Integrated Geophysical Investigation Helps Improve A Landfill Closure Design

This paper describes the geophysical investigation conducted at the Fort Hunter Liggett<br>(FIX) Landfill in Monterey County, California. The geophysical data were obtained using<br>a custom survey system that facilitated the simultaneous acquisition of ground penetrating<br>radar (GPR), electromagnetic (EM), and magnetic (MAG) data. This paper also describes<br>how the investigation results helped to improve a landfill closure design.<br>The preliminary closure design concept for the landfill incorporated a standard regulatory<br>cover constructed over 54 acres - the estimated landfill extent as determined from aerial<br>photographs. The preliminary design was problematic for a number of reasons, including<br>its high cost and its impact on plant species protected by county ordinance.<br>To mitigate these drawbacks, an alternative design concept was developed. Included in<br>the design work was a geophysical investigation to verify the overall extent of the landfill<br>and accurately delineate individual refuse trenches within the landfill. GPR EM and MAG<br>data were obtained simultaneously by towing geophysical sensors from a custom-built<br>non-metallic trailer.<br>The investigation determined that the landfill encompassed approximately 36 acres, as<br>opposed to the 54 acres estimated from the aerial photographs. Eleven large refuse<br>trenches were delineated, as were several small outlying pockets of debris, including a<br>cache of buried 55-gallon drums. As a result of the geophysical investigation, the area to<br>receive cover was significantly decreased, thereby reducing the amount of cover material<br>required (hence the overall cost), and sparing many of the protected native trees. Potential<br>on-site borrow sources were also identified.