Assessment, Distribution and Regional Geochemical Baseline of Heavy Metals in Soils of Densely Populated Area: A Case Study

To understand the content, pollution, distribution and source and to establish a geochemical baseline of heavy metal elements in soil under the influence of high-density population, the concentrations of heavy metal elements Cr, Mn, Co, Ni, Cu, Zn, As, Cd, Hg, Pb and Fe were determined in 23 soil samples in Suzhou University, and geo-accumulation index, enrichment factor, principal component analysis, spatial analysis and regression analysis were completed. The results showed the following: The elements Cu and As were slightly polluted, while the other heavy metal elements were not. The elements Cd, Cu, Ni and As in soils were mainly caused by agricultural activities of chemical fertilizer, whereas the elements Zn and Hg were impacted by the chemicals and batteries. The heavy metal elements in the north were lower than in the south of the campus, as a whole. The enrichment of elements Cu, As and Cd was caused by the east–west river on the campus, and the enrichment of the elements Mn, Ni and Zn was induced by the reservoir. Biochemical experiments and vehicle parking influenced the spatial enrichment of Cr, Co and Pb, while domestic waste led to the spatial differentiation of Hg concentrations. The regression curve between heavy metal elements and Fe was established, and the background values of the heavy metals Cr, Mn, Co, Ni, Cu, Zn, As, Cd, Hg and Pb are 50.90, 489.37, 11.76, 37.74, 55.70, 58.22, 20.07, 0.09, 0.08 and 24.13 mg/kg, respectively.

[Determination of Heavy Metal Geochemical Baseline Values and Its Accumulation in Soils of the Luanhe River Basin, Chengde]

A total of 351 surface soil samples (0-20 cm) were collected from the Luanhe River Basin in Chengde City (a typical area of concentrated mineral resources) and analyzed for 12 heavy metals (Cu, Ni, Cd, Cr, Pb, Zn, Hg, V, Ti, Mn, As, and Co). The geochemical baseline values of the heavy metals were determined using the reference element method and the cumulative frequency curve method. Furthermore, the spatial structure and distribution characteristics of the heavy metals were assessed based on PCA and geostatistical analysis. The accumulation of heavy metal pollution in different types of soil and in association with different land use patterns was also evaluated using a geological accumulative index. The results showed that the geochemical baseline values of V, Ti, Cd, Pb, Mn, and Co in the surface soils of the Luanhe River Basin were higher than their background values for Hebei Province. In contrast, the geochemical baseline values of As, Zn, Cr, Cu, Ni, and Hg were lower than their background values. The average accumulation index of the surface soils followed the order of Cd > Pb > Cu > Ti > Mn > Zn > Cr > Ni > Co > V > Hg > As. More than 80% of the soil samples were categorized as having no accumulation or moderate accumulation of Pb, Ti, V, As, and Co, while over 70% of the soil samples were categorized as having no accumulation or moderate accumulation of Hg, Mn, Ni, Cu, As, Cd, and Cr. With respect to different soil types, the average accumulation index of heavy metals followed the order of fluvo-aquic soil>cinnamon soil>brown soil. With respect to land use types, the accumulation index of heavy metals followed the order of industrial and mining land > shrub forest land > agricultural land > woodland and grassland. The accumulation of Pb and Cd in the surface soils of agricultural land was relatively high in comparison to the other elements, with 27.69% and 25.38% of the samples being above the moderate accumulation level, respectively. The iron group elements Ti, V, Co, Ni, and Cr are likely to derive from naturally high geological background sources, while the spatial patterns of Cd, Pb, Zn, Mn, Cu, and As were associated with the combination of parent material and anthropogenic inputs. The accumulation of Hg was mainly influenced by human activities.

[Pollution Characteristic of Ni in Sediments in the Three Gorges Reservoir]

The Three Gorges Reservoir (TGR) has achieved the target of water storage of 175 m for eight consecutive years until October 2017. To study the temporal and spatial variation of nickel (Ni) in different water periods under 175 m operating conditions in the TGR area, probe the impact of a large-scale water conservancy project on the Ni enrichment, and establish the pollution evaluation system of Ni in the TGR area, we collected 173 surface sediment samples from the TGR area during four consecutive water periods from December 2015 to June 2017 and measured the Ni concentration by using inductively coupled plasma mass spectrometry (ICP-MS). The mean values of Ni in the four water periods are higher than the background value of Yangtze sediment and soil. At the spatial scale, the Ni content in the mainstream exhibits a clear upward trend from upstream to downstream and is lower than that of the tributary. The tributaries of the lower reaches shows a notably higher Ni content than the tributaries of the upper and middle reaches. At the temporal scale, the water period exerts an insignificant effect on the Ni content. The Ni content is relatively stable and shows a downward tendency at a 175 m water level. The regional geochemical baseline (RGB) value of Ni was obtained through building a geochemical baseline model in the area. The RGB values of Ni in sediments during four consecutive water periods are 47.0, 44.2, 42.9, and 41.9 mg·kg^-1, respectively. The Ni contents in the middle and lower reaches of the mainstream and tributary are significantly affected by human activities. Moreover, the pollution evaluations based on global Ni background values, local background values, and geochemical baseline values as reference values were compared and the geoaccumulation index and potential ecological risk of Ni were determined to comprehensively assess its pollution risk. The assessment data indicate that Ni in the aquatic environment of the TGR area is almost uncontaminated and poses a low ecological risk, except for samples in regions around Fengdu County and Guizhou Town in the Zigui County along the mainstream, which were uncontaminated to moderately contaminated. Relative to global and regional background values, the pollution assessment results obtained using the RGB as a reference value are more scientific and better match the temporal and spatial variation of the study area.