LANDFILL CLOSURE IS AN EXPENSIVE process for landfill operators, so cost controls are always welcome. One way to minimize expenses is to use frequency-domain electromagnetic (EM) terrain conductivity mapping. The technology has proven to be an effective and inexpensive aid in locating a landfill's footprint and estimating the in-place waste volume.

EM terrain conductivity surveys have been used in landfill investigations for more than 20 years. The inexpensive surveys can delineate waste, conductive fluids and buried metal, and provide a three-dimensional overview of buried waste. As organic material in a landfill degrades, it produces a terrain conductance signature that is elevated above background conditions. The elevated signature then can be used to locate waste, identify the waste boundary and provide a rough estimate of waste depth. Then, the landfill footprint can be used with the terrain conductivity estimation to calculate the waste volume.

An EM tool consists of a transmitter coil that radiates an electromagnetic field and is integrated with a digital global positioning system receiver to collect real-time spatial and EM measurements. When eddy currents are introduced into the earth, they generate a secondary electromagnetic field that is proportional to the magnitude of the current in the coil. The magnetic field is captured by the receiver in the form of an output voltage, which provides a reading of soil conductivity.

Electrical conductivity of soil is a function of the porosity, permeability and fluids in the pore spaces. Waste degredation generates conductive leachate that fills pore spaces and can be detected easily with an EM terrain conductivity meter. Case studies of regional landfills have confirmed that a relationship exists between measured waste terrain conductivity and waste thickness. Therefore, this relationship can be used to estimate waste volume without seismic reflection surveys or intrusive methods, such as borings.

Soil conductivity values vary widely. Additionally, the relationship between conductivity and waste thickness can be affected by pockets of ferrous and non-ferrous debris, old waste, pit-burned waste, low field-saturated conditions, thick soil cover, extensive construction and demolition debris, and a weak background conductivity characterization. Nevertheless, a linear relationship between terrain conductivity and waste depth has been verified, establishing that thick accumulation of field-saturated waste generates a stronger conductivity reading than a thin waste deposit. The conductivity-waste depth relationship also has been tested against boring-derived waste depths and seismically derived waste depths and found to derive waste thickness and volume within 15 percent of the actual measurements.

The proposed capping and closure of a former sand quarry in the Northeast was hampered by the inability to determine the landfill's footprint with conventional drilling. But EM mapping of the site helped to determine the landfill's footprint. The data also showed that a plume was migrating from the site. So the technology helped the landfill operator to place gas vent wells in locations to maximize landfill gas extraction.

EM terrain conductivity mapping has been proven to calculate waste depth and volume non-intrusively, and to determine an accurate landfill footprint with a reasonable degree of accuracy.
— Peter J. Hutchinson and Laura S. Barta The Hutchinson Group Murrysville, Pa.