Research On The Migration And Transformation Mechanism Of Naphthalene In The Shallow Groundwater And Its Simulation And Forecast

Nowadays, with the rapid development of the industrialized economy, the petroleum industry plays more and more important roles in several countries. It brings great benefits to the human and at the same time serious harm. Petroleum falls on the ground and comes into the soil through the leakage during the exploitation and transport, which poses great risk to the groundwater. The petroleum contaminated groundwater draw much attention for its toxicity and difficulty to be removed. While due to the complexity of the petroleum contamination, more research focus on the usual component of benzene, toluene, ethyl benzene and xylenes (BTEX) according to the existing literatures, but less on the PAHs. PAHs are highly poisonous of carcinogenesis, teratogenicity, and mutagenesis with strong persistence and accumulation in the environment, which poses high risk to the entironment and the human health. Especially the PAHs with two benzene cycles, which is of relatively high solution, usually act as the main pollution sources in the aquifers. Base on all of these considerations, it is quite necessary to carry out the research on the migration and transformation regulation of the PAHs in the groundwater.This paper carried out the research on the migration and transformation regulation of naphthalene in the shallow groundwater and its simulation and forecast, with the support of the subproject-the migration and transformation regulation of the petroleum pollution in the shallow groundwater and its natural attenuation of the State Key Scientific and Technological Project of Water Pollution Control and Disposal-the Key Remediation Technology and Project of Petroleum Contaminated Groundwater along the Song Hua River. The paper researched on the environmental behavior and the inherent regulation of the migration and transformation of naphthalene in the shallow groundwater, with the respect of providing reference for the research on the migration and transformation of other PAHs pollution in our country.There are some researches on the environmental behavior and the characteristics of naphthalene by foreign scholars, including the volatilization, adsorption and biodegradation, abiotic degradation (hydrolysis, photodegradation, etc) in the medium of the air, water, and soils. Well, the migration and transformation regulation of naphthalene in the shallow groundwater is rarely seen in the literature, especially when several PAHs components coexisting. Therefore, the topic of the paper is of great significance, and its results will provide important guidance to the research on the PAHs in groundwater.The paper selected a contaminated site in an oil field in the Northeast of 8000m2 as the study area, taking naphthalene as the research object, to research on its migration and transformation both in the single-component solution and the mixed component solution coexisting with two other target pollutants in the contaminated field-benzene and phenanthrene and finally carry out the simulation and forecast, by the combination of experiments and numerical simulation technology. The research not only supplies the data support for the quantifying the migration and fate of naphthalene in the groundwater environment, but also provides scientific basis for the contaminated groundwater purification and remedy scheme in the research field. The research possesses important reference value for the research on the migration and transformation of other polycyclic aromatic hydrocarbons in groundwater.According to the current literatures, the process of naphthalene photolysis is not going to happen in the underground environment. Secondly, the Henry constant of naphthalene is 1.88×10-2Pa·L/mol, so the volatilization is not included in this paper, considering the hydrogeological conditions of the research field. Therefore, the experiments for the naphthalene adsorption and biodegradation research were carried out with the uncontaminated soils near the contaminated field as the experimental medium. The experiment environment was set as the temperature of 8-10℃, pH value 7-8, dark, to simulate the underground environmental characteristics of the research field.The result of the adsorption kinetics experiments show that the adsorption of naphthalene on the four aquifers medium (gravel sand, coarse sand, medium sand and silt) in the contaminated sites follows the pseudo-second order kinetics equation both in the single-component solution and the mixed component solution. And the adsorption rate of naphthalene on the four aquifer medium is in the order of silt> medium sand> coarse sand> gravel sand, both in the single component solution and the mixed component solution. The adsorption of naphthalene on the silt is the slowest while the fastest on the gravel sand. In addition, on the comparison with the naphthalene in the single component solution on the four aquifer medium, the adsorption rate of naphthalene is quicker than in mix components solution with the coexistence of benzene and phenanthrene.The result of the adsorption thermodynamic experiments show that the the Henry linear adsorption model can be used to describe the adsorption of naphthalene on the four aquifers medium in the contaminated site both in the single-component solution and the mixed component solution. Furthermore, the adsorption capacity order of naphthalene on the four aquifer medium is silt> medium sand> coarse sand> gravel sand both in the single component solution and the mixed component solution, the adsorption capacity of naphthalene on silt is the largest, while smallest in the gravel sand. Moreover, on the comparison with the single-component solution, the adsorption capacity of naphthalene increases in the mixed component solution with the coexistence of benzene and phenanthrene.The results of the biodegradation experiments show that the biodegradation process of naphthalene accords with the first-order decay kinetics model on the four aquifer medium both in the single component solution and the mixed component solution. And the decay rate order of naphthalene on the four medium is silt> medium sand> coarse sand> gravel sand, both in the single component solution and the mixed component solution. Naphthalene was more rapidly degraded on the gravel sand than silt on which the decay rate of naphthalene is the slowest. Besides, on the comparison with the single-component