某废弃煤矿区土壤重金属污染风险评价

Risk assessment of heavy metals in the soil of an abandoned coal mine area

煤矿在开采过程中产生大量煤矸石、洗煤水等废弃物,其中的重金属污染物经沉降、径流、淋滤等作用迁移、扩散至周边水土环境中,对生态环境构成威胁。为研究某废弃煤矿区土壤重金属污染状况,评价重金属污染生态风险,检测了该矿区煤矸石、水、土壤样品中Cu、Pb、Zn、As、Cr、Ni、Hg、Cd元素质量分数,利用单因子指数法和潜在生态危害指数法分别分析了土壤重金属污染状况和潜在生态风险;重点分析了矿区洗煤厂下游土壤重金属污染特征,并采用改进BCR连续提取法分析了洗煤厂下游土壤样品重金属元素(Cu、Zn、Cd)形态,借鉴风险评价编码法评价了重金属元素的生物有效性和环境风险。结果表明:①以平均值计,废弃煤矿区煤矸石中重金属质量分数较低,仅Cr元素质量分数稍高于土壤中的质量分数,且煤矸石中重金属元素质量分数均未超过相应的农用地土壤风险筛选值,对矿区土壤重金属污染影响小。②废弃煤矿区土壤重金属超标元素为Cu、Zn、Cd、Ni、As,Cu、Zn、Ni和As元素污染较轻微,Cd元素污染较严重;Cu、Zn、Ni元素污染区主要集中在矸石山西部和东南部地块,在空间分布上有一定重合;As元素污染分布在矸石山东北部地块,Cd元素污染在矿区广泛分布,以中度–重度污染为主。③Cd、Hg元素存在潜在生态危害风险,Cd元素潜在生态危害风险程度高,重金属元素综合潜在生态危害风险等级较高,主要为中等-强风险。④洗煤厂下游区域沿水流方向土壤重金属元素总体呈递减趋势,Cd元素污染较严重,Cu、Zn、Ni、As元素污染集中在洗煤厂下游200 m内,剖面样品中垂向方向上重金属元素质量分数呈递减趋势,在垂向上没有累积。⑤矿区地下水水质较好,矿井排水重金属元素质量分数及pH满足Ⅲ类地表水水质标准限值,矿井水外排对周边土壤重金属污染影响小。⑥Cu和Zn元素酸可提取态占比低,生物有效性低,Cd元素酸可提取态占比高,呈高度风险–极高风险,生物有效性强,环境风险高。基于矿区重金属污染现状,建议进一步加强土壤重金属污染防治工作,加强源头治理、农产品食品安全监测和Cd元素污染调查研究工作。

During the coal mining process,a large amount of waste such as coal gangue and coal washing water is generated.The heavy metal pollutants in those wastes migrate and diffuse into the surrounding soil and water environment through sedimentation,runoff,leaching and other processes,posing a threat to the ecological environment.In order to study the heavy metal pollution in an abandoned coal mine area and to evaluate its ecological hazard risk,the contents of Cu,Pb,Zn,As,Cr,Ni,Hg,Cd elements in the coal gangue,water and soil samples of the mining area were detected.The single factor index method and potential ecological hazard index method were used to analyze the heavy metal pollution and its potential ecological hazard risk.The heavy metal pollution in the soil downstream of the coal washing plant in the mining area was mainly studied.The improved BCR continuous extraction method was used to analyze the forms of heavy metal elements(Cu,Zn,Cd)in the soil samples and the risk assessment coding method was employed to evaluate the bioavailability and environmental risks of heavy metal elements.The results show that:①on average,the heavy metal contents in the coal gangue of the coal mine area are relatively low,with only Cr element content being slightly higher than that in soil.Moreover,the heavy metal element contents in the coal gangue do not exceed the risk intervention values for the soil of agricultural land,which has a little impact on heavy metal pollution in the soil.②The heavy metal contents in the mine area’exceed the standard,including that of Cu,Zn,Cd,Ni and As elements,and the Cd pollution is relatively serious.The pollution areas of Cu,Zn,and Ni elements are mainly concentrated in the western and southeastern parts of the gangue hill,with a certain overlap in spatial distribution.The As element pollution is distributed in the northeastern part of the gangue hill,while the Cd element pollution is widely distributed in the mine area,with moderate to severe pollution.③The Cd and Hg elements have potential ecological hazard risk,with a higher risk for Cd element.The comprehensive potential ecological hazard risk(RI)of heavy metal elements is mainly moderate to strong.④The contents of heavy metal elements in the soil downstream of the coal washing plant show an overall decreasing trend along the water flow direction,with the Cd element pollution being more severe,and the Cu,Zn,Ni,As element pollution concentrated within 200 m downstream of the coal washing plant.The heavy metal element contents of the profile sample shows a decreasing trend in the vertical direction,with no accumulation in the vertical direction.⑤The groundwater quality in the mine area is good,and the pH and the contents of heavy metal elements in the mine water meet the limit values of Class III surface water quality standards.The discharge of mine water drainage has a little impact on heavy metal pollution in the soil.⑥The proportions of acid extractable Cu and Zn elements are low,with low bioavailability,while that of Cd element is high,presenting a high-extremely high risk,with strong bioavailability and high environmental risk.Based on the current situation of heavy metal pollution in the mining area,it is recommended to further strengthen the prevention and control of heavy metal pollution,strengthen the source control,monitoring of agricultural food safety,and the investigation and research on the Cd element pollution.