The phase II project for a hazardous waste landfill in Shanghai was classified as a Class I construction project, which could potentially cause groundwater contamination. The EIA level for groundwater was Class I. Greenment applied a numerical model to evaluate the potential impact on the groundwater environment.
The main potential groundwater contamination in this project was the leakage of landfill leachate to the surrounding groundwater in the phreatic layer. The assessment factors were identified to be mercury, arsenic and cadmium based on the analysis of contaminants in the leachate. Considering the operation time of the landfill site, continuous leakage over15 years was simulated to evaluate the impact on groundwater quality. Visual MODFLOW was used for the groundwater modeling. The area was horizontally subdivided into 10-meter grids (2.5-meter grids in the source area). Two layers were used to represent the unsaturated zone and unconfined aquifer consisting of fill and silty clay, respectively.
The input parameters were obtained from the results of geotechnical tests and empirical values, and were calibrated by the software or manually. The simulated groundwater contour was consistent with the monitoring results of groundwater level and flow direction.
Solute transport was simulated based on the groundwater flow model. The migration of contamination in fill/silty clay layer and sandy silty layer at different times could be obtained from these simulations. The Class IV limits of “Groundwater Quality Standard” (GB/T14848-93) were used to represent the leading front of the plume. The following figures show the migration of the mercury plume in fill/silty clay layer in 5, 15, 50 and 100 years after the leakage.
Based on the simulation results, it is estimated that the furthest extent of contamination caused by leachate leakage in the fill/silty clay layer was 33.7 m down gradient from the landfill site and the longest duration of impact was 250 years. In the sandy silt layer, the furthest extent of contamination and longest duration of impact were 222.8 m and 500 years, respectively. The extent of contamination could not reach the nearest surface water in the down gradient. Therefore, the impact of this project to groundwater quality was minimum and the risk to the groundwater environment was acceptable.