Studies On Hydrodynamic Characteristics Of Oil-contaminated Aquifer Medium

With the growing demand for oil around the world, petroleum has become an important kind of organic pollutants in groundwater system. Oil pouring or leaking occurs inevitably during petroleum exploitation, refining, transportation and utilization, which can result in serious pollution in the underground water and soil environment. Especially for the low-density oil (e.g. fuel oil, lubricating oil), its migration ability is stronger due to the low viscosity coefficient, which has greater pollution potential once leaked into underground environment.This study is based on petrochemical area of Zibo City. Standard sand and typical soils in this area were taken as representative aquifer medium. CD15W-40 oil, 0 # diesel and 93 # gasoline were taken as representative low-density oil products. The changing rugulations and influence mechanism of surface wettability, capillary characteristics and water-salt dynamics characteristic of oil-contaminated aquifer medium were researched systematically, which can provide important references for evaluation, control and in-situ rehabilitation of oil-polluted sites. The main new understandings and conclusions obtained in this research were shown as follows:(1) Determination method of water and oil content of oil-polluted aquifer medium were established. The traditional drying method for water content and extraction-ultraviolet spectrophotometry for oil content were improved by measuring oil drying loss coefficient and new calculating formulas were also put forward. The new methods can solve the problems caused by mutual interference of oil and water, which can support the subsequent theoretical research with accurate data. Inspection test results showed that, the measuring error of these new methods was just about 0.5%, which mainly result from inevitable instrument errors. Therefore, these improved methods are proeved to be accurate and reliable. In addition, it won't be influenced by oil types or volatility and thus have wide application range. (2) The surface wettability of oil-contaminated aquifer medium variation rule was obtained. The water repellency of oil- contaminated aquifer medium with different engine oil and diesel oil contents was determined. Results showed that the water repellency level increased dramatically, with the increase of oil pollution degree. Once the oil content reaches a certain critical value, the wettability will begin to change from hydrophilic into hydrophobic. The critical engine oil content of quartz sand and coarse sand is respectively around 0.5% and 0.3%, while their critical diesel oil content is respectively around 20% and 14%. The critical engine oil content of clay is about 8%, while diesel pollution hasn't significant influence on clay. In addition, composition of medium and oil both have important influence on the variation characteristics of wettability of aquifer medium.(3) The wettability reversal feature of oil-contaminated aquifer medium was also studied, and the critical water content of wettability reversal was obtained. The results showed that when the water content reaches a certain critical value, the surface wettability will change again, from hydrophobic surface to hydrophilic surface, and the repellency of medium is disappeared. Low water content can reverse the wettability of oil-contaminated sand, the threshold value of which is about 0.4~0.8%. With the increase of oil content, the critical water content of engine oil-contaminated clay increases, and the value of water and oil contents is equal approximately.(4) The capillary water rising dynamics and capillary zone water vertical distribution rule were studied systematically. The results showed that, with the increase of oil content, both the capillary water rising height and water storage capacity reduced significantly. The reason is problebly that some oil was absorbed on the aquifer medium surface, which can increase the interface wetting contact angle and enhance the surface hydrophobicity. Besides, some oil can exsit among the medium particles and thus block the effective capillary water rising channels. In addition, compared with diesel pollution, engine oil pollution has stronger influence on medium capillarity, and the influence of oil pollution on coarse sand is much stronger than clay. (5) The mathematical expressions of capillary water rising height and rate were established as well. Under different conditions of engine oil and diesel contents, logarithmic relationship between capillary water rising height of sand and time can be expressed by quadratic polynomial, and that of clay can be expressed by linear relationship. Accorading to logarithmic relationship between capillary water rising height and time, the relationship between capillary water rising rate and time can also be deduced. The relationships between capillary water stability height and oil content of sand and clay were power and quadratic polynomial respectively.(6) The water-salt dynamics of porous media polluted by oil were studied. Through hydrodynamic dispersion experiment, permeability, effective porosity, diffusion coefficientand and dispersion variation were obtained. The results showed that the effect of diesel pollution on sand effective porosity and permeability was more significant than that of engine oil. With the increase of oil content, effective porosities of quartz sand, coarse sand and silt showed linear decreases. Permeability decreased gradually with a sluggish tendency and permeability coefficient of clay was two or three orders of magnitude lower than that of quartz sand and coarse sand with the same oil content. Dispersion had no apparent change. Hydrodynamic dispersion coefficient of oily silt was one order of magnitude lower than that of oily quartz sand and coarse sand, because pore water velocity in silt was lower and the role of mechanical dispersion was smaller. Relative to oil pollution, the inherent properties of porous media had a more significant influence on its water-salt dynamics.