| As an important part of water resources, groundwater is the reserves and strategic resourcesfor social development. However, with Chinese rapid economic growth in recent decades,groundwater pollution is getting worse, which not only significantly aggravated the contradictionof water shortage, but also greatly threaten ecology and human health. Among numerouspollutants, there is growing concern about petroleum due to its high toxicity. Especially,pollution caused by petroleum became more and more universal and serious in shallowgroundwater and soil environment system, calling for an imminent remediation. Biodegradationis the only process that can reduce pollutants content in the environment, and the mechanism ofpetroleum pollutants biodegradation in groundwater therefore has important scientificsignificance for contaminated water remendiation and water sustainable management.A site in NE China was selected as a particular case characterised by a long history of oilcontamination. Forensic identification of biodegradation on this contaminated groundwater sitewas studied by multidisciplinary knowledge from traditional contamination hydrogeology andemerging molecular biology. Integrated groundwater modelling, hydrochemical and PCR-DGGEmethods were applied for investigation of the hydrogeologic, hydrogeochemical and microbialcharacteristics of groundwater system. Controlling factors were then recognized by the means ofmulti-factors statistics. Based on these, influences of in situ air sparging on biodegradation werealso discussed.Main results of this paper are described in detail as follows:(1)10-month monitoring was carried out for variations of pollutants throughout theinterested site. The average TPH concentration of5.21mg/L largely exceeded the value inrelevant national standards. Pollutants contained alkanes, aromatic hydrocarbons andnon-hydrocarbons. The aromatic hydrocarbon concentration is much higher, at between0.53~2.13mg/L with benzene concentration of between0.010.31mg/L, xylene of between0.08~0.98mg/L, more than their limitations in of drinking water standard (GB/T5750.8-2006).(2) Dynamic changes of contaminants in time and space showed TPH concentrationdecreased during the whole monitoring period. Rapid drop occurred in late2009till early2010 while natural attenuation of pollutants got weak after March2010. Variations of petroleumcomponents showed the ratio of alkanes and aromatic hydrocarbons, and that of hydrocarbon andnon-hydrocarbon declined with time, indicating the occurrence of biodegradation but in relativeweak degree. biodegradation was also confirmed by biomarker data along the groundwater flowpaths but didn’t dominate as the above two ratios increased along flow lines.(3) According to the distribution of TPH in this site, two cores of high contaminants can bedistinguished. To find out the contamination patterns of this field site, TPH concentrations of soilin vertical profiles at typical points were analysis. Results showed two sorts of treads existed.One is continuous declination with depth, and the other is obvious peak near groundwater level.So there should be two patterns: first, vertical infiltration, mainly happens near oil-water pond inthe SW of this site; Second, horizontal spread, is the main path in the middle and downstream.(4) Characteristics of groundwater chemistry at the pollution site were also studied. Wefound that although groundwater there suffered different degrees of oil pollution, the major ionscomposition was relatively stable and defined the water types as HCO3-Na and HCO3-Ca.Moreover, high concentration of HCO3in groundwater may be resulted from biodegradation ofpollutants there according to chemical balance calculation.(5) To further understand the biogeochemical process at this site, redox conditions andelectron acceptors in groundwater were investigated. Eh indicated a reducing environment ofiron-manganese and/or sulfate reduction stage. Breakthrough of potential REDOX indicatorrevealed dissolved iron and manganese increased after January2010, while sulfate in themonitoring period dropped but slowly in2010. These reflect the site was experiencingiron-manganese and/or sulfate reduction process;(6) PCR-DGGE method was used to study structure and diversity of bacteria and archaeacommunity in this site after March2010till July2010. Analysis indicated that bacterialcommunity composition at Z1near the oil-water pond varied most with the lowest similaritycoefficient of only0.261between March2010and July, while higher coefficients could be foundamong points along the same flow lines and the highest similarity coefficient was0.714.Furthermore, diversity indices reflecting changes of species composition and abundance werecalculated. Using the calculated results, ANOVA analysis was performed in various time andspaces respectively. Results showed that bacteria diversity index had no significant changes withtime and space, while the archaea presented significant relevance with time.;(7) Main bands in the DGGE profiles were excised, re-amplified and sequenced for insight into microbial species composition in groundwater. Results of sequence alignment showed thereare three main classes in groundwater: Betaproteobacteria, Gamaproteobactera and Flavobacteria.Betaproteobacteria was dominated and most species were anaerobic. Detected species relevant tobiodegradation and biogeochemical environment are Dechloromonas sp., Gallionella sp.,Thiothrix sp, etc.(8) Principal Component Analysis (PCA) was performed to reduce the dimensionality ofcomplex pollutants, hydrochemical and microbial data sets.9pollution factors,4chemicalfactors and8bacterial factors were extracted, reflecting the pollutant source types, groundwaterchemical/biogeochemical environment, and flora of different characteristics respectively.Canonical Correspondence Analysis (CCA) was then carried out using the above extractedfactors to investigate relationships among them. Thereby, CCA ordination diagram could bedivided into three partitions of benzene series degradation, alkanes degradation and backgroundlevel according to the pollutants, and five partitions of aerobic or weak reduction,iron-manganese reduction, sulfate reduction, methanogenesis and weak degradation according tothe degradation processes. Controlling factors of biodegradation were judged by the location ofpoints representing groundwater samples in the CCA partitions diagram. Analysis implieddegradation ability in upstream turned quite limited but mainly with several alkanes undermethane-producing process probably due to long-term pollution, while downstream was mainlyexperiencing aromatic hydrocarbons degradation under iron-manganese and sulfate reductionprocesses;(9) Based on the above natural biological degradation mechanism research, influences ofin-situ biodegradation on pollutants biodegradation were further studied. First, after13-d singlewell AS pilot test, TPH in groundwater of interested zone reduced by80%while differentdecrease degrees varied with different petroleum components, significantly in aromatichydrocarbons. Second, due to air injection, groundwater REDOX condition transformed fromanaerobic to aerobiotic. Oxygen oxidation reduction parameter Eh increased significantly andsuch change was most obvious in N1nearest to air injecting well. Third, changes of microbialcharacteristics during the AS presented species abundance rather than species composition wasaffected by AS. Finally, aromatic oxygenase gene was quantificationally tested by Realtime-PCR,and the results revealed13-d single well AS pilot test could only promote aerobic biodegradation within4mradius around AS injecting well to the largest extent, which is far less than physicalinfluence area by AS. |
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