With the rapid economic and social development of China, the urbanization rate and peoples standard of living have continued to rise, leading to a steady increase in municipal solid waste generation and a year-on-year escalation in leachate production. In 2020, the leachate generation volume from various municipal solid waste treatment facilities across China reached approximately 50 million tons. Due to constraints imposed by the level of economic and social development and technical conditions at the time, for a considerable period in the past, most municipal solid waste treatment in China relied on direct landfilling; this resulted in the construction of numerous primary landfill facilities characterized by low seepage prevention standards, inadequate environmental protection facilities, and suboptimal operational management practices—making the historical issue of leachate a matter that cannot be ignored.
Waste leachate is a type of wastewater characterized by complex pollutant composition, high concentration levels, and significant environmental hazards, which forms during the collection, transportation, storage, and treatment of municipal solid waste—resulting from moisture content inherent in the waste, water generated by the decomposition of organic matter, surface precipitation, the water retention capacity of the landfill cover layer, and the influx of groundwater into the landfill facility.
We have been commissioned to conduct geophysical exploration for a municipal solid waste landfill project. Available data indicate that the landfill has already been closed, but the leachate levels are showing a continuous increase, raising suspicions of potential leachate leakage. The current objective is to conduct an investigation of the landfill area using geophysical exploration and drilling methods to identify the cause of the increasing leachate levels, pinpoint potential leakage points, and assess the overall condition of the site; the survey scope is illustrated in the figure below.
Based on the pollutant characteristics of the survey area, the survey specifications, the current state of instrument development and operational feasibility, as well as successful case studies of relevant methodologies, the natural potential method (SP), the charging method (CP), and the high-density resistivity method are selected as the primary geophysical investigation methods for detecting pollutants in the survey area.
The overall workflow comprises several stages, including data collection from the survey area and field reconnaissance, geophysical exploration, data analysis and interpretation, and the delineation of the contaminated area.
Analysis of self-charging and charging methods
Based on the data results of this project, the combined results from the potential gradient method and the charging method are presented in the figure below; the contour map shows that there are 4 leakage zones within the site.
The high-density survey profile results clearly illustrate the resistivity distribution within the field.
By conducting a comprehensive analysis of the geophysical survey data in the investigation area and performing 3D meshing of these data, it becomes possible to clearly visualize the morphology, area, and volume of low-resistivity anomaly layers using the 3D data.
Based on the two-dimensional resistivity profiles and measurement results, the resistivity distribution pattern at the landfill site exhibits distinct characteristics. Based on the field conditions and the high-density profile data, the relatively low-resistivity anomaly corresponds to water-rich landfill material.
The field work scope for this project includes natural potential measurements, charging method measurements, and high-density electrical surveying.
1. Based on the results obtained from the natural potential and charging methods:
There are 5 areas with leakage; based on the ERT survey line analysis, there is 1 area with damage to the bottom membrane on the north side.
2. Based on the high-density survey line results:
Groundwater infiltrates into the leachate treatment pond within the dam from the damaged area of the northern base membrane; based on the ERT survey results, it can be determined that a dominant groundwater flow path exists on the northern side; during the high-flow season, groundwater enters the landfill through this dominant flow path and ultimately converges into the leachate treatment pond within the dam.
