ENHANCING SURFACE IMPOUNDMENT CLOSURES TO SUPPORT GROUNDWATER COMPLIANCE

The Disposal of Coal Combustion Residuals (CCR) from Electric Utilities final rule promulgated by the U.S. Environmental Protection Agency requires surface impoundments and landfills at the coal-fired power plants it regulates to undergo groundwater monitoring.

When groundwater monitoring results show any constituents of potential concern listed in the CCR rule at statistically significant levels that exceed groundwater protection standards (GWPS), the unit’s owner or operator is obligated to take action. Unless the excess GWPS levels can be attributed to an alternate source or sampling, analysis or evaluation error, nearby property owners and the general public must be notified and the nature and extent of contaminants that exceed GWPS must be characterized. A corrective measures assessment and groundwater remedy selection must also be completed. These can come at potentially high legal risk and financial costs.

Groundwater compliance requirements should be considered early in the planning and design process, typically during a feasibility study or impoundment closure alternative evaluation. From alternative cover systems and grading designs to waste consolidation and in situ treatment, the options that emerge can be screened for feasibility, cost, effectiveness and overall value. In some cases, the potential cost and risk of groundwater contamination and associated corrective actions justify substantive modifications or enhancements to impoundment closure methods.

While early consideration of groundwater impacts can reduce future risks and cost, the converse is also true. If groundwater issues are not considered during planning and design, project costs and risks can grow — particularly if corrective actions are required after closure is complete. Coal ash removal from a saturated zone, groundwater extraction, in situ water treatment and other corrective actions may, for example, require the costly removal or penetration of cover systems and double-handling of waste.

Addressing groundwater issues from the beginning, in other words, can be far less costly than retrofitting a closure or implementing active groundwater remedies in the future.

IMPOUNDMENT CLOSURE ENHANCEMENT EVALUATIONS

Impoundment closure enhancement evaluations can be of great value to CCR unit owners and operators wishing to mitigate the potential costs and risks associated with GWPS exceedances and other groundwater compliance issues. These evaluations provide a way to assess the short- and long-term effects of impoundment closure activities on groundwater quality.

While the objectives vary from project to project, these evaluations are typically designed to:

• Limit or eliminate direct CCR contact with the uppermost aquifer, as defined in the CCR rule.
• Minimize the potential need for post-closure groundwater corrective action.
• Promote a decrease in CCR constituent concentrations in groundwater.
• Preclude or eliminate pathways for potential contaminant transportation and receptor exposure.

Impoundment closure enhancement evaluations typically involve the following steps.

1. DATA REVIEW

The process begins with a comprehensive review of currently available site data. This can include reviews of data related:

• Groundwater quality
• Site hydrology, groundwater analytical and geochemical parameter data
• Hydrogeological data such as groundwater elevation measurements, hydraulic conductivity estimates and potentiometric surface depictions for multiple aquifer units over multiple seasons and/or years
• Subsurface lithology
• Stream/surface water stage data
• Water supply data
• Geotechnical data and reports
• General site conditions
• Impoundment construction drawings
• Preliminary closure plans
• Historical operational data and future operational plans

2. DEVELOPMENT OR REFINEMENT OF THE CONCEPTUAL SITE MODEL

Data and literature review results are used to develop a conceptual site model (CSM) for the site. A CSM reflects all the factors that impact groundwater movement, as well as the nature, extent and transport of potential contaminants.

Consisting of site maps, cross sections, three-dimensional models and other data presentations, a CSM assimilates information concerning site-specific subsurface conditions and the nature, extent, fate and transport concepts of potential contaminants, along with potential receptors and exposure pathways.

Most CCR sites are located in sedimentary depositional environments, which are often characterized by relatively young and dynamic alluvial systems. An Environmental Sequence Stratigraphy (ESS) analysis can often add considerable value to the CSM by identifying hydrostratigraphic units (HSUs) that constitute preferential groundwater flow paths.

Because HSUs can significantly impact the fate and transport of contaminants, they can be critical in maximizing the performance and cost-effectiveness of impoundment closure enhancement and groundwater corrective measures.

A site’s CSM and groundwater compliance objectives can be further refined by identifying potential receptors and evaluating potential exposure pathways, including any potential groundwater-to-surface water interaction.

3. IDENTIFICATION OF POTENTIALLY VIABLE ENHANCEMENT MEASURES

Given its expansive content, the CSM provides an ideal basis for identifying potentially viable impoundment closure enhancement measures.