solution, the decay rate of naphthalene increases in the mixed component solution, witch leads to the more decreased of naphthalene with the coexistence of benzene and phenanthrene.At the same time, based on the combination of the hydrogeological investigation, the literature review and data collection, the paper established the migration simulation experiment system for naphthalene both in the single component solution and the mixed component solution. Through the column simulation experiments, the migration of naphthalene in the single component solution was studied. By the breakthrough experiments of the tracer ions-chloride and naphthalene in the columns which were filled with the four aquifer medium in the study area, the retardation of naphthalene migration by the four medium are quantified with the parameter Rd. To the migration of naphthalene in the coexistence with both benzene and phenanthrene, the topography, geomorphology, geology, and the hydrogeology of the contaminated area was simulated in the seepage trough. Besides, transporting velocity of naphthalene and necessary hydrogeologic parameters were obtained through the tracer testing in the seepage trough. The migration experiment of naphthalene in the mixed component solution started at 9:30 on August 20, 2010, and the sample were collected schedually according to the sampling hole distance and the changing trend of the naphthalene concentrations. Currently, the monitoring of the seepage trough is still in progress, the samples are being collected everyday, because the migration of phenanthrene is far slower than naphthalene in the seepage trough due to its hydrophobicity Philippines for the hydrophobic. The data used in this research ends in the 6th October 2010.The results of the migration experiment for naphthalene in the single component solution showed that the concentration of naphthalene in the groundwater from every sampling point of each column increased gradually as the time went on and finally stopped and stabilized when a certain concentration level was reached. Naphthalene firstly broke through in the gravel sand column, which took about 6 hours, and its migration in the coarse sand, medium sand and silt took relatively much longer time, respectively, 294hours, 680hours, and 1433hours. Along the groundwater flow direction in the four columns, the maximum of the naphthalene concentration of each sampling points differ to each other after their increase of a period of time. In the gravel sand column, the maximum concentration of naphthalene in each sampling point is close to each other after their stabilization. While to the other three medium columns, the maximum concentration of naphthalene decreased gradually towards the direction of the groundwater flow in the columns. And the retardation coefficient order of naphthalene in the four aquifer medium is silt> medium sand> coarse sand> gravel sand. Among them, the silt retards the migration of naphthalene seriously with the largest coefficient of 9.91, and the gravel sand retards slightly with the least coefficient of 1.75.The results of the migration experiment for naphthalene in the mixed component solution showed that the concentration of naphthalene in the groundwater from every series of sampling point along the groundwater flow direction in the four aquifer medium increased gradually as the time went on and finally stopped and stabilized when a certain concentration level was reached. During the experiment, the concentration of naphthalene increased rapidly in the gravel sand layer, and reached the maximum concentration firstly. The concentration of naphthalene in the other three medium of the coarse sand, medium sand and silt layers increased relatively slowly, and the maximum value appearance of naphthalene concentration lagged than in the gravel sand layer. In the horizontal direction, the maximum value of naphthalene concentration appeared from one sampling point to the next one along the groundwater flow direction. Besides, the maximum concentration of naphthalene decreased gradually along the flow path from the inlet section to the outlet section of the seepage trough. Among the four medium layers, the maximum concentration of naphthalene from each sampling point varied less in the gravel sand layer, and more in the coarse sand layer and the medium sand layer, with the largest decrease in the silt layer. In the vertical direction, as the experiment time went on, the concentration distinction at different section with the different distance (17.5cm, 32.5cm, and 47.5cm) from the tank inlet side uniformed gradually from the early days of the experiment (20d) to the later period of the experiment (40d).Based on the research outcomes of the migration and transformation of naphthalene in the shallow groundwater in the former part of the paper, an analytical model was then built up reflecting the influence of the adsorption and biodegradation on the migration of naphthalene. And the solution of the model was then carried out, according to which the concentration of naphthalene in the groundwater was predicted at different sites away from the contamination sources along the groundwater flow at different time in the contaminated field.The forecast results show that the concentration of naphthalene in the groundwater of the contaminated field will migrate along the groundwater flow direction to the northwest part of the contaminated field. And it finally takes about 9 years to attenuate to the level of the limited concentration.With the time goes on, the naphthalene in the groundwater will migrate to the north edge of the contaminated field (near Z9 borehole), the concentration of naphthalene in the silt layer is below the cleanup standard of naphthalene in groundwater. While in the other three medium, there is a long period when the concentration of naphthalene is higher then the standard. And after about 10.1 years, the naphthalene in the groundwater will migrated to the location 3200m away from the contamination sources, which pose great risks to the downgradient groundwater in the northwest of the contamination field, and the pollution will be finally cleand up through natural attenuation at that time. So the remediation of the contaminated groundwater in the field was strongly suggested to ensure the water supply for the residents both around the field and in the downgradient. Furthermore, the gravel sand layer in the contaminated field was suggested to be the main targeted layer to be remediated.