Distribution, Source And Migration Of Petroleum Hydrocarbons In Soil, Air And Water In The Wastewater-irrigated Area

Aliphatic hydrocarbons and aromatic hydrocarbons are major components of petroleum hydrocarbons. Aliphatic hydrocarbons constitute the bulk of an oil. Generally, normal/isoprenoid alkanes and total resolved/unresolved (UCM) petroleum hydrocarbons were quantified on the gas chromatography (GC), whereas the terpane, sterane, and aromatic hydrocarbon were determined by GC/MS (mass spectrometry). They can be used to assess total oil concentrations and petroleum contamination in air, water, soil and sediments. Geochemical parameters based on them can also provide information on petroleum degradation, depositional environment and petroleum source. The phospholipid fatty acids (PLFA) technique can be used to elucidate different strategies employed by microorganisms to adapt to changed environmental conditions under wide ranges of soil types, management practices, climatic origins and different perturbations (such as contamination of heavy metals and petroleum hydrocarbons).Principal component analysis (PCA) with software CANACO4.5can be efficiently used to simplify the complicated data set by reducing the number of dimensions without losing information contained in the complicated data set. In geochemical studies, pollutants from various sources as well as the air, water, soil, and sediment samples containing pollutants from various sources can be partitioned into different groups. Redundancy analysis (RDA) with software CANACO4.5can be efficiently used to explain one data set by another data set. And the results from PCA and RDA can be visualized with the bounded software CanoDraw for Windows. In addition, compound-specific isotope (CSIA) has been used to identify various sources of pollutants in recent years because of their stability and specificity.In this paper, we used the concentrations of normal/isoprenoid alkanes and total resolved/unresolved petroleum hydrocarbons quantified on the GC to effectively evaluate the levels of petroleum contamination and degradation, as well as identify the petroleum sources of atmospheric deposition, surface water, groundwater, and soil in the Hunpu wastewater irrigation area, located in the southwest of Shenyang City, a heavy industrial city, in China's northeast. We used the concentrations of terpane and sterane quantified on the GC/MS and PLFA to complementally evaluate the levels of petroleum contamination and degradation, as well as identify the petroleum sources of surface soil in the Hunpu wastewater irrigation area. We used the concentrations of normal/isoprenoid alkanes and UCM combined with multivariate statistical analysis (PCA and RDA) with the CANOCO4.5software to differentiate pollutants from various sources as well as the air, water, and soil samples containing pollutants from various sources, to explain the different distributions of hydrocarbons in soil by the basic soil properties, and to calculate the different contributions of the aliphatic hydrocarbons in groundwater and air to the aliphatic hydrocarbons in surface soil. Moreover, we used carbon isotopic composition of n-alkanes to complementally identify the petroleum sources of atmospheric deposition, surface water, groundwater, and soil. We used CSIA combined with multivariate statistical analysis (PCA and RDA) differentiate pollutants from various sources as well as the air, water, and soil samples containing pollutants from various sources, and to calculate the different contributions of the aliphatic hydrocarbons in groundwater and air to the aliphatic hydrocarbons in surface soil. Finally, we compared the results from CSIA with those from chemical fingerprinting analysis. The results were shown as follows:The aliphatic hydrocarbon concentration was highest in the samples obtained from the upland field near an operational oil well and the lower reaches of the Xihe River (Σn-alkanes in surface soil samples.30.4±3.2μg g-1and TAH in surface soil samples:151.9±19.9μg g1;Σn-alkanes in soil profiles:11.0±1.4μg g-1and TAH in soil profiles:190.8±12.1μg g1); it was lowest in surface soil samples in paddy fields where wastewater irrigation promoted the downward movement of hydrocarbons (Σ-alkanes:20.9±1.0ug g-1and TAH:82.8±4.2μg g-1). The surface soil in Hunpu region was found contaminated by heavy petroleum from oxic lacustrine fresh water or marine deltaic source rocks rather than petroleum from anoxic lacustrine, saline, marine evaporitic or marine carbonate depositional environment. Hydrocarbons from light oil and biogenic hydrocarbons were also identified in these samples. Geochemical parameters also indicated significantly heavier contamination and degradation in the upland fields compared with the paddy fields.PCA based on PLFAs showed various microbial communities and their different response to petroleum contamination between upland and paddy fields, and the PLFA compositions of upland fields were more similar than those of paddy fields, especially I-2U (near abandoned oil well more than ten years ago) and I-4U (near operating oil well). PLFA concentrations of15:0,3-OH12:0, and16:1(9) were significantly higher in paddy fields, whereas i16:0and18:1(9) c were significantly higher in upland fields (p<0.05). Poly-unsaturated PLFA (18:2ω6,9; indicative of hydrocarbon-degrading bacteria and fungi) was also significantly elevated in upland fields. These results were important for knowing about various microbial responses to petroleum contamination in different fields and implementing the following works on remediation.Geochemical parameters based on the concentrations of normal/isoprenoid alkanes and UCM combined with results from PCA indicated that the soil profiles from I-2U and I-6U (with nearby oil wells still operating or abandoned) had rich hydrocarbons (Σn-alkanes:1.1-16.4μg g-1dry wt. and TAH:10.9-161.2μg g-1dry wt.), obvious characteristics of petroleum hydrocarbons, and were grouped together by PCA. The hydrocarbon concentrations of the samples from I-5P and I-7P were also high and they were grouped together because of wastewater irrigation. Another group contained I-1P and I-4U (Σn-alkanes:0.5-4.0μg g-1and TAH:1.6-23.8μg g-1), which had a small hydrocarbons. Biogenic hydrocarbons dominated in most samples and no obvious pollution source was detected. I-8U was affected by atmospheric deposition and I-3P irrigated with water from the main canal had a similar pattern. The result from RDA indicated that the compositions