Solutions for a complex world: Subsurface Characterization for Brownfields
Article by: Shawn Calendine | hydroGEOPHYSICS
At first glance, brownfield sites may seem like easy targets for revitalization and improvement by municipalities and developers. Removing old structures and clearing the surface of a site is relatively straight forward. Demolition and site clearance costs can quickly and easily be assessed and compared against new construction in light of the economic vitality of the area.
However, brownfield sites often contain hidden subsurface infrastructure and contamination associated with previous commercial or industrial usage. It is these unknowns that present a great challenge to revitalizing a site. They inhibit development potential, causing these sites to languish in cities and communities across the US.
Brownfield assessment methods of choice are often invasive, involving soil sampling from exploratory drilling, excavating, and potholing. Water samples may be acquired from surface water features, stormwater runoff, and monitoring wells. These methods can be effective in determining potential contamination and discovering subsurface infrastructure; however, their application is often based on inadequate information and basic presumption.
Additionally, the lack of clear and targeted assessment can lead to failure to detect subsurface contamination and dangerous unknown infrastructure, escalating costs, and creating environmental health and safety issues for site workers and the public.
Geophysical characterization techniques can provide clarity in assessing brownfield sites.
Geophysics offers a better, cost-effective, and safer approach to assessing brownfield sites. Geophysics has long been used for subsurface mapping. Now, advancements in data acquisition and analysis techniques have expanded geophysical capabilities to include assessment of increasingly complex settings such as brownfield and superfund sites.
While there are many examples of geophysical methods being applied successfully on these types of sites, the practice is often overlooked by environmental consultants and engineers for more traditional invasive assessment methods. This means that an advanced, cost-effective, safe, and accurate assessment tool for understanding subsurface conditions of brownfield sites is underutilized or not used at all.
Understanding Geophysics for Brownfields
At its core, geophysics is the study of the earth’s internal dynamic geologic structure. Geophysicists use scientific fields such as physics, mathematics, and chemistry to appreciate the physical properties of the earth. Geophysics’ major asset is its scalability, meaning, geophysical methods can be applied to an object as large as the entire earth or on a smaller scale such as a brownfield or superfund site.
At a site level, environmental engineers and consultants can use geophysical methods as target recognition tools to discriminate and map subsurface anomalies. For example, geophysical data can provide greater detail regarding specific information such as size, depth, and intensity, compared to randomly placed drill holes and investigative digging.
Furthermore, geophysics involves a broad range of methodologies with each technique isolating specific properties or characteristics of the earth or objects buried in its surface. Choosing the appropriate geophysical method for a brownfield is always site-dependent because of the potential for both infrastructure and contaminates to be co-located. Complimentary geophysical methods offer a uniquely competent set of assessment tools that can be layered together to reinforce the confidence of targets(results?), while also delineating other targets with properties specific to a single geophysical method.
Four Methods to Consider
Electromagnetic (EM) surveys to measure changes in the electrical conductivity of the subsurface caused by the presence of fluids, metals, and differing soil types. These various materials can be distinguished by their differing electrical characteristics through the process of electromagnetic induction. An electric current is induced in any conductive material in the subsurface by a time-varying magnetic field produced by the instrument. The induced current varies based on material and fluid properties, and these differences can be measured, interpreted, and mapped. The resulting maps show graduated contours of contrasting resistive and conductive features that may be targeted for investigation.
The image below shows an EM contour map of an abandoned brownfield; the site had been a gas station and ice cream shop. What is interesting about this image is that at some point the underground storage tank (UST) was actually removed and the hole was filled with imported soil from a different location. The EM survey was sensitive enough to pick up the different soil types thus delineating the former site of the UST. The contour map also shows locations of other anomalies common in brownfields, which were identified based on subsequently targeted excavations based on the results.
EM techniques typically employ portable, rapid, non-invasive equipment that can be operated by a single person, and can be coupled with a Global Positioning System (GPS) device for precise locating. Multiple acres of data can be collected per day by an operator. This can be significantly increased if the equipment can be towed via UTV or ATV.
Magnetometry is the study of the Earth’s magnetic field. For brownfields, magnetics is a passive survey method that measures the contrasts in localized magnetic responses due to the presence of ferrous materials containing iron, nickel, cobalt, and their alloys. These types of surveys can discriminate magnetic field anomalies in the first few meters of the subsurface identifying typical brownfield targets such as tanks, pipes, drums, foundations, and other related objects.
Magnetic fields typically generated by brownfield targets are frequently measured with a dual-sensor magnetometer called a gradiometer. Gradiometer surveys are becoming increasingly common in environmental/engineering site investigations as they are particularly sensitive to ferrous materials sometimes found in the near-surface of brownfield sites.
The image below demonstrates Electromagnetic Induction (EM) and Magnetic Gradiometry geophysical methods used to characterize the subsurface infrastructure of a Hanford tank farm site. The image shows magnetic and electro-magnetic data layered together around the outside of the TY tank farm. Here the combined data delineate and reinforce the locations of subsurface structures.
Ground Penetrating Radar
Ground Penetrating Radar (GPR) is a geophysical technique that uses pulsed electromagnetic waves to image shallow soil and ground structures. The method has been increasingly applied for brownfield investigations since the 1980s and is used extensively for locating utilities and subsurface tanks. However, GPR is not limited to locating metallic subsurface objects because it is also sensitive to boundaries between differing soils and objects that have contrasting electrical properties.
