| Groundwater is prone to be polluted by petroleum hydrocarbons falling to the ground and leaking from oil pipeline as well as the discharging of oily waste water in the petrochemical production area, reserve base, gas stations and other places. Petroleum hydrocarbons pollutants have been listed as the potentially toxic pollutants which are given to priority in the environment. Research and development of safe and dependable performance, easy operation, high efficiency and low-cost remediation technologies is the key to large-scale solution of petroleum hydrocarbons pollution of shallow groundwater. Petroleum hydrocarbons pollutants usually exist as non-aqueous phase liquids (NAPLs) in the groundwater. The main eleven types of remediation methods can be divided into two classes including en-situ and in-situ remediation methods. In-situ Air Sparging (AS) remediation technology has become the first choice for the NAPLs polluted groundwater due to the outstanding advantages of low cost, high efficiency and in-situ operation over other remediation technologies. AS is an innovative in-situ treatment technology that uses injected air to remove volatile organic compounds (VOCs) and strengthen micro-biological degradation from the saturated zone. A large number of experimental studies and applications from domestic and foreign experts and scholars show that AS technology is obviously to remove the NAPLs pollutants from groundwater. In China, however, AS technology research is still in the laboratory study and small pilot stage, it is urgent to form a simple and cost-effective AS technology system for the treatment of NAPLs polluted groundwater.NAPLs pollutants removal from groundwater by AS is a multi-phase mass transfer process. Influencing factors include the geology and hydrogeology, hydrogeochemistry of polluted areas, and the nature of NAPLs pollutants, climate conditions, as well as the operations characteristics of AS system, etc. This paper mainly studied four aspects associated with AS remediation system:①The mass transfer processes of NAPLs pollutants and remediation mechanisms during the remediation process of AS system play an important role in optimizing system in AS running.②The factors, which have an effect on air flow distribution and the remediation efficiency of AS system, is of great significance to forming the definite applicable standards for the AS technology.③Consequently, it will be of importance in guiding the site design of AS system to integrate the multiphase flow model of the AS remediation process into the AS system.④However, the current design and operation of AS system relying on empirical method often result in a substantial cost increase and low remediation efficiency of the system. Therefore, on the basis of the field investigation of the polluted sites, an AS remediation program is designed for polluted area of Jilin oilfield Kanxiatun, coupling with the AS Technology Integration Research Methods. In addition to provide a complete remediation program, pollution prevention and control measures, and remediation effects of forecasts for the relied projects, research results can also provide a scientific basis and act as a standard example for large-scale remediation of the NAPLs polluted groundwater by AS technology.The research contents and results in detail are as follows:(1) Experimental study on remediation of crude oil polluted groundwater by ASFirstly, DO methods is used to study the AS depth and AS flow rate influencing on the range of air flow in the sand aquifer during the AS process. DO in the upper aquifer is easy to reach saturation during the AS process. However, DO saturation in the lower aquifer changes greatly with the AS depth changes at the air injection rate of 300 ml/min. As a general rule, the greater the AS depth, the deeper the air flow can reach. Even at the same AS depth, DO saturation varies widely with the AS flow rate. The experimental results show that the larger AS flow rate, the wider the air flow can reach. Because the well-sorted sand medium is used in the experiment, the air distribution is relatively homogeneous in the aquifer as air is injected. DO saturation was basically symmetrical distributed to the axis line through the AS point when the AS depth and AS flow rate changed, which indicates that the air flow was roughly symmetrical distributed of U-axis in the aquifer during AS process.As the best AS conditions is determined by DO methods, remediation by AS is carried out at the air injection rate of 150 ml/min from the bottom of the aquifer, taken the simulated wastewater confected by crude oil from Jilin Fuyu oilfield as the research object.150 days later, the TPH concentrations in different parts of the sand aquifer are significantly reduced. The maximum removal rate is up to 84.3%, and the minimum removal rate reaches 59.8%. The removal law of TPH in the horizontal direction is that the closer from the axis through AS point, the faster the removal rate of pollutants is; the farther from the axis, the slower the removal rate of pollutants is. The removal law of TPH in the vertical direction is that the pollutants in the central part of the aquifer remove first, and then the upper part of the aquifer, following by the lower part of the aquifer. The removal law of pollutants is consistent with the airflow distribution model, i.e. the removal rate of pollutants is fast in the dense airflow region, and the removal rate of pollutants is slow in the sparse airflow region.The removal rate of pollutants has slowed down significantly since the ambient temperature dropped to below 10℃from the 30th day. It indicates that the volatilization and microbial degradation of the physical interaction results in the removal of pollutants at the first month. The microbial degradation becomes the main mechanism at the last 4 months. The fall of ambient temperature has a serious impact on microbial activity. It is found that the TPH concentration decays on an exponential model by plotting fitting curve at the moderate temperature during the AS remediation process. If the TPH concentration goes down to 0.05 mg/L below, it will takes about 6-7 months to complete remediation in the airflow dense region and 7-8 months in the airflow sparse region, and more than 2 years in the region that airflow hard to reach.