The identification and development of enhancement measures typically requires a multidisciplinary team of solid waste engineers, groundwater characterization and remediation professionals, regulatory specialists and professionals from other disciplines, such as geotechnical engineers, risk assessors and permitting specialists. This team identifies and conceptualizes the initial closure enhancement options so they can be screened and evaluated for feasibility and compatibility with site conditions and project objectives (see step 4). This process typically results in a shortlist of a dozen or more closure enhancement alternatives.

4. SCREENING EVALUATION PROCESS

Enhancement measures are site-specific but may include alternative cover systems, alternative grading designs, waste consolidation, in situ solidification/stabilization, in situ treatment, flood protection modifications and hydraulic containment, with or without pretreatment and/ or beneficial reuse options (see Figure 1 and Figure 2).

After identifying enhancement measures, the project team conducts an initial screening process, reviewing alternatives on the basis of relative cost, constructability, effectiveness, certainty of performance and other site- or owner-specific criteria. The screening typically results in three to seven or more viable closure enhancement alternatives that, after securing of owner and stakeholder agreement, are carried forward for detailed evaluation.

Design, construction, operation and maintenance, monitoring, compliance and reporting requirements over the 30-year post-closure care period are also developed for each option, as defined in the CCR rule.

Impoundment closure enhancements considered during detailed evaluation process include:

• Cover system enhancements have been constructed to reduce subsurface infiltration under close-in-place scenarios.
• In situ treatments have been implemented using permeable reactive barriers or other delivery techniques.
• Monitored natural attenuation includes closure features and enhancements that support this groundwater management approach.
• Relief wells and levee toe drains used for flood protection have been modified to minimize or eliminate the discharge of potentially impacted groundwater-to-surface water bodies.
• Slurry walls and groundwater extraction have been employed to provide hydraulic containment. Groundwater extraction strategies may also include pretreatment and beneficial reuse options, or an evaluation of hydraulic capture via existing facility production wells.
• Surface grading and drainage modifications have been used to increase stormwater conveyance efficiency and reduce subsurface infiltration.
• Waste consolidation and in situ solidification/stabilization have been implemented to remove or stabilize CCR below or near the upper limit of the uppermost aquifer, as defined in the CCR rule.

In addition to evaluating these alternatives using typical feasibility study criteria, detailed evaluations compare the cost/benefit of implementing these measures during impoundment closure, with that of implementing them later when groundwater corrective action may be mandated by regulation. Delaying implementation could also result in the need to penetrate, retrofit and/or remove the surface cover for investigation, remedial actions or cap improvement, resulting in the rehandling of CCR and fill material.

The CSM serves as the basis for these detailed closure enhancement evaluations and can also be used for data gap analysis and future design and planning tasks. A data gap analysis can identify information needs and the relative cost/benefit of obtaining the data. A data gap analysis effort, for example, could recommend subsurface investigations for the collection of physical, chemical and/or geochemical data, risk assessment studies and/or treatability studies. Hydrologic and groundwater modeling tools may also be used to help improve understanding of potential contamination fate and transport concepts, as well as to predict the performance of closure enhancement measures over time.

Newly acquired data can then be assimilated into the CSM and used as the basis for impoundment closure enhancement selection and design, as well as any required future groundwater monitoring and corrective action.

Life cycle costs are also estimated for each alternative that undergoes detailed evaluation. Cost estimates should be comprehensive and include anticipated engineering design, procurement and construction, project and construction management, engineering during construction, O&M, monitoring and reporting requirements, as well as costs related to general conditions, escalation, contingency and other indirect costs.

Following a detailed evaluation, it is often instructive to conduct a comparative analysis of alternatives (see Figure 3).

CONCLUSION

To identify and mitigate future corrective action risks and costs associated with the federal CCR rule, groundwater quality and compliance issues should be considered during impoundment closure planning and design.

Studies that evaluate closure options and assess the long-term financial liabilities associated with potential groundwater impacts play an important role in the planning process. By assessing risk, identifying corrective measure concepts and forecasting potential costs, these studies provide a framework for evaluating future groundwater monitoring data and the potential need for corrective measures.

In addition, closure enhancement studies allow the project team to reconsider potential groundwater impacts and associated risks before proceeding to detailed design and implementation. Enhancement evaluations at sites where groundwater protection standards are exceeded or are presumed to occur can also be helpful in developing a closure approach. In some cases, closure design features or enhancements that improve groundwater quality and mitigate risk can be incorporated at little or no cost.