North Carolina contains hundreds of properties that served as landfill sites in the past; these former landfills have long since been inactive, and many of the sites are overgrown with no specific knowledge of the extent of the buried waste. In North Carolina, the Department of Environmental Quality (DEQ) has established a program to assist in assessing and remediating these pre-regulatory landfills. The DEQ definition of a pre-regulatory landfill is, “any land area, whether publicly or privately owned, on which municipal solid waste disposal occurred prior to January 1, 1983, but not thereafter, and does not include any landfill used primarily for the disposal of industrial solid waste.”
Geophysical surveys are a common and effective method to determine the limits of buried waste. Specifically, most investigations will incorporate an electromagnetic survey to measure ground conductivity across the area thought to contain buried waste. This survey is typically performed using an electromagnetic terrain conductivity meter. Pyramid Geophysical Services uses a Geonics brand EM31.
The EM31 terrain conductivity meter measures apparent ground conductivity and provides metal detection down to a maximum depth of 15 to 17 feet below surface. The EM31 method determines electrical properties of the earth materials by inducing electromagnetic currents in the ground and measuring the secondary magnetic field produced by these currents. The two data sets that are collected can be used to observe changes in ground conductivity that are the result of something such as the presence of buried waste in a landfill.
The final product generated from an EM31 survey is typically a plan-view color contour map of conductivity variance across the site. The presence of buried waste (relative to the surrounding native soil) will cause an increase in the ground conductivity. Other factors can also result in increased ground conductivity, such as changes in soil type and moisture content; these factors must also be taken into account when interpreting the limits of buried waste. The results can also be analyzed to specifically identify buried metal such as drums or metallic debris.