Determination of Heavy Metal Baseline Values and Analysis of Its Accumulation Characteristics in Agricultural Land in Chongqing

Abstract

In order to understand the accumulation of heavy metals of agricultural land in Chongqing and make clear priority control elements, the soil heavy metal baseline values need to be determined. Based on geochemical baselines, a total of 214 surface soil samples of six soil types were collected according to stratified sampling. Under strict quality control, the As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn content in each sample was measured. The baseline values of eight heavy metals were determined using mathematical statistics, iterative culling, and cumulative frequency curve methods. The accumulation of heavy metals in agricultural soils in Chongqing was evaluated using the geochemical baseline pollution and geological accumulation index methods. The mean baseline values of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn in agricultural land in Chongqing were 5.83, 0.25, 66.78, 25.45, 0.069, 29.90, 26.18, and 78.44 mg·kg-1, respectively, which agreed with the historical findings. Evaluation of the geochemical baseline pollution index method showed that the accumulation of As, Hg, and Zn in soil was the most prominent, with 14.65%, 11.82%, and 3.88% of the samples reaching severe accumulation, respectively. The proportions of moderate accumulation of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn were 12.12%, 6.50%, 3.79%, 6.50%, 15.27%, 5.29%, 5.45%, and 17.96%, respectively, of which As, Hg, and Zn were relatively large. The geological accumulation index indicated that more than 93% of the samples of Cd, Cr, Cu, Ni, and Pb were in a state of no accumulation. The proportions of mild accumulation of Hg, Zn, and As were 26.60%, 21.84%, and 21.21%, respectively. Only 5.56% and 0.49% of the samples had moderate accumulation of As and Zn. Different soil types have different properties. All soil types in the sample had different degrees of accumulation of the eight kinds of heavy metals. The accumulation of heavy metals in paddy and purple soils was lower than that in other soil types. For the soils of different geological and geomorphic types, the Ki values of As in the limestone middle and low mountain area of Daba Mountain in Northeast Chongqing and the low mountain area of the limestone of Wuling Mountain in Southeast Chongqing were relatively large; the rest were not significantly different, and all belonged to the non-accumulation and mild accumulation ranges. Both evaluation methods showed that the main accumulated heavy metals in agricultural land soil in Chongqing was As, followed by Hg and Zn, and management and prevention of the accumulation of these elements should be strengthened.