Intrusive site investigation (consisting of drilling and sampling), typically completed in the Phase II portion of Environmental Site Assessments (ESAs), provide important information for identifying, quantifying and characterizing contamination on sites during ESAs. However, completing the drilling and sampling program in an ESA, without prior mapping of the site with appropriate geophysical surveys (Figure 1), to properly identify the contaminated areas, contaminant plume(s), lateral and vertical extent of the plumes, and more, is like slinging a shot in the dark and hoping that it hits the target.
Using appropriate geophysical surveys to pre-map a contaminated site, as a precursor to intrusive investigations, prior to executing the drilling and sampling program, and using the results of the surveys to plan, guide and direct the intrusive investigations, is like turning on the light in a dark room to see things very clearly. Geophysical surveys help to see what is going on within the subsurface very clearly, and can be described as are the “light in the dark of the subsurface” when investigating contamination on a site. They provide useful imagery that show the location, vertical and lateral extent, and properties of contaminant plumes, making it easier to plan, guide and direct the drilling and sampling program, to know exactly where to drill, how deep and wide to go to collect samples in areas of interest.
Geophysical surveys are typically classified as part of the Phase II ESA, but since they take the guess work out of planning and executing intrusive site investigations in the Phase II ESA, they can be completed in tandem with the Phase I ESA. This way, important information that will help to plan the intrusive investigations in the Phase II ESA are obtained.
Geophysical surveys are sometimes seen as optional in the ESA process, and skipped entirely, which should not be the case. Geophysical surveys should not be seen as an optional part of the ESA process, but as a crucial component that must be done carefully and thoroughly, in order to properly characterize a contaminated site and develop a robust conceptual site models for corrective actions, site clean-up, remediation, rehabilitation or closure.
There are several geophysical survey methods that can be used to delineate contamination on sites. The type of contaminants on the site, which can be identified from the previous land use, or items on the site that can interfere with signals, and other site-specific considerations, will determine the appropriate method or combination of geophysical survey methods that are selected for use on a site. Examples of geophysical surveys for contaminated sites include the electromagnetic (EM) survey and electrical resistivity tomography (ERT) (Figures 2, 3).
A geophysical survey, such as the EM survey, measures the combined electrical conductivity of the soil matrix and pore fluids. Contamination is detected from the measured terrain conductivity, as increased conductivity will increase terrain conductivity. EM surveys thus help to define the extent of soil and/or groundwater quality impacts. An ERT measures the electrical resistivity (inverse of electrical conductivity) of the terrain and provides cross sectional imagery of the distribution of electrical resistivity in the subsurface. The ERT survey will provide information on the depth, thickness, and changes in geologic conditions, as well as the delineation of contaminant plumes.
When used together, the results of the EM and ERT surveys (for example), provide detailed information on both the lateral and vertical extent and distribution of contaminants or substances of potential environmental concern. Examples of the results and imagery that can be obtained from the EM and ERT surveys are presented in Figures 2 and 3. Figure 2 shows the lateral distribution of contaminants or substances of potential environmental concern on a site as obtained from an EM survey, while Figure 3 shows the vertical profile of the site as obtained from ERT line 3 going north west to north east in Figure 2.
Given the importance of geophysical surveys for delineating contamination on a site, essentially, any ESA or investigation of a site with known or perceived contamination that is completed without geophysical surveys should be deemed incomplete. Owners and regulatory bodies reviewing Phases I and II ESA reports, should demand that geophysical surveys be completed, and the results of the surveys be used to inform, guide and direct the ESA process. This way, contamination and substances of potential environmental concern are well identified and characterized, for subsequent corrective/remedial actions, site rehabilitation, clean up, closure, or future end use.
Geophysical surveys could help prevent incomplete site clean-up, and any consequent environmental legacy and liability issues that could ensue later on. In addition, geophysical surveys provide cost savings on site assessments, since the intrusive investigation will be targeted and guided by the results of the surveys. Hence, unnecessary and incorrect drilling and sampling locations, and associated costs will be avoided. Similarly, since the intrusive investigation will be based on what the geophysical survey has revealed to be the actual areas of concern or potential concern to target, and not on guesses, the added costs relating to remobilization to a site to complete additional intrusive investigation will be avoided.
Terrasyntec will work with you to plan and execute detailed environmental site investigations and assessments, and develop suitable remedial and corrective actions as needed. Please forward your enquiries and questions to technicalservices@terrasyntec.com or fill out the form on our Contact page to engage with us.