Study On The Migration Mechanism Of Petroleum Hydrocarbon And Chlorinated Hydrocarbon Pollutants In The Groundwater System In The Eastern Area Of A City

The problem of pollution of petroleum hydrocarbons and chlorinated hydrocarbons in groundwater systems has become a global problem.After the founding of the People’s Republic of China,industry has developed rapidly and a large number of pollutants have entered the groundwater system through various channels.Petroleum hydrocarbons and chlorinated hydrocarbons are persistent in the pollution of groundwater.Once they enter the groundwater system,they are difficult to remove in a short time and will migrate and diffuse with the flow of groundwater.During the migration process,the concentration and spatial distribution of pollutants change due to adsorption and degradation.Therefore,it is of great significance to study the migration mechanism of petroleum hydrocarbons and chlorinated hydrocarbons in the groundwater system.This article takes the eastern urban area of a city as a research area.Through hydrogeological survey,water sample collection and detection,indoor experiments,field dispersion experiments and numerical simulation,we explored the pollution sources,pollution pathways,spatial distribution,medium adsorption characteristics,characteristics of aeration zone migration,biodegradation and simulation of pollutant migration in the groundwater system of petroleum hydrocarbons and chlorinated hydrocarbons in the study area.We also studied the migration mechanism of petroleum hydrocarbons and chlorinated hydrocarbons in groundwater systems.The main conclusions are as follows:（1）Pollution characteristics of petroleum hydrocarbons and chlorinated hydrocarbons in the groundwater systemThe pore water and karst fissure water in the study area were both polluted by petroleum hydrocarbons and chlorinated hydrocarbons.Among the 28 sampling wells in the study area,petroleum hydrocarbon pollutants were detected in 19 wells and the detection rate was 67.86%.The highest average concentration is 0.27 mg/L,which appears at the J15 sampling point near refinery B.Chlorinated hydrocarbon pollutants were detected in 22 sampling well sites,including 11 types of carbon tetrachloride,chloroform and trichloroethylene,among which carbon tetrachloride was the most serious,with a maximum concentration of 30.2μg/L.The main pollution sources of petroleum hydrocarbons are steel plant A,refinery B,fertilizer plant C and petrochemical plant D.The sources of chlorinated hydrocarbon pollution are mainly divided into three categories:chemical pollution sources（54.28%）,electromechanical pollution sources（26.36%）and pesticides pollution source（9.53%）.Petroleum hydrocarbons and chlorinated hydrocarbons as a whole are distributed in a north-south band.（2）Research on migration of petroleum hydrocarbon and chlorinated hydrocarbon in the groundwater systemWithin the concentration range set by the experiment,the adsorption of two pollutants by silty clay and limestone medium is mainly linear adsorption.K_{d toluene}>K_{d carbon tetrachloride} carbon tetrachloride shows that the adsorption capacity of two media for toluene is greater than carbon tetrachloride,K_{d silty clay}>K_{d limestone} limestone shows that the adsorption capacity of limestone is less than silty clay.It shows that the Quaternary in the study area has a certain"blocking effect"on the infiltration of pollutants.Once the pollutants enter the karst fissure water aquifer,the migration and diffusion rate of the pollutants will be greatly accelerated.Soil column leaching experiments show that the clay layer in the study area has a higher retention capacity for petroleum hydrocarbons and chlorinated hydrocarbon pollutants.In general,the clay layer has a greater retention capacity for petroleum hydrocarbons than chlorinated hydrocarbons.The results of the soil column leaching experiment show that the longer the storage time of the soil,the lower the concentration of pollutants in the leachate,which means that the longer the residence time of the pollutants in the aeration zone,the smaller the impact on the pollution of the groundwater system.The results of the undisturbed water leaching experiment of the undisturbed soil column show that in the intermittent inflow mode,the retention rate of pollutants by the clay layer is greater than that in the continuous inflow mode,which means that the sewage in the study area is more likely to enter the groundwater through the clay zone of the aeration zone under continuous discharge.Biodegradation is an important destination for the transport of pollutants in the groundwater system,but the contribution rate of biodegradation to the removal of petroleum hydrocarbons and chlorinated hydrocarbon pollutants in the experiment is not high,both of which are less than 5%.The main reasons are:（1）The duration of the experiment is short,some contaminants are not fully degraded.（2）There are some difficult-to-degrade components in petroleum hydrocarbon and chlorinated hydrocarbon pollutants in sewage.（3）Over time,the sterilized group gradually became a bacterium environment and the substances consumed by the microorganisms in the unsterilized group could not be replenished in time,resulting in a smaller concentration difference between the two groups.（3）Simulation study of petroleum hydrocarbon and chlorinated hydrocarbon migration in groundwater systemThe relevant hydrogeological parameters were obtained through the field dispersion test.Establish a groundwater flow model and a pollutant solute migration model to simulate and predict the migration of groundwater petroleum hydrocarbons and chlorinated hydrocarbon pollutants and analyze their migration rules in the groundwater system,in which carbon tetrachloride is selected as the chlorinated hydrocarbon characteristic pollutants.The results show that over time,the concentration of petroleum hydrocarbon pollutants and carbon tetrachloride in the groundwater system of the study area will be reduced to a certain extent,but there is still a large gap with the national standard.Their development direction in the pollution plume in the next two decades is the same as the direction of groundwater flow in the study area and they mainly spread northward.However,the extent of the reduction of the range of pollution plumes is not obvious,which shows that the pollution of petroleum hydrocarbons and chlorinated hydrocarbons in the groundwater system is persistent.The simulation prediction results show that the decay rate of petroleum hydrocarbon pollutants in the groundwater system in the study area is faster than that of carbon tetrachloride.