| With the rapid development of modern society and the economy, oil has become one of the most important mineral resources for human survival and development. However, due to natural and man-made reasons, petroleum hydrocarbons and heavy metals, from oil drilling, mining and transport processing, are emissioned to soil, water and air and cause severe damages to the ecological environment. Especially heavy metals in soils are of high toxicity, persistence and accumulation characteristics, and lead to serious threat to soil microorganisms, plants, animals and even human beings. So far, there are many researches about petroleum hydrocarbons pollutants in water and soil pollution, but few about the pollution of heavy metals in oil-polluted soils, and their distribution patterns and potential sources. The Yellow River Delta has abundant oil and gas resources, and therefore sufer from oil and heavy metal contaminations. Combined with fragile ecological environment caused by soil salinization in the Yellow River Delta, the situation is conflict with the national strategic needs of the construction of the Yellow River Delta Efficient Ecological Economic Zone. Therefore, it has become an urgent solved research topic to study heavy metals pollution status, distribution and sources in oil-polluted soils of the Yellow River Delta, and model a comprehensive and effective method for soil environmental quality assessment.This article relies on the environmental nonprofit industry research projects of the Chinese Ministry of Environmental Protection. Gudao oilfield was chosen as the study area in this study. The contamination level of heavy metals in oil-polluted alkaline soils and their spatial, temporal and speciation distributions were analyzed, and discussed their relationship of with oilwell exploitation. Source apportionment analysis and modeling an environmental quality assessment method were also worked out. These work provides technical and theoretical supports to the further development of standard and criterion of oil-heavy metal compound contamination, strengthen environmental management and improving soil environmental quality by the government and supervision department. The main study contents are as follows:(1) Using microwave digestion, the pretreatment of oil-polluted alkaline soils was optimized with HNO3(5mL)+HF(3mL)+H2O2(2mL) composite acid system. This method digested more complete, with better precision (RSD=1.5-7.5) and better reproducibility (recovery of85-110%) than hotplate digestion recommended by the national standard. It is applicable to the determination of heavy metals in the oil-polluted alkaline soils. Using the modified BCR sequential extraction method, the extraction effect of oil-polluted alkaline soils. Nitric acid digestion was added to digested the organic acid in the extracting solutions. The results showed that the method is applicable to the speciation analysis of oil-polluted alkaline soils, and the precision and reproducibility are better. The sum of each proportions is equal to95%-113%of the total heavy metal content, suitable for eco-geochemical investigation and evaluation of research work requirements.(2) In this paper, the optimized method were used for the analysis of Cu, Zn, Pb Cd Cr Ni, V, and Mn in the soil of50km2area of Gudao oilfield. The results showed that the mean values of the8heavy metals were all lower than the national secondary standard. However, maximum concentrations of Cu, Zn, Cd and Ni values were above the national primary standard, and the means of Zn, Cd and Ni were higher than the background values of Shandong Province. The enrichment factors showed that it was relatively high enriched for Zn and Cd, and low enriched for Cu, Pb and Ni in the Gudao oilfield, which means a potential threat to the local soil environment.By correlation analysis, principal component analysis and cluster analysis, apportionment analysis of soil heavy metals in Gudao oilfield was carried out. The results showed that, Cr, V and Mn are likely to come from natural causes; the sources of Cu, Cd and Ni are affected by the oilfield development; Pb and Zn are from other anthropogenic sources such as farmland farming behavior, traffic emissions and atmospheric deposition.(3) The contour maps for the eight kinds of heavy metals were drawn by kriging interpolation method. From these maps, the distribution patterns of Cu, Cr, Ni, V and Mn are all similar with that of the well density, but only the distributions of Cu and Ni have direct relationships with the oil pollution. The distribution of Cd is related with the soil salinity. High concentrations of Pb are mainly found in the soil surrounding the main trunk road, while higher Zn concentrations only exist in the farmfields.In the soil of the edge of oilfields, the concentration of heavy metals first increase and then decrease in the direction of increasing distance from the oilwell hole (except Mn), and the maximum concentration of Cu, Zn, Pb, Cd, Cr and Ni were found at the distance of20-40m. In the oil-polluted alkaline soil profile, the oil pollution and oilfield development activities affect the distribution of most heavy metals, and cause significantly increases of concentrations of Cd and Ni (P<0.05).Concentrations of heavy metals in soils around the oilwells with different development age were analyzed. The results revealed that the concentrations of heavy metals showed a significant change (except for Cu and Cr) with time increase. The concentrations of Zn, Cd, Ni and Mn significantly increases with oilwell developing time (P<0.05), which showed that the concentrations of Zn, Cd, Ni and Mn were significantly affected by the oilfield exploitation behavior.(4) Using modified BCR sequential extraction method, the4fractionations of the8heavy metals in oil-polluted alkaline soils were analyzed. It is found that the proportion of extractable Zn, Pb, Cd, Ni and Mn are relatively high, especially the bio-availability of Cd and Zn. The existence form of Cu, Cr, and V is mainly the residual fraction, which means high stability. In the oil-polluted alkaline soils of Gudao oilfield, the migration abilities of heavy metals are in the order Cd> Mn> Zn> Ni> Pb> Cu> Cr> V. Cd is the most labile, bioavailable and mobile element of the8kinds of heavy metals, which make Cd the most serious threat to the surrounding environment. But V is the hardest to transform and the most stable element.In the soil profiles, the organic fractions of8heavy metals showed a significant surface enrichment phenomenon, which indicate that the speciations of heavy metals in soils have been affected by oil pollutions. With the develop time of oilwells increase, the total contents and extractable fractions of Cd and Mn are all significantly increase in the soils around oilwells (P<0.01). This suggests the distributions of these two elements are significantly affected by oilwell exploitation activities. But for the heavy metals of Pb, although the extractable proportion increases with time, its total content is negative. This result means the speciations of Pb are affected by the oil field development, but its source was irrelevant.(5) Considering the petroleum pollutants, heavy metals and alkaline simaltaneously, take these three pollutants as The soil island of oil region organics, heavy metals and salts taking these three kinds of pollutants as assessment factors, a integrated pollution assessment index (IPI) was modeled based on the Nemerow evaluation method. This assessment method is a comprehensive evaluation of the ternary complex pollution, and could evaluate the heavy metals contaminations in oil-polluted alkaline soils comprehensively. Using the modeled index (IPI), the assessment of soil environmental quality of Gudao oilfield were carried out. The result shows that the soil environmental quality grade is V, and the soil environmental quality level is heavily polluted. |
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