The subsurface cannot be seen or understood perfectly in three dimensions. Direct information on subsurface conditions is limited to the information gathered from drilling wells. The problem often lies in predicting the conditions between wells and creating the conceptual stratigraphic framework, like predicting what a 1,000-piece jigsaw puzzle should look like with just a few pieces in hand. Yet, a prediction must be made to design a remedy. The accuracy of this prediction can determine the success or failure of any subsurface-related project. Historically, the petroleum and groundwater remediation industries have had very different approaches to creating these subsurface conceptualizations.
Aquifers — which host groundwater resources — and oil reservoirs — which host hydrocarbon resources — are primarily composed of sediments such as sand, gravel, silt and clay. Water, oil or gas flows within the pore spaces between the individual sediment grains. In the case of groundwater contamination, as clean groundwater flows through highly contaminated source zones, contamination is dissolved and carried away from the source, forming contaminant plumes. However, within source zones and groundwater plumes, contamination can exist in multiple phases (aqueous, nonaqueous liquid, solid or sorbed, and gaseous) that interact with geologic materials and each other.
Plumes require remediation to prevent or mitigate their migration to drinking water wells, springs, streams and other natural resources. Volatile groundwater contaminants can also threaten air quality within structures and dwellings located above the plumes. Groundwater remediation systems attempt to control, and ultimately remediate, contaminant plumes to keep them from expanding and endangering humans and the environment. The prevention of further migration was historically achieved through pump-and-treat methods, in which the plume was hydraulically controlled by operating pumping wells that conveyed contaminated groundwater to a central location for treatment. More recently, chemical and biological treatment methods have been implemented in situ, within the aquifer, to destroy or immobilize contaminants in place. Biological methods typically involve the injection of a food source to stimulate naturally occurring microorganisms to metabolize harmful compounds into less toxic daughter products, while chemical methods require a reagent to destroy or immobilize contaminants on contact.
Groundwater remediation project designs have historically suffered from the same flawed assumption: The subsurface layers are continuous. In reality, the subsurface is composed of highly permeable zones or channels, responsible for most of the fluid movement. These channels are encased in less permeable silt and clay deposits, through which water moves much more slowly.
Over time, contaminants in groundwater diffuse into these less permeable silt and clay deposits, creating secondary contaminant sources throughout the plume. As time goes on, contaminant concentrations within the permeable zones become less than those trapped within the low permeability materials. This causes contaminants within these secondary source zones to back-diffuse in a slow-drip fashion into the more permeable channels, providing an ongoing source of groundwater contamination for generations to come.
By contrast, instead of assuming a layer cake subsurface, the oil and gas industry begins with the base assumption that the subsurface is highly heterogeneous and anisotropic. The term for this is “permeability heterogeneity,” and it is a feared and respected factor in terms of the economic viability of oil and gas development projects. It is well known that this complexity has the potential to make or break billion-dollar developments, especially in hard-to-reach areas such as the deep ocean. To address this critical risk element, the petroleum industry has invested billions of dollars in corporate research labs and universities worldwide to develop conceptual tools such as facies models and sequence stratigraphy. These tools have proved extremely useful over several decades and have stood the test of time and financial performance, representing the current best practice for subsurface characterization.
Applying these principles and employing professionals from the petroleum industry within the groundwater remediation space is a game changer for the groundwater remediation industry. Contaminated sites where true progress toward site closeout has not been possible due to complexity and uncertainty can instead be set on a different path. For such sites, alternative management strategies may instead be developed to strike a balance between the practical realities of the subsurface and the need to protect human health and the environment. This approach will instead help responsible parties and remediation professionals avoid the selection, implementation and maintenance of expensive, ineffective remedies that are incompatible with site geologic conditions.