(2) Study on the factors of AS technologyThe factors including AS flow rate, AS depth and AS operation mechanism is investigated to indentify the impacts on the remediation efficiency of AS system. Mixed sand with the size ranging from 0.075 mm to 2.0 mm after the high-temperature sterilization was chose as the aquifer medium, and the simulated wastewater confected by toluene with pure-water was taken as the research object. The air is injected into the bottom of the aquifer for AS remediation, at the rate of 15 ml/min,30 ml/min and 60 ml/min, respectively. The pollutants in the lower part of the aquifer have been removed more than 50% in the different three air injection rate after 60 minutes. And the faster of air injection rate, the greater the removal rate of pollutants is. AS remediation are carried out repeatedly at different depths of the toluene polluted aquifer at the same air injection rate of 30 ml/min. The results show that the part below the AS point of the aquifer are extremely slow to remove, and the farther away from the AS point the pollutants are more difficult to remove. Therefore, it is better to inject air below the pollution plume or at the bottom of the aquifer for actual application of AS system. Compared with continuous AS mode, Intermittent AS mode has the advantage of saving energy, but requires longer time to complete remediation, which make it not obvious advantage taken the two factors into consideration.The environment ambient temperature has an obvious influence on the efficiency of AS system under the absence of microbial degradation. AS remediation from the bottom of the aquifer which is composed of mixing sand medium in the air injection rate of 30 ml/min.240 minutes later, the pollutants concentration decay to 24.8 mg/L in the lower part of the aquifer when the environment ambient temperature is 8.0℃. At this time the pollutants removal rate has reached more than 99% when the environment ambient temperature is 30.0℃, and the concentration is only 5.4 mg/L. It will need more than 720 minutes for the pollutants removal rate reach above 99% when the environment ambient temperature is 8.0℃. At the same AS conditions with the air injection rate of 5 ml/min, the air flow ways vary with the permeability of aquifer medium. The pollutants removal rate in the coarse sand aquifer is fastest, followed by the medium sand aquifer, and the pollutants removal rate in the fine sand aquifer is slowest. This is because the different air flow ways affect the air saturation distribution in the aquifer, thus affect the gas-liquid mass transfer area.By plotting the fitting curve based on the availability of the experimental data in each group, the result indicted that the pollutants in the aquifer also exist exponentially decay under the absence of microbial degradation during the AS remediation process. There are 10 curves correlation coefficient above 0.95 in all of the 17 fitting curves, and 13 curves correlation coefficient above 0.9, only one curve correlation coefficient below 0.85. Comparing the half-life, it can be seen that the AS system at the air injection rate of 30 ml/min appears the highest economic benefits under the current experimental conditions. The efficiency ratio of intermittent AS mode is better than the continuous AS mode to a certain degree, but intermittent AS mode required far longer time than the continuous AS mode to complete remediation. The lower of the ambient temperature, the greater the half-lifeλof pollutants is, and much longer time it requires to complete remediation. The larger size sand aquifer is most appropriate to remediate by AS system; its half-life is smallest and required shortest time to complete remediation.(3) AS technology integration and remediation mechanismBefore AS system is designed, it is necessary to bore a hole on site and analysis the geology and hydrology conditions of the polluted sites; take water sample from drilling wells to analysis the type of pollutants and polluted condition; and investigate the hydro-geochemical characteristics and climate surveys. AS technology is applicable to the remediation of the NAPLs polluted shallow aquifer whose rock lithology is silt or silty clay soil and do not have thick and continuous pink cap. AS technology always preferably uses in the unconfined aquifer layer whose groundwater depth of burial is less than 15 m, and the soil permeability is greater than 10-14 m2. NAPLs pollutants can be removed through AS technology when the pollutants saturated vapor pressure is greater than 5 mmHg and the Henry's constant is greater than 1.013 (Pa·m3)/mol. The efficiency of microbial degradation related to the hydro-geochemical characteristics of the polluted area. If the concentration of ferrous ions is too high, it will affect the remediation efficiency of AS system. In order to rationally determine the AS system startup time, we have to obtain the seasonal, long-term and short-term changes law of the ambient temperature of the polluted area.NAPLs pollutants in groundwater generally dispersed inside of the soil interspaces, clay cranny and interlayer. Firstly, these pollutants move into the air channel from high concentration area to low concentration area through molecule diffusion, and then removed by convection-dispersion. The main mechanism of pollutants removal at the initial stage is that the pollutants transfer from the water phase into the gas-liquid interface by convection and diffusion, and then evaporate into the gas phase flowing out of the aquifer with air. The second mechanism of pollutants removal is that the NAPL phase VOCs dissolution into the aqueous phase if there is NAPL phase VOCs pollutants in the aquifer. The third mechanism of pollutants removal is the pollutants adsorption/desorption during AS remediation process because soil medium surface and organic matter on the surface have adsorption for the pollutants.