Research On The Migration And Transformation Mechanism Of Petroleum Characteristic Contaminants In Shallow Groundwater

With the world's industrial take-off, the oil demand is constantly increasing. Inthe process of oil exploration, mining and transportation, pollution accidents of spilled,and leaks often occur, and cause great harm to ecological environment, difficult torestore in a short-term. Petroleum contaminants enter the soil environment, andleakage into the aquifer,serious threat the safety of groundwater environmental.The northeast of China is China's major oil producing areas, is rich in oil. Thesame time, it's China's old industrial base, and rice cultivation base, in the oilpollution event of groundwater, it will take great harm. From the previous studies, theresearch of composite pollutants migration and transformation is focused onpolycyclic aromatic hydrocarbons and combined pollution of BTEX. Molecularstructure, chemical properties, mechanism action of these components has a certainsimilarity. The migration and transformation of aromatics and chlorohydrocarboncomposite pollutants is not clear, especially the research of aromatic hydrocarbonsand chlorohydrocarbon pollutants adsorption on the organic matter and themechanism of biological degradation have great significance.This paper selected an oil pollution sites in the northeast of China as the studyarea, which is about800m2and originally was low-lying reed land. Because of all theoil pipelines are buried underground throughout the year, corrosion and leakage leadgroundwater polluted in the slow leakage process, continuous over a long period oftime. The investigation results show that the lithology of shallow aquifers1-2m wasdiscontinuous sandy silt, and the main body of aquifer is silt with fine sand interlayer.The main groundwater recharge sources of the study area are precipitation infiltrationand lateral runoff recharge, main discharge items are the lateral flow out andevapotranspiration. The groundwater level is shallow, hydraulic gradient is about3-5‰. In the basis comprehensive analysis of the existing data and research results,through the hydrogeological survey, the soil and groundwater sampling and testing, to find out the space distribution characteristics of the petroleum pollutants in thegroundwater,and through the indoor static experiments,as well as the seepagecolumn simulation experiments, to reveales the pollutants migration andtransformation mechanism in the groundwater and soil. Builds the solute transportmodel of the characteristics pollutants in the groundwater, and forecasts the changesof the pollutants concentration.Adsorption is one of the main functions of petroleum pollutants naturalattenuation, which affects the flow and retention of the pollutants in the aquifers andother transformation processes, such as the biological degradations. This paperthrough the static tests studied the adsorption mechanism and the pollutantscompetition adsorption mechanism in the medium of vadose zone and aquifer, and thecontribution of the composition of aquifer medium (mineral and organic matter)to theadsorption.Use the medium of the vadose zone and the aquifer, such as sandy silt, silt, finesand to carry out the dynamics adsorption experiments and the thermodynamicsadsorption experiments. The dynamics adsorption results show that the adsorptionbalance time of sandy silt is the shortest, the adsorption balance time is about6h; theadsorption balance time of silt is about16h; the adsorption balance time of fine sandis about24h. The features of three pollutants adsorption on the media of the sites canbe describe as linear model, this time we use Henry model to analysis the mediumadsorption ability. These three kinds of organic pollutants is easy to be adsorption inthe smaller particles of the media. The order of the pollutants mobility fordichloromethane> trichloromethane>benzene, the dichloromethane is easier to movewith groundwater, and the benzene is easier to be adsorption.The experiment of pollutants competitive adsorption shows that when benzeneand chlorinated hydrocarbons coexist in groundwater, with the concentration ofbenzene in solution increasing, adsorption of chlorinated hydrocarbons in the aquifermedia has increased. The impact of chlorinated hydrocarbons to the adsorption ofbenzene is expressed as when the benzene concentration is higher, chlorinatedhydrocarbons inhibit the adsorption of benzene in the aquifer media.Organic matters play an important role in adsorption of pollutants in the aquifer media. By comparative experiments of the simulating silt, organic matters account forapproximately1%of the total mass of the aquifer media and the contribution rate tothe adsorption of pollutants has reached10%-40%.Biodegradation is one of the main characteristics of pollutants in the migrationand transformation process of natural attenuation. In this research, the study hascarried out from the site intensity of microbial