Geology is the static framework that is the primary control on groundwater flow and contaminant migration, and this is why a sound geologic model is the foundation for remediation success.
The remediation process often begins by defining the site conditions based on the dynamic processes affecting contaminant behavior, such as groundwater elevation. This helps to interpret flow direction and contaminant chemistry data, which ultimately interprets contaminant plume extent.
In most applications of groundwater science and hydrogeology, simplifying assumptions are made when analyzing and modeling dynamic systems. Often, the media through which flow occurs are assumed to be homogeneous. Contacts between hydraulic units are assumed to be planar and horizontal. However, there are few geologic systems with such characteristics. Subsurface heterogeneity is more often the rule than the exception. To achieve successful remediation, the key characteristics of the subsurface must be identified and honored over the course of site analysis and remediation, and that means a representative geologic model.
PROGRESS — Progressive Remediation Strategies — is an integrated methodology that emphasizes a data-driven, process-based conceptual site model (CSM) to accurately understand and quantify complex contaminated sites. PROGRESS uses the site geologic model as the foundation for this approach and directly addresses the greatest uncertainty facing any remediation project — the subsurface itself.
Key Components of a Geologic Model
A geologic model is guided by geologic observations and principles pertaining to scale, observed styles of geologic origins, and indicators of environment and time. The essential components of the geologic model are:
- An understanding of the regional geology and how key elements of it relate to site position and scale.
- Site-specific data that describes the details of the geologic materials present.
- Geologically defensible correlation of those materials in 3D based on understanding of their genetic relationships and the relationship to the regional geologic sequence of events.
The geologic model implicitly informs the understanding of site hydraulics because the physical characteristics of geologic materials determine parameters such as hydraulic conductivity. The geometry of the geologic deposits defines the spatial distribution and variability of the hydraulic parameters. Often, expanding from a geologic to a hydrogeologic understanding of the subsurface requires an iterative process whereby correlations are tested by comparison with head data and other hydrologic characteristics. However, the subsurface plumbing (or geology) must be properly connected before water can run through it to understand the flow mechanics and dynamics (or hydrogeology/hydraulic characteristics).
Building a Geologic Model
The essence of the geologic model uses the knowledge of regional geology, genetic depositional relationships and stratigraphic interpretive methods to develop a geologic framework. This framework reduces uncertainty and improves remedy selection, design and performance. However, the geologic model is only successful if it meets project objectives — including cost and schedule. Therefore, input from the remediation team and stakeholders is critical to selecting the appropriate levels of model scale and accuracy.
Step 1: Research the regional geology with an intent to focus on components critical to the site-specific evaluation. The order and manner that geologic correlations are made must honor the geologic system and sequence of the depositional evolution of the subsurface. In sedimentary systems, this is conducted by identifying the depositional environment and associated facies models to define expected grainsize patterns in the subsurface data.
Step 2: Focus the analysis of the geologic materials. Datasets can include borehole log descriptions, geophysical logs and a variety of direct-push methods including cone penetrometer test logs, hydraulic profiling tool profiles, and electrical conductivity logs. Data must be spatially posted accurately with care taken to recognize the impact of vertical exaggeration on one’s ability to interpret scale and geometry, relative to what was learned in regional research.
Step 3: Develop geologic interpretations that are typically represented on geologic cross sections and maps to depict the three-dimensional geologic framework. In this step, interpretation is guided by geologic observations and principles, and the process of incorporating the genetic relationship of geologic materials into the correlation/interpretation is completed. Three-dimensional visualization tools can be of value with the caution that the geologic interpretation must be guided by the geologist and a not a computerized algorithm.
Environmental Sequence Stratigraphy (ESS) is a common method used to develop geologic models for complex contaminated sites. This methodology can be applied to clastic sedimentary geology, as presented in the U.S. EPA Technical Issue paper.
The geologic model is not a one-size-fits-all concept. One site may be situated in a river valley, making fluvial sediment geometry and the ability to recognize the size and scale of buried stream channel deposits critical. Another site may be situated on crystalline bedrock, making understanding fracture network geometry and relationships to geologic structure (faults and folds) key to establishing a framework that supports successful remediation.
No matter the site, the geologic model should provide the static geologic framework that supports the development of the CSM and continuously evolve as new subsurface data is collected to further inform and refine the model.
This post is one of a series explaining Progressive Remediation Strategies (PROGRESS). Through PROGRESS, enhanced modeling and predictive analytical tools are used to optimize technology selection and design. But this is just one component of PROGRESS and its comprehensive, next-generation approach to remediating complex sites.
