The EPA estimates there are some 4,000 active and over 10,000 closed municipal waste landfills across the United States. In many cases, the information on active and old solid waste landfills can be limited, or completely absent in some cases. In addition, the majority of closed landfills are unlined, allowing leachate and contaminant plumes to migrate off-site and pollute groundwater. These issues can make ongoing monitoring or remediation efforts inefficient or problematic at best. Geophysical methods can assist in landfill characterization or monitoring by providing non-invasive tools that can characterize both the spatial limits and waste depths in closed and active landfills, image and constrain plumes, investigate landfill gas production, and assess void and subsidence issues.
Geophysical methods for landfill characterization can characterize both the spatial limits and waste depths in closed and active landfills
During the past 25 years, HGI has developed a variety of geophysical methods for landfill characterization that have proven to be efficient and effective for characterizing subsurface geology and hydrogeology associated with these structures. As a result, geophysical characterization of landfills is now a routine practice. Even so, due to the inherent uniqueness of each site, it is necessary to carefully select the method and appropriately design the survey parameters in order to derive adequate benefit. In many cases, particularly when multiple objectives are desired, it is best to use complimentary geophysical methods.
HGI designed the Geophysical Operations Cart (G.O. CartTM) to increase survey efficiency and imaging accuracy by integrating multiple subsurface mapping instruments on one ATV or hand-towed trailer. The platform is engineered to handle rugged terrain and is constructed of non-metallic materials that do not interfere with metal mapping surveys. Instruments are coupled with GPS navigation and a heads-up display allows a single operator to rapidly cover large areas with high data resolution. The precise spatial control allows for time-based monitoring of changes within the landfill subsurface by repeating surveys on a periodic basis and then comparing the differences. These differences may indicate plume migration, gas accumulation, or meteoric water infiltration.
Delineating the spatial boundaries of active and closed landfills can be achieved using a combined Electromagnetic Induction and Magnetic survey, typically mounted on HGI’s dedicated G.O. Cart. These two methods provide an assessment of the subsurface to differentiate between natural background soils, fill or cover material, and the solid/liquid waste placed in the landfills. The example shown to the right highlights a successful spatial mapping survey conducted on a closed landfill, approximately 8 acres in size. The elevated electromagnetic conductivity response to the waste materials (yellow to brown shades) allowed the boundary of the landfill to be mapped (shown by the red outline). It can be seen how this mapped boundary differs to the assumed pre-survey landfill boundary (Red line), allowing for an update in landfill extent and waste locations. In addition, the spatial mapping can provide areas of interest for follow up with an Electrical Resistivity survey for further subsurface characterization and volumetric estimation. HGI’s system is also highly effective for validation of post-closure engineered cover integrity.
Geophysical methods such as Electrical Resistivity surveying allow us to collect high-resolution and continuous electrical cross-sections through active and closed landfills to characterize the subsurface. The image to the left shows an example from a landfill delineation and volumetric estimation investigation at a closed landfill. The cross-section provides the variation in depth of the conductive landfill waste material (white dashed line), the thickness of the placed cover material (black dashed line), and highlights where leachate plumes are present (orange dashed line) and potential issues relating to their migration off-site. In addition, the method is sensitive to the presence of perched water in the landfill materials (which can impact drilling activities), can indicate areas that have elevated decomposition potential, and can be used to identify and track landfill gas migration within the subsurface.
A variety of techniques can be applied to define landfill properties, leachate circulation, or regional contamination from offsite seepage. HGI offers a suite of services designed to investigate landfills from every angle. Depending on the project’s objective and desired resolution, methods for characterization include electrical resistivity, electromagnetic induction, magnetic gradiometry, and seismic refraction. The table below offers some guidance in choosing a method that may be suitable for your particular needs.
|Method||Purpose||Depth-of Investigation (ft)||Resolution||Acquisition-rate (lineal coverage)||Cost|
|Magnetic Gradiometry||Define boundary / find iron||30||Low||1-10 miles/day||Low|
|Electrical Resistivity||Define moisture characteristics||1000||High||>1 mile/day||Moderate|
|Induced Polarization||Define clay or organic material||1000||High||>1 mile/day||High|
|Electromagnetic Induction||Define boundary / find metal||15||Low||1-10 miles/day||Low|
|Transient Electromagnetics||Define moisture characteristics||1000||Low||>1 mile/day||High|
|Seismic Refraction||Define strength characteristics||100||Moderate||>1 mile/day||Moderate|
|MASW||Define strength characteristics||100||High||>1 mile/day||High|
HGI has completed landfill investigation services for City of Tucson, City of Phoenix, City of Albuquerque, US Army Corps of Engineers, Davis Monthan Air Force Base, as well as multiple private consulting firms.
Example: Resistivity Imaging of a Municipal Landfill
Below is an example of a landfill characterization using High Resolution Resistivity (HRR), one of HGI’s signature methods. The two transects are approximately 700 ft long and separated by 100ft. The data show, as interpreted from the contrasts in high and low electrical resistivity values, the edges of the landfill as well as the bottom edge. Smaller structural features are also seen, such as faults and underlying basement basalt.
The figure below shows results for the same site using Total Field Magnetics.