Delineation Of Groundwater Nitrate Pollution Based On Electrical Resistivity Tomography

Water resources are the material basis for human survival,and groundwater,as an important part of water resources,is inadvertently polluted in human activities.Due to the complex hydrogeological conditions of groundwater,it is difficult to control and repair.Once it is polluted,the environmental and ecological damage caused is often difficult to reverse.As an interdisciplinary subject,environmental geophysics has been widely used in the field of groundwater pollution investigation.It mainly uses geophysical methods and principles to study the changes of physical characteristics between groundwater pollution areas and surrounding rock materials.In the process of solving related problems such as groundwater pollution investigation,geophysical methods and technologies play an increasingly important role with their advantages of high efficiency and accuracy.This paper mainly studies the application of electrical resistivity tomography(ERT)in the investigation of groundwater pollution.On the base of summarizing the theory of ERT,the data collection method is optimized,and the data points of field data collection are increased,to further improve the accuracy of the ERT.A total of 32 DC profiles were laid out in the study area with a total 4.5 km.Multiple survey lines were laid out inside the plant and in the groundwater flow to investigate the spread of groundwater pollution plumes.On the basis of ERT data collection,water chemistry data collection was carried out,and 21 groundwater samples were collected,including one local agricultural irrigation water sample and 13 soil samples.At the same time,using GMS numerical simulation software to build a hydrogeological model,which a solute transport model is built to study the transport and transformation of pollutants in the region with time.The specific research work and the results obtained are as follow:(1)After optimizing the array and calculating its model resolution,it is found that the traditional dipole-dipole array displays low resolution below 14 meters,while the resolution of the whole region is higher than 0.9 after 40 iterations of the CR method.Through the numerical simulation of the replica of the classic case and the groundwater pollution model with stratum boundary,it is found that the optimized array has higher resolution than the traditional array.The measured data again verifies that the optimized array is better than the traditional array.Based on the time factor,the CR method with 40 iterations is selected for field data collection.(2)The ERT inversion results show that there are many low-resistivity anomaly areas in the study area,some low-resistivity abnormal areas in the north and west of the study area,and the other areas are relatively high-resistivity abnormal areas.Through the inversion results,it is found that the contaminated area in the study area has not yet spread to the surroundings,but there is a tendency to spread westward.Combined with the geological data,it is found that the high resistivity anomaly area in the northern part of the study area is low-permeability.To a certain extent,the diffusion of pollutants has been blocked,and the shallow domestic water in the northern residential area has not yet been affected.After ERT profiles were detected,groundwater and soil samples were collected.Synthesizing the inversion results and the results of water chemistry,the resistivity distribution in the study area is consistent with the results of water chemistry,and the results of ERT and water chemistry are mutually confirmed.Finally,the industrial wastewater discharged by the fertilizer plant in the study area is determined as the pollution source of the local water pollution.(3)After the pollution source is identified by ERT,the hydrogeological model was constructed by GMS.After the recognition and verification of the model,the solute transport model was built on the hydrogeological model.The simulation results show that,with the passage of time,pollutants gradually spread with the direction of water flow.We found that at the first the pollution did not spread to the low permeability area,and the direction of pollution diffusion is north-northwest,but with the increase of diffusion range,pollution spread to low-permeability area,the direction of transport diffusion gradually shifted to the west-northwest to spread.In the prediction of 2039,the pollutants have been spread to about 350 m away from the pollution source,with an area of about 0.17 km2.