NONINVASIVE MAPPING OF LEACHATE LEVELS AND FLOW SYSTEM DELINEATION IN CLOSED LANDFILLS

The shape and size of landfill leachate plumes are often controlled by areas of high recharge and preferential infiltration within the landfill. Geophysical methods have proven useful in noninvasively mapping leachate levels within landfills as well as water levels in surrounding unconfined aquifers. This information can then be used to generate flow nets and other quantitative models that predict leachate movement within and beyond the landfill. Leachate-saturated municipal refuse typically appears in geoelectrical models as a low resistivity layer (3 to 7 ohm-m) in the basal part of the refuse. Resistivity soundings made at the Mallard North landfill in northern Du Page County, Illinois, mapped a leachate surface that generally matches the leachate surface defined by monitoring wells. Geophysically-determined leachate levels were within 1-2 m of measured levels at depths of 4 to 10 m below the top of this landfill. Seismic refraction surveys mapped water levels outside the landfill where groundwater was not highly conductive.

Geoelectrical methods are also being used to map leachate levels, identify areas of enhanced recharge, and to understand the flow system within two large oil-shale tailings landfills in Maoming, southern China. The landfills average 6-7 km long, 2-3 km wide and 5-6 m high. Water reacting with sulfides in the waste produces acidic leachate that mobilizes heavy metals -- these metals have polluted an unconfined aquifer used by nearby villages. Leachate head maps are being combined with water levels around the landfills to estimate volume of leakage and leachate migration directions. GIS may also be used to correlate EM conductivity anomalies, the position of hydraulically-active fractures on the landfills and the position of leachate level mounds / depressions within the landfills by superimposing these data sets as separate thematic layers.