Transformation Mechanism Of Nitrobenzene In Groundwater And Soil And Its Simulation And Forecast In Jiamusi Area

As the result of explosion accident of the Jilin Petrochemical Company of the Chinese Petroleum Co.Ltd (called the Songhuajiang River Pollute Accident as follows) in November 13, 2005, the Songhuajiang River were contaminated by the organic pollutants to a large extent. Because of the special hydrogeological conditions along river sides, the groundwater had been inevitably contaminated by the polluted river water in many source sites where groundwater has close hydraulic relation with the surface water. Owing to the relatively slow renewal rate of the groundwater, the bioaccumulation, high toxicity and low degradability of the pollutants and its long persistence of the organic pollutants in the environment for more than several years to few decades, the seepage of the polluted water into the groundwater system, the pollutants may exist in the groundwater will threaten the local ecosystem system, the health of residents, animals and so on.In order to suatain the safety of the water supply and the sustainable utilization of groundwater resources in the areas along the Songhua river, the Project of "the Effect Evaluation and Countermeasure of the Songhuajiang River Pollution Accident on Ecosystem Environment" was initiated by National Environmental Protection Agency on Dec. 13, 2005, under support of the specific item "Simulation and Forecast of Nitrobenzene in Riverside Groundwater" of this project, this dissertation is depended on one of this item's special tasks, to develop the research of particular pollutant nitrobenzene's migration and transformation in underground environment.The characteristics and environmental fate of nitrobenzene have been documented, such as volatilization, sorption, biodegradation and non-biodegradation (hydrolysis, photo degradation and so on) of nitrobenzene in the media of air, water and soil. However, most of these relevant researches on the nitrobenzene environment behaviors are mainly focused on the media of atmosphere and surface water and the interest on the disposal of the organic waste water with high concentration nitrobenzene, with few researches on quantatively characterizing the migration and transformation of the low concentration nitrobenzene in underground environment.On the basis of present situation of Songhuajiang River pollution and the characteristics of groundwater movement on both sides of the Songhuajiang River, the Jiamusi city which takes the groundwater as the sole source of water supply is selected as the case studuing area to mathmatically simulate the migration and transformation regulation of nitrobenzene in soil and groundwater through experiment research.This research is of obvious significance not only to provide data and dependable science basis for forecast and prevention of the groundwater pollution after the Songhuajiang River Pollute Accident, to lay a foundation for the security influence evaluation,but also to supply an important experience for the related research on other organic pollutants.The research area is located in the groundwater source site on neighbouring river in Jiamusi city. Based on the result of the field investigation and the features of the simulated area (topography, geomorphology, geology, hydrogeology, etc), experimental system was set up in the laboratory. By tracer testing in the seepage trough, the transporting rate of nitrobenzene and the necessary hydrogeologic parameters were modeled. This experiment began at 9:40 Feb. 21, 2006 and ended at 9:40 Mar. 13 2006 with the total duration of 480 hours. On the consideration of sample points' location and the changing trend of nitrobenzene concentration, the test sampling time gap was assured about 2～4 hours, and 1,500 water samples were collected in 116 batches.The results of nitrobenzene's transport experiment in the groundwater and soil showed that the peak of nitrobenzene with highest concentration of groundwater in the horizontal direction from the inflow section to the outflow section appeared respectively in each section, but the height of the peak is becoming lower with the increasement of groundwater flow path along the direction of groundwater movementvalue. The distribution of nitrobenzene concentration in the groundwater is changed from the inhomogeneous state in the upper section to the homogeneous state in lower section with the experiment time duration and the increasement of solute transport distance. During the duration time of this experiment, the nitrobenzene's concentration is initially increasing on each observation point and then decreasing after the appearence of the peak. After 300 hours, the experimental result showed that the nitrobenzene concentration of each layer in the aquifer has dropped down to 1-5% of the initial concentration and after 480 hours, the nitrobenzene's concentration of each cross-section has been under the detected limit of 0.001 mg/L.It is well documented in many literatures that nitrobenzene can not be hydrolyzed and the possibility of photolysis is low in underground environment.The volatilization influence can also be ignored in the condition lower than 20℃with the Henry constant of 20 Pa. Thereby, the experiment about the mechanism of nitrobenzene's migration and transformation in this research is mainly on the adsorption and biodegradation. The sediment silt and sand in the aquifer along the bank of the Songhuajiang were taken as the experimental medium. Considering the underground environmental characteristics, the experiment was carried out to explore the mechanism of the adsorption and biodegradation in the underground environment.The adsorption experimental results showed that the nitrobenzene's adsorptions of silt and sand is in the accordance with the Langmuir equation of equilibrium and adsorption and silt has higher adsorptive capacity than sand.The nitrobenzene will take 16 hours to reach adsorption equilibrium in silt while 12 hours in sand.The biodegradation experimental result showed that the process of nitrobenzene's biodegradation in different conditions satisfied the Monod equation and the q_max value was varied in the range of 0.018～0.046/h. The initial concentration of nitrobenzene in groundwater has little efect on the nitrobenzene's biodegradation and the biodegradation capacity in unsterilized medium was higher than sterilized medium in both media of sand and silt, especially in the unsterilized silt medium.Based on the result of nitrobenzene's migration and transformation from the research, a coupling model composed of convection, dispersion, adsorption and biodegradation about the nitrobenzene's transport was established. And the packages of the HST3D module and the PHREEQC module of the PHAST were applied to respectively simulate the groundwater flow regime, solute transport process, adsorption, and biodegradation. After considering the hydrogeology condition and the parameters of the water-bearing medium in the seepage trough, the simulated area was divided into two parts and subdivided into the isometric cells of 54×2.5cm. According to the sampling time, the time lengh in the simulatation moel was assigned 1 hour and the stress period was 480 hours long.The observation data (from the 10 sampling wells) of the nitrobenzene's concentration during the experiment was applied to the caliberate and validate the numerical simulation model. The simulation result showed that the nitrobenzene-adsorption capability of silt was higher than of sand and the linear adsorption coefficient of silt and sand was 0.526cm³/g and 0.123cm³/g respectively, and the retardation coefficient of silt and sand was 2.3and 1.54 respectively. It is also true that the biodegradation of nitrobenzene in silt is better than that in the sand. The maximum specified matrix consume rate of nitrobenzene in silt and sand was 0.04/h and 0.025/h respectively.After the modification and adjustment of the field hydrogeological parameters according to the scale effect, the calibrated model of nitrobenzene transport based on the inner experiment mentioned was used to forcast the process of the nitrobenzene's migration and transformation in Jiamusi area after the accident with the observed data of nitrobenzene concentration in groundwater.The length of the forecasting area is subdivided into cells of 135×5m isometric units and the forecast periods last for 180 days from Dec. 10, 2005 to Jun. 8, 2006 with the time step of one day. The prediction result showed that nitrobenzene had entered into the aquifer and the peak of the concentration of nitrobenzene in groundwater was becoming lower and lower along the direction of groundwater movement from the riverside to the center of the depression cone. However, the concentration peak value was lower than allowed limit of the national standard (0.017mg/L).Although the concentration peak value was lower than allowed limit of the national standard after the accident, more attention should be paied to the remediation of groundwater pollution to cotroll the hidden troubles of Songhuajiang River contamination and some technologies of the groundwater remediation are suggested to develop.