Theme: 05. Fractured Rock and Complex Geology

Topic Area: 5d. Process-Based Conceptual Site Models (CSMs) for Informing Remediation

Formatted Title:

Breaking the Uncertainty Logjam: Leveraging CSMs to Make Remedial Progress in Bedrock Aquifers

Background/Objectives:

Over the last several decades, attitudes toward remediation of fractured rock aquifers have evolved from abject pessimism to a new pragmatism. Yes, fracture transport is complex, matrix diffusion poses real challenges, and injected substrates sometimes wind up where you don’t want them, but examples of real progress continue to mount. These successes have several common threads. First and most critical is a thoroughly considered conceptual site model (CSM) constructed from a robust empirical dataset of the site-specific aquifer characteristics that govern mass storage and transport. The CSM can focus remedial decision-making on what is possible and create a realistic framework for goal setting that balances both risk and cost. The new pragmatism embraces risk-based remedial targets (where permitted), focusing on achievable mass flux reductions and protection of receptors and not the technically impracticable restoration of all matrix pore water. Most successful bedrock remedies are adaptively designed and executed. Because no fracture system can ever be explicitly mapped, over-confidence in the predesign CSM can be fatal. The most successful remedies plan to pivot, using careful observations of performance data of the remedy to guide next steps. This presentation examines how these common elements influenced outcomes of over twenty bedrock remediation sites and provides a case study of in situ bioremediation (ISB) in bedrock to illustrate how a pragmatic approach can achieve remedial goals.

Approach/Activities:

Remedial performance from over 20 fractured rock sites will be reviewed to provide insights on remedial performance based on lithology (matrix/fracture porosity), remedial strategy employed, and overall remedial goals. The importance of characterization data, CSM development, and remedy design will then be described using an example from the southeastern United States. The site has a TCE plume in a shallow fractured limestone aquifer. Borehole and surface geophysics, continuous water-level monitoring, and vertical profiling were employed to characterize the bedrock and identify the primary flux pathways from the source area. Pathways were tested again during a pilot test combining injections of carbon substrate and fluorescent dyes. The tests provided the needed understanding of substrate injectability, fracture interconnectivity, and transport velocities to build out a highly targeted injection network and advance to full-scale remediation.

Results/Lessons Learned:

The success of remediation at the bedrock sites reviewed hinged on multiple factors. Some were immutable site characteristics, e.g., the magnitude and accessibility of the source, maturity of the plume, and characteristics of the rock. A second set reflected varying definitions of success: acceptance of risk-based closure or solutions that manage risk at reduced long-term costs. CSMs were essential elements in all cases; not by eliminating uncertainty but by establishing a sufficient understanding to allow remediation to start. At the example site, stakeholders were able to pivot to ISB from a failed pump and treat only through additional investigation to refine the CSM. While uncertainties remained, the team proceeded with a phased remedy implementation; at each step refining the CSM, adjusting the approach. Full-scale implementation ultimately required only five injection wells to reduce TCE source mass from 50 mg/L to below detection within less than 6 months.

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Keri Brill