陕北某化工企业周围污灌区土壤-作物系统重金属积累特征及评价

Accumulation Characteristics and Evaluation of Heavy Metals in Soil-Crop System Affected by Wastewater Irrigation Around a Chemical Factory in Shenmu County

对陕西北部某化工企业污灌区土壤一作物系统的重金属积累状况及空间分布进行了深入研究．结果表明，污水排放导致了周围农田土壤中Cd和Cu的聚集，其中Cd的积累量超过国家土壤环境二级标准；土壤Cu、zn和Pb的单因子污染指数均小于1，Cd的单因子污染指数为1．21，属轻度污染，4种重金属综合污染指数为0．74，属警戒线等级．在剖面上，所有重金属元素均表现出明显的表聚现象，主要聚集在土壤表层0～10Cm范围内，在空间分布上，污水排放企业污灌区土壤和作物重金属Cu、zn和Cd的强烈聚集区出现在企业排污口附近100m范围内，而Pb集中在200m范围内，并随着距企业距离的增加重金属含量呈降低趋势．在企业污水灌溉的影响下，玉米籽粒中Cu、Pb和Cd的平均含量分别为4，74、0．129和0．036mg·kg^-1，明显高于对照区，其中P11达到5．7％的超标率．玉米籽粒中重金属单因子污染指数均小于1，大小为Pb〉Cu〉Zn〉Cd，综合污染指数为0．53，属安全清洁等级；4种重金属元素中，除Cd在作物中与土壤中有效态和全量之间均呈现极显著的正相关关系外，其余元素作物中和土壤中含量相关性均未达到显著水平．因此，该化工企业周围，尽管由于污灌引起重金属在土壤中的累积，但由于在农作物中的积累有限，暂时未对人体健康造成威胁，但由于交通及烟尘引起的Pb的超标应引起重视．

Soil heavy metals Cu, Pb, Zn, and Cd, are regarded as “chemical time bombs” because of their propensity for accumulation in the soil and uptake by crops. This ultimately causes human toxicity in both the short and long term, making farmland ecosystems dangerous to health, In this paper, accumulation and spatial variability of Cu, Zn, Pb and Cd in soil-crop system affected by wastewater irrigation around a chemical factor in northern Shaanxi province were analyzed. Results showed that wastewater irrigation around the chemical factory induced significant accumulation in soils compared with control areas. The average concentrations of available Cu and total Cu were 4. 32 mg·kg^-1 and 38.4 mg·kg^-1, which were twice and 1.35 times higher than those of the control area, respectively. Soil Zn and Pb were slightly accumulated. Whereas soil Cd was significantly accumulated and was higher than the critical level of soil environmental quality （ Ⅱ ）, the available and total Cd concentrations were 0. 248 mg·kg^-1 and 1.21 mg·kg^-1, which were 10 and 6.1 times higher than those of the control areas, No significant correlations were found between available and total heavy metals except between available Cd and total Cd. All the heavy metals were mainly accumulated in the top layer （0-10 cm）. Spatially, soils and plants high in heavy metal concentration were distributed within the radius of about 100 m from the waste water outlet for Cu, Zn and Cd and about 200 m for Pb, and decreased exponentially with the distance from the factory. Affected by wastewater irrigation, contents of Cu, Pb and Cd in maize were 4.74, 0. 129 and 0. 036 rag. kg- 1 which were slightly higher than those in the control area. The content of Zn was similar to that in the control area. Affected by the vehicle exhaust, the over standard rate of Pb was 5.7% in maize. All the heavy metals did not show significant correlation between soil and crop, except Cd. The square correlation coefficients were 0. 83 and 0.75 between soil available and total Cd with maize. Therefore, the chemical factory contributed to the accumulation of heavy metals in the soil around it, but the contribution to the crop was limited, and thus temporarily caused no threat to human health. The reason for the lower accumulation was the high soil pH and low soil organic matter content. But more attentions should be paid to the higher accumulation of Pb in the maize caused by the vehicle and dust.