CONTAMINANT DETECTION AND IDENTIFYING HYDROGEOLOGICAL COMPLEXITY FROM SIMPLE GEOPHYSICAL SURVEYS AT LANDFILLS NEAR MAOMING (SOUTH CHINA) AND BEIJING

Mapping the extent of groundwater contamination beneath landfills and hazardous waste sites is a major challenge, particularly in developing countries. Monitoring wells are scarce, expensive, and commonly fail to define the full extent of contamination and its three-dimensional character. Geophysical methods offer a powerful noninvasive tool for identifying subsurface contamination in these situations, especially when combined with other hydrogeological and remote sensing data within a Geographic Information System (GIS) framework.

This study examines groundwater contamination near Maoming in Guangdong Province, southern China. Oil shale mining and retorting northwest of Maoming have produced 50 million tons of waste that were dumped in two huge landfills averaging 6-7 km long, 1-2 km wide and 5-6 m high. Rainfall and surface water percolating through the waste has led to pollution of adjacent shallow aquifers by low pH landfill leachate containing heavy metals and organic compounds.

Reconnaissance surface geophysical measurements, combined with basic hydrological observations, suggest hydrogeological complexity in these aquifers. A resistivity sounding made over the north Maoming landfill identified the leachate level and suggests leachate-saturated tailings have a bulk resistivity of less than 50 ohm-m. Another sounding identified a very low-resistivity layer (less than 10 ohm-m) at 5-6 m depth in a village with contaminated wells about 1 km southwest of the north landfill – the conductive layer probably represents leachate within the confined aquifer. Other conductive zones were identified near abandoned wells approximately 600 m east of this landfill where a different sequence of layers was encountered. Resistivity profiling also suggests lateral changes in lithology and/or thickness.

Some of these methods may be applied to landfills surrounding Beijing in the near future, to optimize placement of monitoring wells and estimate water balances. About 7000 waste sites exist within and around Beijing, 70% of which have a diameter of more than 50 meters. These pose a substantial hazard to groundwater. Geophysical surveys are being planned for the X35 landfill, located in the southeastern Beijing suburb of Daxing, underlain by relatively shallow unconfined and confined aquifers.