ERT-based Joint Diagnosis Technique For The Leakage Detection Of Vertical Barrier Of Contaminated Sites

The impermeable system is the main barrier to prevent contaminants from leaking out of the contaminated site.If the barrier is broken,contaminants can seep into the surrounding soil and contaminate the soil and groundwater,as well as endangering ecological safety.Vertical barrier technology is one of the widely used technologies to control the migration of pollutants in polluted sites.But the leakage of vertical barrier wall may be caused by improper construction,corrosion,geomembrane damage,bottom contact and other reasons of vertical barrier walls.Detecting the leakage of vertical barrier wall is a key issue in the prevention and control of contaminated sites.The research was carried out based on the National Natural Science Foundation of China(41877240;41672280)and National Key Research and Development Plan of China(2018YFC1802300).Firstly,the application premise of geophysical method for leakage detection of vertical barrier walls in contaminated sites was analyzed,and a combination of temperature tracing method,chloride ion tracing method,and drilling sampling to measure true resistivity based on electrical resistivity tomography(ERT)method was proposed.The ERT method numerical simulation study was carried out for different leakage conditions of the contaminated site,and the indoor small-scale model test research was carried out based on the numerical simulation results.Finally,two typical contaminated sites were selected for engineering case studies.Based on the test results,the following conclusions can be drawn:(1)The distribution law of apparent resistivity of the soil outside the vertical barrier wall under different seepage conditions and the measurement inversion effect of the ERT method for different seepage conditions were studied through forward and inverse numerical simulation.The numerical simulation results show that the larger the size of the pollution plume and the greater the difference in resistivity between the pollution plume and the surrounding medium,the higher the detection resolution;the ERT method had better detection results for massive and horizontal inorganic pollutants,and better detection results for massive and vertical organic pollutants;The ERT method was better for detecting contamination plumes at shallow burial depths,and when the plume was located at the boundary between surface soil and homogeneous sandy soil.The numerical simulation results provided a theoretical basis for subsequent indoor small-scale model tests and engineering case studies.(2)According to the requirements of the leakage detection test of the vertical barrier wall of the polluted site,an indoor small-scale model test device was designed and manufactured.The model test device includes two parts: the pollutant transport system and the data acquisition system.The small-scale model test device can realize the measurement of resistivity profile,temperature field,chloride ion concentration field,soil real resistivity and other parameters;the electrical resistivity tomography instrument was modified,and small-scale electrodes,cables and connectors were self-made,so that the modified instrument can meet the requirements of indoor small-scale model tests.(3)The detection effect of the ERT method on the vertical leakage of inorganic pollutants,point and vertical leakage of inorganic pollutants,and horizontal leakage of organic pollutants was studied through indoor small-scale model tests.The test results showed that the resistivity profiles of the ERT method at different times could accurately reflect the leakage point location,pollution plume concentration,pollutant migration path and other information under different leakage conditions.In the case of a combination of various leakage forms,the larger the area of the leakage point,the more obvious the detection effect of the ERT method on the point.The ERT method could accurately identify the migration process of light non-aqueous organic pollutants to the soil surface after leakage.The closer the ERT line was to the blocking wall,the higher the detection resolution.(4)In the indoor small-scale model test,the detection effect of the ERT method was further verified by the results of the temperature tracing method,the chloride ion tracing method and the sampling measurement method of the real resistivity measurement method.The test results showed that: after adding the thermal tracer,the temperature response speed of the temperature measurement point closer to the leakage point was faster,and the temperature change range was larger;after adding salt tracer,the chloride ion concentration near the leakage point rose rapidly to a high point,then decreased slowly and finally stabilizes;the results of temperature tracking and chloride ion tracking were the same as the leakage point position and pollutant migration law measured by ERT method,which proved that temperature tracking method and chloride ion tracking method had a good supplementary verification effect on ERT measurement results;the real resistivity value obtained by soil sampling proved that the resistivity value measured by ERT method was more reliable.(5)Based on the results of numerical simulation and indoor small-scale model tests,two typical contaminated site barrier walls,compound contaminated sites and organic contaminated sites,were selected to conduct the ERT method integrity assessment study.The actual engineering site detection results showed that the ERT method had good response to low or high resistance anomalies in the subsurface,and some areas of resistance anomalies match the results of the preliminary ground investigation report,and the integrity of the vertical barrier wall can be further determined subsequently based on the monitoring results of other methods.The ERT survey line laid perpendicular to the vertical barrier wall proved that the vertical barrier wall had the effect of keeping out contaminants.