Spatial variations, health risk assessment, and source apportionment of soil heavy metals in the middle Yellow River Basin of northern China

The ecological and environmental problems in the Yellow River Basin of northern China have attracted wide-spread research attention. This study focused on the soil heavy metals (HMs) in the middle Yellow River Basin; the spatial variations of soil HMs were analyzed using geostatistical techniques, the pollution status and health risks were comprehensively assessed through multiple methods, and the sources of soil HMs were apportioned using a positive matrix factorization (PMF) model. The results showed that the spatial patterns of soil HMs in the study area with a high coefficient of variation (>20 %) exhibited obvious regional zoning, at a low level in the north and a high level in the south. The soil with significantly high HM content was mainly located in the vicinity of industrial cities and usually featured large enrichment factor values (>1.5), especially with respect to Hg, Cd, and Pb, indicating some influence of anthropogenic activities on soil HMs. Based on the comprehensive assessment method, it was found that soil in the study area was clean and safe on the whole, but relatively severe pollution and relatively high risks were found in the strip area of urban agglomeration, including in Tongchuan and Weinan. Further, Hg, Cd, and Pb - with a cumulative contribution of >85 % - were the primary elements to cause soil pollution, while As and Pb were the primary objects leading to health risks, with a cumulative contribution of >95 %. After quantitative analysis using a PMF model, five sources of soil HMs were identified in the south and their contributions decreased in the following order: natural source (43.25 %), industrial source (21.09 %), mixed source (13.15 %), agricultural source (13.11 %) and traffic source (9.40 %). Moreover, some key areas were delineated both with the combined pollution of soil HMs and the relatively high health risk for children. These key areas should be paid more attention to and the Hg, Cd, Pb, and As should be the main objects of prevention and control in the middle Yellow River Basin.