Geophysical plume mapping is a technology used to detect and delineate contaminate plumes in the earths subsurface typically associated with industrial sites, municipal solid waste disposal sites, superfund sites. Subsurface contaminate plumes are of great concern because in most cases these environmental hazards will eventually lead to groundwater contamination unless successfully remediated.
Plume mapping is a technology used to detect and delineate contaminate plumes in the earths subsurface.
Organic and inorganic contaminants of sufficient concentration whether in the vadose zone or in the groundwater, can be mapped from the surface with geophysical methods such as two dimensional and three dimensional electrical resistivity imaging. These methods map the distribution of resistivity of subsurface materials. The resistivity image provides general information on subsurface characterization of buried waste and contaminated soil. Underground soil or water that has been contaminated by pollutants usually has a significantly lower resistivity value. It is this contrast between resistive and conductive features in areas where contaminate plumes exists that enables geoelectric resistivity technology to identify and delineate the extent of contaminated soil.
HGI is an expert at plume mapping through the employment of hydrogeologists, who ensure that geophysical interpretations match hydrological expectations.
Plume mapping success will depend on a number of factors:
- Host formation
- Geophysical method
- Site complexity
HGI has recently adapted the electrical resistivity method to accommodate highly complex areas, such as those with underground pipelines, tanks, metal fences, and other infrastructure that normally would interfere with mapping the subsurface. The solution has been to use the infrastructure directly as sensors, and steel cased wells are perfect electrodes for passing electrical current or measuring voltage. Horizontal piping could also be used as long as the positions are known. HGI has used well-to-well (WtW) resistivity, a.k.a. long electrode resistivity, on several environmental projects to characterize and monitor plumes in the subsurface. Additionally, we have used the method to track fluid injections into heap leach piles and oil reservoirs for secondary recovery of resources.
Example: Long Electrode Resistivity Mapping of an Underground Waste Storage Tank Facility
An underground storage tank facility was imaged using electrical resistivity. The image below shows a site photo that is highly complex and the overhead view of the site shows all of the pipes, tanks, and buildings at the facility. Steel cased wells, originally emplaced to conduct periodic borehole logging, were used as long electrodes. Fortunately, a large number of wells existed – 77 in total. The long electrodes were aided by surface electrodes along the periphery of the facility to enhance the coverage. Each well had a turn at passing electrical current while all others measured voltage using a pole-pole array. The resulting dataset was inverse modeled to construct a spatial distribution of electrical resistivity values that mimicked the likely distribution of waste in the subsurface. In particular, the far south tank shows a very low resistivity distribution, and the 3D view of the data shows the plume beneath the tank to extend deep into the subsurface.