Environmental Sequence Stratigraphy (ESS) helps contextualize site data within a stratigraphic framework to confirm observations, highlight anomalous features, and help identify and close hydrogeologic data gaps. EA held a webinar to explore the underlying principles of ESS and its growing role in the development of conceptual site models (CSMs) for contaminated sites. An overview of the principles of ESS, how it differs from traditional approaches, and areas of application were presented, as well as case studies with a series of depositional environments and areas for innovation. The webinar video can be viewed below.
Summary of Environmental Sequence Stratigraphy Webinar
Dan Walker, PhD, AM, ASCE, EA Senior Geologist and Practice Lead for ESS, led the webinar and introduced ESS as a significant shift in environmental site characterization, emphasizing its potential to enhance the understanding of contaminant fate and transport for per- and polyfluoroalkyl substances (PFAS) and other conservative contaminants that form large, dilute solute plumes such as chlorinated solvents and perchlorate. ESS is not merely a rebranding of existing techniques but represents a fundamental change in approach that can support development of robust CMSs concerning groundwater flow and contaminant transport. It helps in understanding the delineation and distribution of contaminants, plume migrations, and plume geometry, making remediation efforts more effective, and has broad applicability, particularly in understanding the fate and transport of contaminants like PFAS.
Theory and Practice
The concept of ESS, rooted in sequence stratigraphy developed for the oil and gas industry, focuses on understanding sedimentary sequences and their environmental implications. ESS aids in developing robust, testable CSMs, focusing on repurposing techniques in the oil and gas sector for environmental contaminant work.
Walker explained the critical role of hydraulic conductivity in understanding subsurface plume geometry, particularly in unconsolidated sediments. Grain size influences porosity and permeability, which are essential for understanding the flow of contaminated groundwater in the subsurface. Coarse-grained sediment (e.g., sand and gravel) have larger pore spaces, and greater interconnection between these spaces, leading to zones of higher permeability. It is critical to understand the spatial geometry of the coarse-grained units, primarily sand and gravel, to predict the morphology of a subsurface plume. These units are key to delineating and distributing contaminants effectively.
The goal of ESS is to place a site into a broader geologic context and build a robust CSM. This involves understanding the depositional history of sediments and the processes controlling their facies distribution. A wide range of information needs to be incorporated, including historical data and publicly available data provided by entities like state geological surveys or water resource agencies to create a testable model that can be updated as new data becomes available. Relying solely on recent contractor reports may not provide a comprehensive understanding of the site.
Application in Environmental Contaminant Work
Walker highlighted the application of ESS in delineating and understanding the distribution of contaminants within environmental sites. A significant portion of contaminant mass is often found within a small percentage of subsurface water-bearing units, emphasizing the importance of targeting these highest flow zones for effective remediation. Understanding the depositional environment and sedimentary sequences can inform the placement of monitoring wells and the design of remediation strategies.
EA is executing innovative approaches to improve ESS applications, including robust desktop analyses and enhanced data collection techniques. From a desktop perspective, time spent reviewing literature and existing data before fieldwork helps build a strong initial CSM. High-resolution photographs of borings and cores provide a permanent record, allowing for better quality assurance/quality control and more accurate geological descriptions.
Benefits of ESS
ESS has the potential to improve site characterization and remediation design and save time and money by providing a more accurate understanding of subsurface conditions, leading to greater effectiveness of remediation outcomes.
For More Information:
Dan Walker, PhD, AM, ASCE
EA Senior Geologist and Practice Lead for Environmental Sequence Stratigraphy
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