment were equivalent. However, contamination levels of heavy metals in stream sediment fell below that in soil. The results from the PMF model pointed out that natural sources contributed remarkably to the contents of heavy metals in soil, while industrial sources had the highest contribution to the contents of heavy metals in river sediment. According to the health risk assessment, among all selected metals, Cr brought about the highest carcinogenic risk, while As generated the highest non-carcinogenic risk. Source-oriented health risk assessment showed that natural sources (37%) and agricultural sources (50%), respectively, contributed most to the non-carcinogenic risks and carcinogenic risks induced by soil metals. Industrial sources contributed most both to the non-carcinogenic risks (46%) and carcinogenic risks (37%) induced by river sediment metals. The current research may offer theoretical bases for heavy metal pollution treatment in the soil-river continuum.