of hydrocarbons in soil were also impacted by the soil properties.Geochemical analysis revealed the presence of biogenic (terrestrial plant wax, microbiota, and hydrobionts) and degraded petrogenic hydrocarbons in the surface water and the groundwater in the Hunpu region. The sources of hydrocarbons in the atmospheric deposition sampled in different seasons were various, but high carbon number hydrocarbons from degraded heavy oil were dominant in the atmospheric deposition samples in the region. Moreover, small biogenic hydrocarbons and hydrocarbons from light oil were identified in the atmospheric deposition samples. The hydrocarbon concentration in water was higher near oil wells and the Xihe River Reach. And the levels of petroleum pollution and degradation were significantly higher in groundwater in October2009[total aliphatic hydrocarbons (TAH):909.3-10343.1μg L-1] than those in May of the same year (TAH:357.0to6802.1μg L-1). For air, the concentrations of Σn-alkanes, unresolved complex mixture (UCM), and TAH were lowest in winter (Σn-alkanes:76.6μg m-2d-1, UCM:147.9μg m-2d-1, and TAH:224.5μg m-2d-1); the n-alkanes were more abundant in spring (841.2μg m-2d-1); and UCM and TAH were more abundant in summer and autumn (UCM:13173.7μg m-2d-1and TAH:13859.9μg m-2d-1).Through PCA, the water and air sampled in different seasons and sites were differentiated based on their degree of petroleum pollution and various aliphatic hydrocarbon compositions. Through RDA, we found that69.5%of the variance of hydrocarbons in the soil could be explained by all measured water and air samples, which indicated that the petroleum hydrocarbons could migrate among different medium and the petroleum contamination of the water and air had an important effect on the surface soil. The66.0%of the variance of hydrocarbons in the soil could be explained by measured water and12.2%of the variance of hydrocarbons in the soil could be explained by the bulk atmospheric deposition, which indicated that the irrigated water had a bigger effect on the surface soil than the air. The variance of hydrocarbon composition in soil explained by the hydrocarbon composition in water collected in May2009was70.5%, whereas the value for the water collected in October was37.0%. The reason is that the farmlands in the Hunpu region are mainly irrigated in May. ⅠThe n-alkanes in the atmospheric deposition (2009), the surface water (October2008), the groundwater (October2008), and the surface soil (October2009) in the Hunpu wastewater irrigation area were from modern C3plants and degraded petroleum and its products, which were indicated by the stable carbon isotope ratios (δ13C) of n-alkanes and the geochemical parameters based on the concentrations of normal/isoprenoid alkanes and UCM. The carbon isotopic composition of n-alkanes in the surface water (Xihe River) was significantly different from that in the groundwater. The groundwater I-3G (collected in the paddy fields) came from areas nearest to the Xihe River and was affected by the Xihe River. Therefore, the carbon isotopic composition of n-alkanes in I-3G was similar with that in the water from the Xihe River and significantly different from that in other groundwater. Higher maturity of δ13C of n-alkanes was discovered in the groundwater Ⅰ-4G collected in the upland fields near the operational oil well. The differences between the carbon isotopic compositions of n-alkanes in water and those in atmospheric deposition samples collected in different periods were significant. The differences between the carbon isotopic compositions of n-alkanes in atmospheric deposition samples collected in different periods were also significant. These results described above were all visualized through PCA of513C of n-alkanes.Through PCA and geochemical analysis of the concentrations of normal/isoprenoid alkanes and UCM, the hydrocarbons from heavy oil with the high boiling points was found dominant in the surface water and the groundwater samples collected in October2008and the atmospheric deposition samples collected in different periods in 2009. Various concentrations and geochemical characteristics of the aliphatic hydrocarbons were discovered in the water and the air samples affected by various pollution sources, which were well visualized in the diagram obtained from PCA.In PCA of δ13C of n-alkanes, high carbon number hydrocarbons, middle carbon number hydrocarbons and low carbon number hydrocarbons from various sources were partitioned into different groups. The samples affected by various sources and environmental factors were more dispersed in PCA of δ13C of n-alkanes, compared with the results from PCA of the concentrations of n-alkanes. Differences between the results from PCA of δ13C and those from PCA of the concentrations of n-alkanes indicated that more geochemical information could be presented by analyzing δ13C of n-alkanes.The results from RDA of δ13C and the concentrations of n-alkanes indicated that petroleum contamination in the irrigated water had a bigger effect on petroleum contamination in the surface soil than that in the atmospheric deposition in the Hunpu region. The δ13C and the concentrations of n-alkanes in the water from the Xihe River and the groundwater I-4G (upland fields, came from areas nearest to the Xihe River) were significantly correlated with those in surface soil in the region. The28.2%of the variance of hydrocarbon concentrations in the soil could be explained by measured water (October2008) and7.7%of the variance of hydrocarbon concentrations in the soil could be explained by the bulk atmospheric deposition (2009). The46.4%of the variance of δ13C in the soil could be explained by measured water (October2008) and24.4%of the variance of δ13C in the soil could be explained by the bulk atmospheric deposition (2009). The variance of δ13C in the soil explained by all measured water and air samples was higher than the variance of hydrocarbon concentrations in the soil explained by all measured water and air samples. And the variance of δ13C in the soil explained by the bulk atmospheric deposition was higher than the variance of hydrocarbon concentrations in the soil explained by the bulk atmospheric deposition. These results indicated that the effect of petroleum contamination in the irrigated water and atmospheric deposition (especially petroleum contamination in the atmospheric deposition) on petroleum contamination in the surface soil was underestimated through analyzing hydrocarbon concentrations compared with δ13C.