(4) Simulation on remediation of NAPLs polluted groundwater by ASTMVOC is a numerical simulator for three-phase non-isothermal flow of water, soil gas, and a multicomponent mixture of hydrocarbons in multidimensional heterogeneous porous media. It is an extension of the TOUGH2 general-purpose simulation program developed at the Lawrence Berkeley National Laboratory. It has been strongly recommended by the China Geological Survey because of its good accuracy and reliability. It can simulate the pollutants behavioral patterns under the "natural" environmental conditions, and explain the water-soluble and volatile differences fir different VOCs that may occur in the NAPLs. It can simulate the hot remediation approach, such as steam injection or resistance heat up and the associated phase transitions and flow effects. A simple model for biodegradation is provided as well.The comparison results between the two-dimensional experiments and mathematical simulation of AS show that the error of simulation results is in an acceptable range. The reliability of the AS remediation model established by TMVOC is good. It can be well predicted pollutants decay law of toluene polluted groundwater during AS remediation process. Numerical model calculation results can meet the variety needs for analysis; especially for study phase change of NAPLs polluted groundwater during the process of AS remediation, micro process of mass transfer and provides an effective means to study the micro change of equilibrium in the polluted area. Numerical simulation plays an important guiding role and can act as reference for the design and implementation of AS system used in the remediation of NAPLs polluted groundwater. What's more, it is also beneficial to cost savings as well as the AS technology standardization and large-scale promotion.(5) Example of AS program designThe drilling data on site indicate that the quaternary phreatic water of the polluted area in Jilin oilfield Kanxiatun occur in the sub-sand, sand, and gravel sand layer. Saturated zone has the thickness of 16.6 m and underlain by a 2.4 m thick layer which is composed of sub-sand clay. The surface layer is mainly composed of the fine sand formed by weathering with the thickness of about 1 m. The soil permeability varies from 10-13 m2 to 10-11 m2. Groundwater recharges by meteoric water infiltration and groundwater lateral runoff supply; groundwater discharge by evaporation of phreatic aquifer, underground runoff (discharge to the river) and artificial exploitation, etc. Underground runoff direction is from southeast to northwest. The average groundwater depth is 3.52 m. The average hydraulic gradient is 1.7%. Polluted area is on the groundwater flow direction that passes through the petroleum wells. The polluted area is typically moderate temperate continental monsoon climate. The average temperature is 4.7℃for the past years. Average annual rainfall is about 435.9 mm. Interannual variations of rainfall varies from 150 mm to 350 mm. Annual evaporation is 1694 mm with the maximum amount of 318.5 mm in May. Drilling data indicate that the chemical characteristics of the pore phreatic water are dominated by heavy carbonate calcium-sodium type, containing heavy carbonate chloride calcium-sodium type in part. The groundwater in aquifer is contaminated by hydrocarbons pollution in various degrees, with the over-limit radio of 82%, and the maximum over-limit radio up to 48.6 times. The pollution plume reached as far as 10 m on the vertical direction below the groundwater table. Field data indicate that the polluted area is suitable for remediation by AS technology. The key parameters AS program required is determined by combined the AS technical integration research method with the reliability of AS remediation model as TMVOC showed. Results show that it is suitable to inject the air at the rate of 12 m3/h for the AS system, and as a result, the effective radius is about 7 m and the remediation range can spread the whole aquifer below the downstream of AS point. According to the equilateral triangle method to design AS wells distribution, the air injection wells distance is 12 m. It is more suitable to start the AS system in the spring as the weather turns warm and the effective remediation time throughout the year is up to 8 months in such a case. LNAPLs are the main pollutants in petroleum oilfield. By simulating the migration laws at unsaturated-saturated zone of BTEX after the leakage in the unsaturated zone, the results indicate partition wall and long-term AS system are good methods to prevent new pollution caused by ground falling oil after complete remediation. AS remediation of NAPLs polluted groundwater is a long-term process, requiring at least 8 months or more to complete. Therefore, it is necessary to periodically monitor the water samples during remediation period and constantly optimize the AS system in order to insure the stable operation and good remediation efficiency of the AS system.
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