3. Landfill Characterization, El Dorado County, CA
A closed landfill located on U.S. Forest Service land is believed to be irregularly shaped and encompass an area of about 30 acres. A portion of the site was filled with clean earth materials, whereas, the remaining area was filled with miscellaneous municipal refuse. However, the limits and thickness of the waste are not known and it is not known whether there is a continuous cap over the refuse. The general geology of the area consists of coarse grained materials overlying moderately weathered granitic bedrock.
A comprehensive geophysical investigation was conducted consisting of the following techniques:
- magnetic and terrain conductivity surveys on a 10-foot grid to determine the location of specific debris and the lateral extent of the refuse;
- a series of electrical resistivity (ER) profiles over the area to determine the approximate thickness of the refuse overlying the granitic bedrock; and
- a series of ground penetrating radar (GPR) profiles to determine the existence and thickness of the cap.
Contour maps displaying the magnetic and terrain conductivity results, show large well defined zones consisting of numerous localized magnetic and conductivity variations. These zones delineate the footprint of the landfill. The limits of the fill shown on both contour maps are remarkably similar and show a sharp contrast with the uniform magnetic and conductivity values of the adjacent clean fill and undisturbed site native soils.
Electrical resistivity profiles over the mapped refuse area show a noticeable change in electrical properties. The refuse is significantly more conductive than the clean fill and underlying granitic bedrock. As shown on the sample electrical resistivity profile, the approximate thickness of the refuse is up to about 50 feet and the lateral limits correlate with the magnetic and terrain conductivity results. Low resistivity zones beneath the refuse may be indicative of conductive leachate that has migrated into the weathered bedrock. The zone of clean fill is represented by noticeably higher resistivity values.
Ground penetrating radar profiles taken over the mapped debris fill show a shallow zone of material extending from the ground surface to a depth of about 2 to 6 feet. This zone is generally featureless and lacks significant radar reflections. Below this zone there is an irregular interface consisting of numerous reflections. The featureless zone and underlying interface is interpreted as representing the homogeneous cap material and underlying top of refuse. The sample GPR profile shows the irregularity of the cap/refuse interface.