Apparent formation factor for leachate-saturated waste and sediments: Examples from the USA and China

The formation factor relates bulk resistivity to pore fluid resistivity in porous materials. Understanding the formation factor is essential in using electrical and electromagnetic methods to monitor leachate accumulations and movements both within and around landfills. Specifically, the formation factor allows leachate resistivity, the degree of saturation, and, possibly, even the hydraulic conductivity of the waste to be estimated from non-invasive surface measurements. In this study, apparent formation factors are computed for three landfills with different types of waste as well as sediments contaminated by landfill leachate. Resistivity soundings at the closed Mallard North landfill in suburban Chicago (Illinois, USA) mapped leachate surfaces that were confirmed by monitoring wells. The resistivity of leachate-saturated waste from resistivity sounding inversions was then divided by the leachate resistivity values measured in-situ to compute apparent formation factors (F a) ranging from 1.6 to 4.9. A global F a of 3.0±1.9 was computed for the entire monitored portion of this landfill. At a nearby mixed laboratory waste landfill, a 2D inverted resistivity section was used to compute an F a of 2.9. Finally, a distinctly different F a value of 10.6±2.8 was computed for leachate-saturated retorted oil shale wastes north of Maoming (茂名), Guangdong (广东) Province, China. Shallow aquifers in the Laohuling (老虎岭) Formation near this landfill are polluted by acidic leachate containing heavy metals and organic compounds. The F a for aquifers containing contaminated groundwater fall in the same range as aquifers with normal groundwater, 1.7–3.9. However, models from inverted sounding curves over these contaminated areas exhibit unusually low resistivity layers, which may be diagnostic of contamination.