In practice, an electromagnetic wave is radiated from a transmitting antenna and travels through the subsurface of the earth. The wave spreads out and travels downward until it hits an object or substrate that has different electrical properties from the surrounding medium. The wave is scattered from the object, and the scattered wave returning to the surface is detected by a receiving antenna.
GPR data are typically collected using a portable GPR system which can be coupled with a GPS unit to locate each data point in a brownfield survey. The system is typically mounted on a cart or sled and is pushed or pulled across the surface of the earth. One big advantage is that GPR data can be displayed as a two-dimensional cross-section on an attached screen in real-time. The data in the image can be interpreted on-site and locations of anomalies can simply be painted on the ground as they appear. The data can also be recorded and interpreted laterally to create a 2-D plan view showing anomaly locations. Below is such a 2D plan view of interpreted linear pipeline location, created from GPR collected inside one of the underground radioactive waste storage tank farms at the Hanford Nuclear Site, WA.
Like both EM and Magnetometry equipment, GPR systems are extremely field-portable and can be coupled with a Global Positioning System (GPS) device for the location of brownfield targets. A single person can operate the equipment and data can be acquired in just about any accessible location.
While magnetics, EM, and GPR are predominantly used for locating actual physical objects, resistivity geophysical methods are highly suited for the location and delineation of subsurface contaminants. Resistivity surveys are concerned with how electrical current flows through soils and rocks and where the current flow may be inhibited or conducted. Thus resistivity surveys can be very sensitive to the presence of fluids associated with contaminants, which tend to be much more conductive than the uncontaminated materials.
The resistivity method uses an electric current that is transmitted into the earth through one pair of electrodes (transmitting dipole) that are in contact with the soil. A resultant voltage potential is then measured across another pair of electrodes (receiving dipole). The measurement made between the single pair of transmitting and receiving electrodes produces a single resistivity value for a small volume of the earth at depth. The depth of the data point is based on the distance between the two pairs of transmitting and receiving electrodes. When numerous electrodes are deployed along a line (which may be anywhere from feet to miles in length and contain potentially hundreds of electrodes), thousands of data points can be measured at various depths and distances along the survey line. These data points are then used to create a map of distributed electrical potentials.
In the case of brownfield contaminants, the pore spaces between grains of the earth may fill up with contaminated fluids changing the electrical properties of the soil. When a resistivity survey is performed over the contaminated area the contrasting electrical properties of the contaminated and uncontaminated regions can be mapped defining potential contaminant locations. These targets can then be considered for remedial action.
The image below shows a 2-D resistivity cross-section of a waste disposal site on the Hanford Nuclear Reservation. This particular site consisted of a series of concrete-lined trenches open at the bottom where low-level radioactive waste was historically dumped and allowed to infiltrate into the soil and subsurface. This radioactive waste was rich in nitrates and other salts which were highly conductive. In the plot, you can clearly see a conductive plume (more conductive values are purples to blues, more resistive are yellows to reds) related to the waste below the areas associated with trenches 33 and 34. . Since this waste was disposed of decades ago we can use the resistivity method to delineate the current boundaries of the contaminants in the subsurface allowing for a targeted remediation effort. We can also monitor the movement of these contaminants within the subsurface by repeating the surveys over time, possibly identifying preferential flow pathways, geological controls, etc.
The Big Picture
The goal for any environmental engineer and consultant is to perform the most complete assessment of a brownfield site by using the industry’s best tools and technologies to optimally pinpoint problems prior to intrusive site investigation. One of the best ways is by layering geophysical methodologies, which allows us to present and interpret resulting visual data in a way that is meaningful and useful to both geophysicists and non-geophysicists alike. Geophysics is a powerful tool that can enhance brownfield cleanup efforts by identifying abandoned infrastructure and possible contamination sources and pathways within the subsurface. Geophysical characterization results contribute to more efficient planning of secondary remedial activities by reducing cost/risk, and improving personnel and environmental safety.
About the Author: Shawn Calendine | hydroGEOPHYSICS
Shawn Calendine is the Marketing & Development Manager at hydroGEOPHYSICS (HGI). Since joining HGI in 2005, Shawn has worked in many positions for HGI, most notably, as a team member managing the leak detection and monitoring (LDM) program for nuclear waste tanks at the Hanford Site in eastern Washington State. Following the LDM program, Shawn moved into field geophysics, gaining high-level experience with HGI’s geophysical technology toolbox.
In addition to work as a marketing consultant, Shawn participates, as a board member for several industry-related nonprofit organizations and authored several papers on geophysical methods relating to liner leak location and resistivity characterization. Over the past 15 years, Shawn has presented on geophysical topics more than 30 times at professional meetings and conferences. Shawn holds Bachelor of Science degrees in both Environmental Science and General Science with a minor in Biology from Portland State University.
Other Articles By Shawn Calendine
- How Geophysics Could Help Prevent Reservoir Breaches Like The Florida Industrial Pond
- Bare Liner Leak Location – The Water Puddle Method
- Pinpointing industrial pond liner leaks – HGI Leak Detection
- Electric Liner leak Location for Industrial Ponds
- Subsurface Characterization for Brownfields – Is There A Better Way?