Theoretical And Experimental Study On Evaluation Of Soil NAPL Pollution By Radon Tracer Technique

In recent years,the situation of soil non-aqueous fluid pollution in China is becoming more and more serious,and the state pays more and more attention to the remediation of soil NAPL pollution.Radon tracer technology plays a very important role in evaluating the pollution investigation and repairing effect of soil NAPL pollution.In this paper,the soil radon as the theoretical basis of a tracer for soil NAPL pollution investigation was analyzed.Influencing factors of equilibrium pore radon concentration in soil with different NAPL saturation and water saturation(soil pore radon concentration represents the radon concentration in soil pore)were analyzed.A gas-liquid two-phase radon distribution coefficient measuring setup was designed.The distribution coefficient of radon between a typical non-aqueous fluid(diesel,lubricating oil)and air was obtained by means of experimental measurement;An experimental setup for measuring pore radon concentration in soil was designed.The soil of University of South China was selected as the experimental sample to study the quantitative relationship between soil equilibrium pore radon concentration and soilpore diesel saturation under dry conditions;A one-dimensional soil test column was designed and made to simulate the pollution process of soil after gasoline leakage.The radon concentration of pore gas in different height of soil test column,the concentration of pore gas TVOC(Total Volatile Organic Compounds)in soil test column and radon exhalation rate on the surface of soil test column was measured at different times after gasoline leakage.The following research results have been obtained.(1)Based on the existing foreign research results,the mathematical model of migration of gas-NAPL-water-soil system was summarized and established according to the distribution relationship between gas-liquid two-phase and the migration theory of porous media.The effect of the water saturation of soil and the saturation of NAPL on the soil equilibrium pore water concentration was analyzed.The concentration of radon in soil equilibrium pores increased with the increase of soil water saturation and decreased with the increase of soil NAPL saturation.Based on the gas-liquid two-phase radon distribution coefficient measuring setup,the radon distribution coefficients of diesel oil and lubricating oil were 14 ±0.56 and 9.5 ±0.71 at 20 ?,respectively.They were significantly higher than the distribution coefficient of radon in water which was 0.252 at this temperature.(2)The results of soil equilibrium pore radon concentrationmeasurement under different soil pore diesel saturation conditions showed that: The concentration of soil pore equilibrium radon decreased with the increase of soil pore diesel saturation,and the decreasing rate decreased with the increase of soil pore diesel saturation.The nonlinear fitting of soil pore radon concentration reduction and diesel oil saturation was carried out,and the R-squared of 0.96,which indicated that soil pore NAPL saturation can be determined by measuring soil equilibrium pore radon concentration;The soil emanation coefficient increases with the increase of soil pore diesel saturation.(3)The experimental results of simulating soil gasoline pollution by one-dimensional soil test column showed that: Before gasoline spraying,the soil test column was in free diffusion equilibrium,and the pore radon concentration of the soil test column increased with the increase of the depth of the soil test column;When gasoline was sprayed,the pore radon concentration in each height layer of the soil test column decreased rapidly,the soil pore radon concentration was negatively correlated with the soil pore gas TVOC concentration.The soil pore radon concentration decreased with the increase of soil pore TVOC concentration and the change trend was the same;When the distribution of radon concentration in the pores of the soil test column was stable,the decrement between the balance of the equilibrium pore radon concentration of the soil test column and the initial equilibrium pore radon concentration of the soiltest column was positively correlated with the TVOC concentration of the soil test column with the stable gasoline migration,and the R-squared of 0.98.It indicated that soil NAPL pollution can be evaluated by measuring changes in soil pore radon concentration.The radon exhalation rate on soil surface was obviously affected by soil NAPL pollution.After gasoline spraying,the radon exhalation rate on the surface of soil test column decreased sharply.The gasoline dropped by 81.3% on the first day after the gasoline spraying.However,with the migration and volatilization of gasoline,the radon exhalation rate on the surface of soil test column increased.After several days of leakage,the radon exhalation rate on the surface of soil test column increased to 80.9% of that before spraying gasoline.It was indicated that the degree of soil degradation and the degree of recovery can be used to qualitatively determine the degree of soil NAPL pollution and the repairing effect of soil NAPL.