Hydrogeologic Evaluations of Fractured Bedrock Systems

Fractured bedrock is a common component of hydrogeologic systems and can pose unique environmental and engineering challenges that are less common in more homogeneous porous media systems. Fractures and other forms of secondary porosity (e.g., dissolution cavities) typically control the hydraulic properties of the rock and are the primary means of fluid flow and mass transport in bedrock. Understanding the interconnectivity of the fractures and the connection between the fractures and overlying geologic units is essential for development of robust conceptual models of these groundwater systems. Those conceptual models help stakeholders select effective remediation strategies for adversely affected sites with fractured bedrock.

Numerous methods and techniques are available to improve the characterization of fractured bedrock. These methods can be used to estimate hydraulic parameters, map the connectivity of fracture systems, and obtain information about the overall conditions of a groundwater system. Techniques range from low-effort, low-cost methods, such as hydrograph analyses, to more involved options, such as large-scale pumping tests. These techniques can often be conducted in tandem to provide more robust evaluations of fractured bedrock systems. Applying these methods and techniques while also understanding their limitations and potential difficulties can improve the evaluation of fractured bedrock systems and save stakeholders time and money when developing effective remedial strategies.

Lessons Learned from Groundwater Modeling to Support Engineered Solutions for Post-Excavation CCR Basin Water Management

For CCR basins constructed in steam valleys in the southeastern United States and planned for closure by excavation, post-closure groundwater control is often required. Numerical simulations suggest that groundwater elevations may intersect planned final grades, creating uncertainties in future groundwater-surface water interactions and requiring proactive engineered solutions supported by a solid understanding of complex geology and hydrology.

This presentation reviews lessons learned from recent projects in the Carolina Piedmont, emphasizing targeted field data collection, conceptual site model (CSM) development, and iterative numerical modeling to guide closure design. High-quality water level and flow data are critical for defensible hydrologic analysis. Case studies show how heterogeneous and transient site conditions, such as shallow bedrock, uncertain historical channel geometry, evolving basin conditions, and water management, affect groundwater responses, reinforcing the need for a well-defined CSM to support reliable model predictions.

Water budget analysis links field data, modeling, and design by quantifying surface and groundwater contributions, evaluating storm and recharge variability, and identifying controlling factors in model calibration. Catchment basin delineation is critical, as the magnitude of inflow directly affects drainage sizing and long-term performance. Integrating those insights into groundwater models improves accuracy when evaluating closure elements such as groundwater collection systems, stormwater channels, sumps, soil backfill quantities, and performance goals.

By comparing results across multiple projects, this presentation highlights recurring challenges, strategies to reduce uncertainties, and practical considerations in developing effective, regulator-ready engineered designs. These lessons can help streamline future monitoring, design, permitting, and performance evaluation for CCR basin closure across the region.

A Cost-Effective, Low-Impact Remedial Approach for Metals: Utilizing Phyto-Enhancement to Accelerate pH Adjustment of Shallow Groundwater

Thomas Colton, P.E., and Jerry Wylie, P.G., will present a unique remediation strategy that accelerates natural attenuation processes. The approach employs pH adjustment of groundwater using agricultural lime coupled with the use of daikon radish to augment lime contact time and to increase depth of soil augmentation. This method is designed to be a more sustainable, less resource-intensive, and less technically challenging remedial alternative with lower estimated overall costs.

The primary corrective action—source removal—has been completed. However, the oxidation of residual sulfide-bearing minerals has resulted in persistent low-pH groundwater, which increases the solubility and mobility of metal constituents of interests (COIs). While long-term natural attenuation is expected, low-pH conditions could persist for decades; therefore, an evaluation of remedial options was completed to accelerate COI attenuation mechanisms.

The selected remedy is the application of agricultural lime (CaCO₃) to the shallow subsurface to increase pH and buffering capacity in groundwater and unsaturated soil, thereby decreasing COI mobility and concentrations. Application dosage rates and details were developed following a baseline evaluation and analysis of subsurface soil conditions.

Although infiltrating rainwater will naturally carry the higher-pH water deeper into the water-bearing unit, SynTerra developed a simple, low-cost enhancement using daikon radish root channels (phyto-enhancement) to accelerate lime movement into the groundwater. This enhancement is expected to expedite the buffering capacity of the water-bearing unit and the time needed to decrease COI mobility. A robust monitoring well program is already established to track performance. The final assessment design, site characterization results, and lime application calculations will be presented, along with initial observations from the novel pilot study.