Perform Nonintrusive Investigation to Determine Waste Extent and Characteristics
Non-intrusive investigation techniques are geophysical in nature. Geophysical techniques measure contrast in material properties in the subsurface and include the following:
- Seismic methods
- Electrical and electromagnetic procedures
- Gravitational field techniques
- Magnetic field techniques
- Ground Penetrating Radar
Geophysical investigations can be a timesaving and cost-effective method for providing qualitative subsurface information for a site. They can be used for screening large areas for potential buried wastes, for focusing resources for intrusive investigation activities on the anomalous areas, and for identifying or confirming the presence and extent of landfills and/or burn dumps. Buried solid waste and metals will (most likely) exhibit different bulk material properties than the surrounding native soil. This will typically allow geophysical instruments to distinguish the waste form the soil.
The results obtained from a geophysical investigation are subjective and rely on geologic interpretation. Geophysical techniques do not directly measure the parameter needed to solve the problem but instead measure contrasts in material properties. For example, seismic methods measure velocities of seismic waves through the subsurface material and electrical methods measure the conductivity (or it’s inverse, resistivity) of a material.
The interpretation of geophysical contacts is based on geologic assumptions: (1) earthen materials have distinct subsurface boundaries, (2) a material is homogeneous (material properties are the same throughout) and (3) the unit is isotropic (material properties are the same in all directions). Since these conditions rarely occur in nature, and almost never occur in solid waste or burn ash, geophysical methods are most often used in conjunction with other intrusive methods in order to more correctly assess the site.
Although geophysical interpretations are not always perfectly accurate, geophysical equipment is very precise. That is to say that the measurements obtained from non-intrusive geophysical techniques are very exact. The raw data is good data. The problem resides in the geophysical interpretation of the data, which are often educated estimations and/or calculated correlations and can lead to inaccuracies. However, when the appropriate geophysical technique is coupled with an intrusive investigation, large volumes of material can be explored accurately and cost-effectively.
Non-intrusive geophysical methods can be utilized as preliminary screening before performing intrusive investigations, they may be implemented as the primary investigative technique, they may be used in combination with intrusive investigation methods such as bore holes or test pits, or they can be used in combination with other non-intrusive geophysical methods. Understanding the specific strengths and weaknesses of each method will allow the investigator to decide how to best utilize geophysical investigation.
The following tables compare various methods and point out strengths and weaknesses:
- Decision Matrix for Geophysical Methods: Adobe PDF, 6 KB or MS Excel, 17 KB
- Characteristics of Geophysical Methods: Adobe PDF, 6 KB or MS Excel, 18 KB
Advantages and disadvantages of seismic methods:
Applications of Geophysical Methods
A variety of non-intrusive investigative techniques can be used to study environmental issues. The location and boundaries of buried wastes can be best approximated using Ground Penetrating Radar, Electromagnetics, and/or Resistivity. The following are site-specific examples of successful use of geophysical investigative techniques.
- Reese Air Force Base Trench-and-fill Disposal Site Located: IT Corporation
- Electromagnetic Survey Used to Map Sulfides and Acid Sulfate Ground Waters at the Abandoned Mine: USGS
- Yucca Mountain (high-level nuclear waste repository) Geophysical Studies: USGS
- Magnetic and Electromagnetic Methods used at Fort Chaffee North P.O.W. Landfill, Arkansas: USGS
- Geophysical Investigation of Chemical Waste Landfill in Northwestern Arkansas: USGS
- Geophysical Characterization of Landfill Flow Path, Winthrop, Maine: USGS
- Surface-geophysical investigation of the University of Connecticut landfill, Storrs, Connecticut: USGS
Site Investigation Reports
A table listing site investigations completed by the CIWMB is available. The investigations are listed alphabetically by site name and include: Solid Waste Information System (SWIS) number, type of plan or report produced, investigation type, types of operations, and whether the site is located in a rural or urban setting. The table identifies site investigations during which geophysical survey operations were utilized.
Valuable Textbooks:
Burger, H.R., Exploration Geophysics of the Shallow Subsurface, 1992. [with] accompanying Macintosh computer software by Douglas C. Burger and H. Robert Burger. Englewood Cliffs, N.J.: Prentice Hall, c1992.
Fowler, C.M.R., 1995. The Solid Earth: An Introduction to Global Geophysics. Cambridge University Press. New York. 76-96.
General Links:
Closed, Illegal, and Abandoned (CIA) Disposal Sites, http://www.ciwmb.ca.gov/LEACentral/CIA/
John Macanas, jmacanas@ciwmb.ca.gov (916) 341-6722