Abstract:

Soil serves as the foundation for grain production and is an important cornerstone for national economic development. In recent years, owing to human activities and the geological background of certain regions, soil heavy metal pollution has become increasingly severe. Heavy metals are highly mobile in the environment, are easily absorbed by crops, enter the food chain, and pose a threat to human health. Effective risk management of contaminated farmlands is crucial to ensure food security. According to the “Environmental Quality Standards for Soils of Agricultural Land-Risk Control Standards for Soil Pollution (Trial)” (GB 15618—2018), farmland soils are classified and managed based on their total heavy metal content. The “Guidelines for the Remediation and Management of Contaminated Agricultural Land” (NY/T 3499—2019) stipulate that for safe-use farmland, pollution source control should be prioritized before remediation efforts. Thus, the primary approach to managing soil pollution risks in farmlands involves classifying environmental quality, tracing and eliminating pollution sources, and implementing pollution remediation. Studies have shown that soil heavy metal pollution is concealed, cumulative, and originates from diverse sources influenced by factors such as the geological background and human activities. Therefore, excessive heavy metal levels in farmland soils do not necessarily indicate exogenous pollution. Conversely, even if soil heavy metal concentrations are within limits, the presence of nearby pollution sources poses potential ecological risks owing to their accumulation. Currently, the classification of farmland soil environmental quality in many regions of China is based on soil pollution characteristics, neglecting the accumulation of soil heavy metals due to potential pollution sources. This oversight makes it difficult to accurately determine whether farmland soil is contaminated by exogenous sources during subsequent risk management, which affect precise source tracing and elimination. Therefore, managing the risk of soil pollution in farmlands is a crucial measure for improving soil environmental quality and ensuring agricultural product safety, and it is key to research technologies for the prevention and control of farmland soil pollution. Datong Town is located in the eastern part of Pengshui County, Chongqing, with an altitude range of 1000 to 1480 meters. The local stratigraphy is primarily composed of the Maokou Formation of the Permian system, characterized by limestone and carbonaceous shale (with coal seams). The farmland is predominantly dryland, with corn being the main agricultural product. As a typical area with a high geological background, its soil heavy metal pollution characteristics are representative of those of the southwestern region of China. A grid-based sampling method considering farmland distribution was used to collect 50 sets of soil and corn samples. The concentrations of the heavy metals Cd, Hg, As, Pb, and Cr, as well as the soil pH, were analyzed. A comprehensive evaluation of soil heavy metal pollution and accumulation was conducted and methods for classifying farmland environmental quality were explored. Precise suggestions for tracing and eliminating pollution sources and remediation measures are proposed to provide technical support for managing the risk of soil pollution in geological high-background areas. The results indicated that the average concentrations of Cd, Hg, As, Pb, and Cr in the soil of the study area were 0.84, 0.12, 17.05, 25.90 and 111.69 mg·kg⁻¹, respectively. The soil pH ranged from 4.46 to 7.85, with a median value of 5.97, indicating that the soil was generally acidic. High soil heavy metal concentrations were concentrated in the southwestern part of the study area, where Datong Town is located. The proportion of soil Cd classification was 10.00% for priority protection, 76.00% for safe use, and 14.00% for strict control, with the majority being safe. The other heavy metals in the soil fall under the priority protection category. The average accumulation index (A) for soil heavy metals was 0.92, with 22% of sites having A≥1, primarily due to the accumulation of Cd and As. The overall exceedance rate of corn samples in the study area was 28%, with mild and severe exceedance rates of 18% and 10%, respectively. The exceedance was attributed to Cd, and the exceedance rate and degree were mainly related to the soil Cd content and pH. When the soil pH exceeded 6.5, the risk of agricultural product exceedance decreased significantly. The classification of soil environmental quality showed that the areas of Class I (priority protection, no significant accumulation), Class II (priority protection, significant accumulation), Class III (safe use, no significant accumulation), Class IV (safe use, significant accumulation), Class V (strict control, no significant accumulation), and Class VI (strict control, significant accumulation) farmlands were 20.73, 0, 369.34, 88.98, 0 and 50.24 hectares, respectively, with Class III being dominant. Class IV and VI farmlands are located in the southwestern part of the study area. The analysis of soil heavy metal sources indicated that excessive levels of heavy metals in the farmland soil of the study area were generally influenced by the geological background. The spatial autocorrelation of soil Cd and As was moderate, with strong spatial variability in their content. Due to exogenous inputs, excessive levels of soil in some farmlands showed a clear increasing trend. It is recommended to strengthen the monitoring of local agricultural inputs and to control those that do not meet these standards. Based on tracing and eliminating the sources of soil heavy metals, calcium-based soil conditioners or calcium-magnesium phosphate fertilizers can be applied according to soil pH and heavy metal pollution levels to achieve “production while remediation” and gradually improve the soil environmental quality in the study area. The methodological innovations in this study were manifested in two ways. First, it identifies areas that are not polluted but have exogenous inputs and includes them in risk areas. This aspect has not been addressed in the “Environmental Quality Standards for Soils of Agricultural Land-Risk Control Standards for Soil Pollution (Trial)” (GB 15618—2018) and the “Technical Guidelines for the Classification of Environmental Quality of Agricultural Land Soils”, posing potential risks to the soil. Second, the method proposed in this study was used to classify the risk categories of farmland soil, and the results were verified by the exceedance of the collected crops, indicating the feasibility of the proposed method. Based on this method, crop samples can be selectively collected according to the classification, in contrast to the sampling method given in the “Technical Guidelines for the Classification of Environmental Quality of Agricultural Land Soils”, which reduces work costs.

Key words: cultivated land, heavy metal pollution, cumulative evaluation, environmental quality classification

摘要：

以重庆彭水县大同镇为例，协同采集土壤-玉米样品50套，分析土壤及玉米样品中Cd、Hg、As、Pb和Cr的含量及土壤pH，综合考虑土壤重金属污染及累积性，开展耕地土壤环境质量类别划分，提出风险管控措施建议，以期为地质高背景区耕地土壤风险管控提供技术方法。结果表明，研究区耕地土壤以安全利用类和严格管控类为主，土壤存在明显重金属累积的点位占比为22%，主要的超标及累积因子为Cd和As，土壤重金属污染及累积问题显著。研究区耕地土壤环境质量类别以III类（安全利用类，无明显累积）为主，IV类（安全利用类，有明显累积）和VI类（严格管控类，无明显累积）耕地均分布在研究区西南部。研究区耕地土壤重金属超标整体受地质背景的影响，重金属累积主要与城镇生活污水、禽畜粪肥和部分复合肥施用等人为活动有关，由于外源输入，部分耕地土壤超标程度加剧趋势明显。研究区玉米样品的总体超标率为28%，超标因子为Cd。当土壤pH>6.5时，农产品超标风险显著降低。建议在耕地土壤重金属溯源断源的基础上，根据土壤及农产品污染程度，施用钙质土壤调理剂或钙镁磷肥进行土壤酸化治理，实现“边生产，边修复”，逐渐改善研究区土壤环境质量状况。

关键词: 耕地, 重金属污染, 累积性评价, 环境质量类别划分