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Mapping Contaminant Plumes With Geophysics
Article by: Moira Poje | Geoscientist II | hydroGEOPHYSICS
Contaminant plumes in groundwater are a widespread environmental problem. Tracking plumes can be complex and daunting while estimating the plume’s extent, source, and impact can have high uncertainty, especially when only a few measurement locations are available. Compliance wells to determine the concentration of a plume’s constituents are often too few due to the cost and complexity of drilling them in numerous strategic locations. Monitoring wells can also be misplaced, missing the plume altogether. Therefore, meaningful spatial interpolation and sampling may lack the resolution for reliable tracking and monitoring of contaminant plume evolution. This high uncertainty leads to inaccurate decision-making and can unnecessarily extend monitoring and clean-up time.
Geophysical methods are well suited to map contaminant plumes
Geophysical methods are well suited to map contaminant plumes and provide qualitative information to define essential characteristics for groundwater modeling and remedial actions. Geophysical methods aimed at hydrogeologically-based problems, including electrical resistivity, induced-polarization (IP), ground penetrating radar (GPR), nuclear magnetic resonance (NMR), and electromagnetic methods (EM), can provide information that is spatially continuous in 2D and 3D. High-resolution temporal information can help target clean-up and validate remedial strategies when geophysical methods are applied repeatedly over time.
Inorganic plumes are relatively straightforward to map, as the higher ionic strength of contaminated porewater compared to the background levels provides a consistent target to pinpoint with electrical and electromagnetic methods. In the image above, a contaminant plume comprising mainly sodium nitrate and mapped by HGI using the electrical resistivity method depicts an enhanced survey resolution of our methodology. The figure reveals the varying contaminant concentrations of a plume, with pink hues representing higher concentrations of contaminant while reds and yellows represent uncontaminated background soils.
Other types of plumes may be more difficult to image directly due to low contaminant concentrations, or the pollutants’ presence does not alter any physical property that geophysical methods can measure. In these cases, geophysics enhances site knowledge, such as mapping lithological sequences or geological structures (faults, fractures, contacts). Non-aqueous phase liquids (NAPLs) are a group of contaminants representing oil-based products that are typically too dilute to measure directly. The second example above shows mapping with resistivity and IP across a bedrock stream and adjacent sediments. The data highlight clayey material that may decrease hydraulic conductivity or preferentially adsorb specific contaminants of concern.
Typical geophysics applications for contaminant tracking include fluid spills, legacy disposal sites, mine reclamation, acid rock drainage, or a containment breach (e.g., from ponds, tanks, or pipelines). Geophysical mapping techniques are non-invasive with low environmental impact and are economical, as measurements are collected rapidly and efficiently across a survey area. Applying a hybrid approach to plume mapping, combining high-resolution geophysical techniques with an optimized drilling campaign, provides a valuable set of tools to your site-characterization toolbox.
HGI is a world leader in applying, monitoring, and understanding the science of plume mapping using geophysical technologies. Contact us for a free consultation on how geophysics can benefit your next project. We’re always happy to chat – follow us on LinkedIn.
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About the Author: Moira Poje | hydroGEOPHYSICS
Moira Poje is a Staff Geoscientist with hydroGEOPHYSICS, Inc. She works on contaminant delineation, void detection projects for mine reclamation, subsurface fluid flow mapping, and depth-to-bedrock delineation via seismic, gravity, and electrical methods.
Moira has presented at the AGU Annual Meeting, Arizona Hydrological Society Annual Meeting, CUR REU symposium, and published in EEGS’s magazine FastTIMES. Moira serves on the Board of Directors for the Environmental and Engineering Geophysical Society (EEGS). Moira holds a B.A. in Geophysics and Planetary Science from Boston University.
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