activity, the site degrading bacteriadiversity, degradation kinetics, impact factors of biodegradation etc. And thedegradation law of the biodegradation on the characteristic pollutants in the aquifermedia and the vadose zone has been revealed.The respiration intensity of the site soil mainly expresses as gradually weakeningwith the depth increasing, and gradually increasing near the water table(approximately1.1m). The overall respiration rate of vadose zone is higher than theaquifer. The intensity of basal respiration in the range of0.2-0.5m of vadose zone isthe highest. In general, the intensity of basal respiration of the well ZK3is higher thanthe well ZK1and ZK6. The concentration of nitrate, ammonia, nitrite in the aquiferand the vadose zone has a certain influence on microbial activity. Under the water-soilconditions of vadose zone and aquifer, the degradation kinetics experiment shows thatthe three pollutants mentioned above fit the first-order decay kinetics model. Thedegradation rate of pollutants in the vadose zone is higher than in the aquifer. By thedegrading bacteria factor experiment, the result shows that the site groundwatercondition of PH, TDS, Cl-are more suitable for the growth of the characteristicpollutants degrading bacteria, too high or too low will cause inhibitory. When thenitrate concentration is lower than51.79mg/L, the number of degradation bacteria andpollutant degradation rate will increase with the nitrate concentration increasing. Theincrease of elevated phosphate concentration can promote the growth ofdichloromethane and trichloromethane degradation bacteria. But excessiveconcentration can inhibit the growth of the benzene degrading bacteria.This research takes the means of PCR-DGGE for analyzing the microbialdiversity of the soil samples of vadose zone and aquifer, then use Ward method tocluster analysis the samples, establishes the system tree of the microbialcommunity.From the cluster analysis results to see the microbial community in the unsaturated zone and the aquifer medium, it can be mainly divided into two groups,group A takes the microbial community of unsaturated zone as the main body,themain depth of distribution is0-1.0m; while group B takes the microbial communityof the aquifer medium as the main body, the main depth of distribution is1.0-1.8meter.The DNA sequencing results show that the microorganism in the unsaturatedzone and the aquifer medium, not only have the soil rhizobium andPseudomonadaceae,but also have the methane degradation bacterium and oilpollution resistant bacteria which are the same as other oil leakage field or oil fieldarea, this indicates that under the long-term effects of petroleum pollutants, a part ofmicroorganism in the unsaturated zone and the aquifer medium have been able totolerate petroleum contaminants, and have the degradation function. Although theDGGE picture has more recognition band, only a limited number of the samples canbe cut and send to test,doesn't find the chlorinated hydrocarbons and the benzenedegradation bacterium, which shows that both of them are not the notable microbialcommunity in the pollution sites, the number is in limited.This research carried out the seepage column simulation experiments about themigration transformation law of the characteristic pollutants in the sites underdifferent pollution ways. Aquifer seepage column simulation experiments are mainused to simulate the characteristic pollutants transport process in the first pollutionway. The experiment results show that the contribution of adsorption to pollutantsattenuation is trichloromethane87%, dichloromethane85%, benzene94%; thecontribution of the biodegradable, trichloromethane is3%, dichloromethane is2%,benzene is3%. This shows that the medium of the aquifer is much thinner, thepollutants adsorption capacity is more, besides, the groundwater flow is slow, thecontact time of the pollutants and the medium of the aquifer is longer. In the naturalconditions, the microbial biomass is small, the quantity of the degrading bacteria isless, which lead that the proportion of the biological degradation function is lower.From the result of the water leaching experiments in the vadose zone, we can see, themigration order of the characteristic pollutants is, dichloromethane>trichloromethane> benzene, and when the oil wastewater penetrate the vadose zone, the high concentration wastewater, will causes harm to groundwater.We use the indoor simulation experiment parameters to forecast theconcentrations of pollutants change. The forecast results show that thedichloromethane is least harmful to groundwater,below the EPA drinking waterstandards after the100d. The hazard of benzene and dichloromethane is higher thantrichloromethane, and below the EPA drinking water standard in the500d and1000d.But to completely remove the dichloromethane need for a long time, more than5000d, so recommended to take the necessary reconstruction measure, such as theAS-BS and so on.