Ecological risk assessment of sediments in Beiluo River, Loess Plateau

With the rapid development of industrialization and urbanization, the wide spread of heavy metal pollution in river sediments is posing a threat to the ecological security. As a typical river on the Loess Plateau, the pollution of Beiluo River is becoming increasingly prominent, whereas no attempt has been made to systematically evaluate the spatial distribution of heavy metals in the watershed. Therefore, the monitoring and analysis of typical heavy metals in sediments of Beiluo River is of great significance to the ecological health of the basin.In June 2021, sediments were sampled from 17 sites in the main and tributaries of the Beiluo River. The spatial distribution of eight heavy metals (Hg, As, Cd, Cr, Cu, Pb, Ni, Zn) was determined by microwave digestion-atomic fluorescence spectrometry and Aqua regia extraction-inductively coupled plasma mass spectrometry, followed by evaluating the degree of heavy metal pollution and the ecological risk assessment using geoaccumulation index, Nemerow pollution index, and potential ecological risk index, respectively. The source apportionment was conducted using Positive Matrix Factorization (PMF) model and Pearson correlation analysis. The distribution characteristics of heavy metals showed that the content of heavy metals in each sampling site varied greatly. In particular, the content of Zn was significantly higher than that of other heavy metals. The highest content of total heavy metals appeared in the upstream B1 (448.46 mg/kg), followed by the downstream B13 (323.43 mg/kg). Some heavy metal contents were higher than the heavy metal background values of Shaanxi Province. The order of coefficient of variation was as follows: Zn (68.21%) > Hg (51.78%) > Cd (49.65%) > Pb (41.61%) > As (31.82%) > Cu (30.77%) > Ni (23.89%) > Cr (23.74%). In the matter of ecological risk assessment, the geoaccumulation index analyses showed that the accumulative conditions of Cd and Zn were moderately polluted, and most of the other heavy metals were at the pollution-free level. The Nemerow pollution index ranged from 0.74 to 3.64, with an average value of 1.37, and the values of more than half of the sites were within 1 to 2, indicating that the overall pollution in the study area was slightly polluted. The mean values of individual potential ecological risk indexes were: Cd (39.05) > Hg (30.37) > As (12.56) > Cu (4.37) > Ni (4.29) > Pb (3.19) > Zn (1.41) > Cr (0.87). The comprehensive ecological risk index ( RI ) values ranged from 51.60 to 199.75, with an average of 96.09. In addition to the intermediate ecological risk at B10 (158.63) in the middle reaches and B13 (199.75) in the lower reaches, risks in all the other sites were low. Conclusions: (1) The results of geoaccumulation index and Nemerow pollution index show that the heavy metal pollution in the upstream B1 and downstream B13 are ranged from the medium to high levels. According to the results of potential ecological risk index, Cd and Hg in the Beiluo River may pose a threat to the ecosystem. In addition to the ecological risks in the middle stream B10 and the downstream B13 that are classified as moderate, the whole basin has slight potential ecological risks. (2) Based on the PMF model and correlation analysis, the sources of heavy metals in the sediments of Beiluo River were analyzed. The results showed that the sources of eight heavy metals could be divided into three categories: traffic pollution sources, coal and oil industrial pollution sources, and mixed sources of agricultural activities and natural parent materials. (3) Based on the results of this study, it is suggested to divide the pollution levels of the Beiluo River basin into regions, focusing on the heavy metal pollution at B1 in the upper reaches and B13 in the lower reaches, especially reducing the input of metal Zn and Hg in the river basin, and taking certain measures to supervise and control related industries involved in heavy metal pollution, so as to eliminate the pollution of heavy metals in the sediments. In future studies, the enrichment characteristics of heavy metals in sediments with different grain sizes will be revealed to provide more comprehensive scientific data for the prediction of heavy metal pollution in sediments and the overall ecological protection of the basin.

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