The Residua Of Oil Pollutants And Its Hydrodynamic Effects On Porous Medium

After entering the porous medium, the oil pollutants can adsorb on the surface of the medium particles under the influence of electrostatic attraction generated by the medium.At the same time, they can also exist in the pores with discrete state through the effects of capillary force produced by the capillary pores.The hydrodynamic properties of the porous medium can be changed dramatically by these behaviors of the oil pollutants, which can affect the flow state of water and nutrition supply during in-situ groundwater treatment (Pump-treat, Permeable Reactive Barrier, Biodegradation and Flushing).Therefore, the study of the residua of oil pollutants and its hydrodynamic effects has great significance and application for the evaluation, control and in-situ remediation of oil-contaminated fields.In this study, diesel and engine oil are taken as testing oils, coarse sand and silty sand are taken as porous media. The properties of sand samples and oil samples were tested firstly. One-dimensional and two-dimensional hydrodynamic dispersion apparatus were used to measure the water-salt hydrodynamic parameters of the sand samples with different oil contents.The residual state of the oil in the sand samples was analyzed, and its hydrodynamic effects were discussed. The conclusions are as follows:(1)Under the condition of saturated seepage, the residual content of diesel in coarse sand is about 2%, while that of engine oil is between 4% and 8%.The residual content of diesel in silty sand is between 3% and 4%, while that of engine oil is about 10%.(2) The effective porosity of the sand decreases linearly with the increase of oil content, and the decrease induced by diesel is greater than by engine oil.(3)The permeability of sand also decreases with the increase of oil content, and the decreasing trend tents to mitigation. The permeability coefficient of coarse sand decreases from 1.90×10-2cm/s to 4.70×10-3cm/s as the diesel content of which increases from 0 to 2%, reduced by 75.26%.The permeability coefficient of coarse sand decreases from 1.90×10-2cm/s to 1.26×10-2cm/s as its engine oil content increases from 0 to 4%, reduced by 33.68%.The permeability coefficient of silty sand decreases from 2.60×10-4cm/s to 6.88×10-5cm/s as the diesel content of which increases from 0 to 2%, reduced by 73.54%.The permeability coefficient of silty sand with 8% of engine oil is 3.76×10-5cm/s, which decreases by 85.54% compared with pure silty sand. The permeability coefficients of silty sand are 2 to 3 orders of magnitude smaller than those of coarse sand under the same oil content.(4) Oil contamination has no prominent influence on hydrodynamic dispersion coefficient and dispersivity. The dispersivity of pure coarse sand is slightly higher than that of coarse sand contaminated by oil, while the dispersivity of silty sand increases with the increase of oil content. Moreover, the hydrodynamic dispersion coefficients of silty sand are one order of magnitude smaller than those of coarse sand under the same oil content. This is because the pore water velocity in the silty sand is much lower than that in the coarse sand, so the mechanical dispersion in the silty sand is weaker than that in the coarse sand.(5) The tracer breakthrough curves of two-dimensional hydrodynamic dispersion tests show that both the peak time of tracer concentration and the peak concentration in the longitudinal direction depend mainly on the pore water velocity. The greater the pore water velocity is, the shorter the peak time of tracer concentration, and the higher the peak concentration; conversely, the longer the peak time of tracer concentration, the lower the peak concentration. Moreover, the diffusion range of the tracer in the transverse direction increases with the increase of tracer migration distance.(6)The results of two-dimensional hydrodynamic dispersion tests show that the longitudinal dispersivity of coarse sand contaminated by engine oil is smaller than that of pure coarse sand, while the longitudinal dispersivity of diesel-contaminated coarse sand is bigger than that of coarse sand. The longitudinal dispersivity values of coarse sand with different oil contents range from 0.133cm to 0.363cm, while the transverse dispersivity values, which are smaller than the longitudinal dispersivity values, range from 9.83×10-3cm to 4.64×10-2cm.In addition, due to the high concentration of the tracer during instantaneous injection, its density is bigger than the aquifer water. So it tends to migrate vertically under the influence of gravity, which is equivalent to enhancing the extent of transverse dispersion, resulting in the calculated values of transverse dispersivity slightly larger.