The Brownfield Challenge
The United States Environmental Protection Agency (EPA) estimates that there may be nearly half a million brownfield sites within the United States, located in industrial centers, business districts, and neighborhoods. By definition, a brownfield site is “a former industrial or commercial site where future use is affected by possible environmental contamination.” These sites also often contain hidden subsurface infrastructure associated with previous usage. The existence of brownfields distresses the fabric of our cities and poses threats to the environment, human health, and the social welfare of our shared communities. However, the redevelopment of brownfield sites can present great social and financial opportunities. Brownfield recovery creates a catalyst for community regeneration by providing employment, housing, and a shared place for social enjoyment, thus enhancing quality of life and conservation of our environment.
In the early 2000s, geophysicists and engineers at hydroGEOPHYSICS began developing and applying cutting edge geophysical methods for characterization of brownfield sites.
Brownfield sites regularly carry great uncertainty due to lack of information and documentation on their historical usage. This lack of historical understanding poses a challenge for redevelopment, requiring broad site assessment by stakeholders before remedial efforts can proceed. Typically, the greatest unknowns are associated with the subsurface, where contaminants can lurk from historic disposal practices and spills, and dilapidated infrastructure may remain unseen.
While geophysics has long been used for subsurface mapping, advancements in data acquisition and analysis techniques over time have expanded geophysical capabilities to include increasingly complex settings. In the early 2000s, geophysicists and engineers at hydroGEOPHYSICS began developing and applying cutting edge geophysical methods for characterization of complex industrial environments, including the Hanford, Savannah River, and Los Alamos US Department of Energy (DOE) Nuclear Sites. These highly dynamic and hazardous sites required a host of geophysical methods be layered together to create a holistic understanding of the subsurface infrastructure and contamination zones.
HGI’s approach is to complete a site specific survey design that balances detection/mapping goals with brownfield site logistics and project costs in order to maximize results.
HGI’s DOE experience set the stage for development of a set of cooperative site characterization techniques that provide clarity in the critical steps of assessing and determining reclamation strategies. These complimentary geophysical methods offer a uniquely competent set of target recognition tools invaluable to industrial and brownfield site investigations. Our experience has shown that using any single characterization method may not capture the full spectrum of subsurface features defining a brownfield, which can contain a broad range of targets needing discrimination. In addition to soil contaminants, different types and sizes of infrastructure, such as reinforced concrete, underground storage tanks, power lines, vaults, voids, and pipes could be located on a single site. This high level of infrastructure, each with differing components and properties, requires a multi-method geophysical approach coupled with a high level of data density.
Choosing the appropriate geophysical method or methods is always site dependent: the Ground Penetrating Radar (GPR) method may be used to discern PVC pipes, buried concrete structures, and small voids; the magnetic and electromagnetic methods may be used to locate buried electrical lines, metal pipes and tanks; the electrical resistivity method may be used to map contaminant plumes and pathways and their relationship to the water table. HGI’s approach is to complete a site specific survey design that balances detection/mapping goals with site logistics and project costs in order to maximize results. Through this process, we analyze and optimize data collection/instrument parameters, survey resolution (distance between survey lines and between data points), and line or grid direction. We perform analyses of site safety parameters and site specific health and safety plans (HASPS) to ensure safe data collection practices. During- and post-survey, the acquired data is processed using state-of-the-art computer modeling software and, at survey completion, the multi-method data results are combined together to present a full visualization of the site.
Four Methods to Consider
Electromagnetic (EM) surveys 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 subsequent targeted excavations based on the results.
img class=” wp-image-3281″ src=”https://www.hgiworld.com/wp-content/uploads/2015/08/Brownfield-EM-Image-scaled.jpg” alt=”EM contour map of an abandoned brownfield.” width=”603″ height=”296″ /> EM contour map of an abandoned brownfield.
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 a 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 figure below demonstrates multiple 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 tank farm. Here the combined data delineate and reinforce the locations of subsurface infrastructure. The image in the following section (Ground Penetrating Radar) is of a single tank in the same tank farm where linear features (pipes) associated with a single tank have been found using the GPR method.
Magnetometry equipment is extremely field-portable and can be coupled with a Global Positioning System (GPS) device to simultaneously acquire magnetic data and GPS data, recording the precise location of each data point. A single person can operate the equipment and data can be acquired in just about any area a person can walk.
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 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 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 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 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.
A Complete Picture
HGI’s goal is to use our geophysical tools and technologies to optimize site assessment and pinpoint problems prior to intrusive site investigation. We do this 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.