Heavy metal pollution characteristics and its response of source-sink relationship in agricultural soil at field scale

Abstract: As one of the most important resources about assuring food security and human development, the agricultural soil and its pollution has been widely reported these years. A certain field in Yangtze River Delta was selected as our study area to analyze the agricultural soil heavy metal pollution characteristics and its main influence factors in case of the same pollution sources. We collected 30 soil samples to analyze their heavy metal concentration (Cd, Pb, Cr, As, Co, Cu, Ni and Zn) and soil physico-chemical properties (i.e. EC, CEC, pH, OC, clay content). The results showed that the average Cd concentration in this field was 3.74 mg/kg soil, which exceeded the China Tier II environmental standard value by 12.5 times. The highest value of Cd even exceeded the Tier II environmental standard value by 44.7 times. The Cd was mainly accumulated at the top soil. The mean concentrations of Cd at the top and down soil were 4.44 mg/kg soil and 2.09 mg/kg soil, respectively. The high values of Cd were mainly distributed at the area nearby the water inlet and outlet. The Pb of soil samples that exceeded the soil background value accounted for 90%, but the Pb concentrations of all soil samples did not exceed the China Environmental Quality Standard (Tier II). The mean Pb concentration of top and lower soil were 33.98, 26.29 mg/kg soil. All soil samples of Cr, As, Co, Cu, Ni and Zn had no pollution. The CV of Cd at the top soil was 64%, which was extremely higher than that of other heavy metals (about 6% of Zn, and about 15% of As). Ni, Cu, Zn had small spatial variation, which indicated that the filed had similar sources. Three pollution evaluation method: geo-accumulation index (Igeo), Hakanson potential ecological index (EI), and Nemerow pollution index (NPI) were used to evaluate the heavy metal pollution level. We found that most of the soil samplers had high Cd Igeo (mean: 5.82), which were identified as "severe contamination". The mean EI of soil samples were 2 566.97, which exceeded 4.3 times of the lowest value of "highest ecological risk". Moreover, the NPI ranged from 2.99-217.02 (mean value: 60.58). All these evaluation showed that the extremely serious soil Cd pollution caused the highest pollution level in this field. Moreover, we used the correlation analysis and path analysis to identify the main influence factors of Cd pollution in a situation of similar sources at a filed scale. The Cd did not significantly correlate with every index of soil physicochemical properties. It can be indicated that the Cd through soil adsorption were extremely lower than the Cd through external source import. The distance from the water inlet and outlet was the most direct and determinant factor at the field scale. The north of this field was drainage outlet. The Cd can flow into the field with water through water inlet. So Cd concentration of the northern field decreased with the distance of water inlet. And water flow velocity slowed down when the water in the field reached the surrounding of water outlet, which caused accumulation of Cd nearby the outlet. Decreasing the Cd of persistent external source import and improving the irrigation and drainage facilities were the efficient way to reduce the severe Cd pollution.