Emissions

Closing Coal Combustion Residual Ponds

Coal combustion residual (CCRs) have historically been managed either in ponds (wet storage) or landfills (dry storage).

By Ken Ladwig and Bruce Hensel, Electric Power Research Institute

Aerial view of a coal combustion residual (CCR) site. Photo Courtesy: EPRI

Coal combustion residual (CCRs) have historically been managed either in ponds (wet storage) or landfills (dry storage). Management in ponds is particularly prevalent in the eastern U.S., where ponds ranging in size from ten aces to several hundred acres have been in operation for decades. In 2015 the US Environmental Protection Agency (EPA) promulgated regulations that established requirements for disposal of CCR.

While the CCR Rule does not explicitly prohibit CCR management in ponds, the strict requirements for ponds, along with the subsequent passage of the Effluent Limitation Guidelines for Steam Electric Plants and the retirement of many older coal-fired power plants, combined to produce essentially the same effect, greatly accelerating pond closures.

EPA did not have permitting and enforcement authority under the CCR Rule, but instead relied on citizen lawsuits. In 2016, federal legislation was passed that provided states authority for implementation of the rule. This change clarified actions for most power companies, which were facing dual and sometimes conflicting regulatory requirements at the state and federal level. However, this is not expected to change the ultimate transition from wet to dry management of CCRs, and the attendant closure of many CCR ponds over the next 5 to 15 years. The remainder of this article discusses closure implementation under the federal CCR Rule (Part 257.100 – 257.104) for ponds containing CCRs derived from bituminous coal-fired plants. State implementation requirements must be as least as protective as the federal criteria.

Assessing the Options

The CCR Rule provides two basic closure options: closure in place (CIP) and closure by removal (CBR). CIP is typically simpler and less costly than CBR, and has been the alternative of choice in the past. CBR might be considered when consolidating CCR from smaller ponds into one location, or if leaving the pond in place would have a significant long-term impact on groundwater and surface water.

With CIP, a cap composed of compacted clay and often a geomembrane, is placed over the former impoundment to limit the amount of infiltration from precipitation or snow melt into the CCR. In some cases, the footprint of the facility may be consolidated prior to capping to 1) remove CCR from undesirable areas, for example immediately adjacent to water bodies, 2) provide material to build up appropriate slope to assure runoff from the cap, and/or 3) reduce the overall CCR management footprint.

With CBR, all of the CCR is excavated and taken to a lined landfill for disposal or to a beneficial use location. The landfill may be on site or off site; off-site facilities may be owned by the power company or by a third party. CBR to on-site landfills will typically have lower cost and lower impact to the surrounding community than CBR to off-site landfills, but on-site landfills are only possible if there is suitable space on the property.

While the cost of CIP is significantly less than CBR in most cases, choosing the most appropriate option for a site also depends on other factors, including but not limited to:

  • Environmental Impacts/Benefits
  • Community Impacts
  • Time to Completion
  • Sustainability Metrics

Integrating these factors with economic considerations and overall company management strategy can be a complex task.

A Decision Framework to Evaluate Closure Options

Given competing advantages and disadvantages between CIP and CBR, use of a scientific, quantifiable decision framework that considers beneficial and adverse impacts over multiple environmental and community pathways can aid evaluation of the two alternatives. Environmental pathways where the relative impact of CIP and CBR on chemical concentrations can be predicted and quantified include groundwater, surface water, and air. Quantifiable community impacts include inhalation of particulate matter generated during construction and material transport, construction worker safety, community safety as a result of increased truck traffic to and from the site, and green & sustainable remediation (GSR) measures such as energy use, greenhouse gas emissions, and raw material consumption.

Because of the multifaceted nature of these variables, a decision framework cannot provide an absolute determination on the best approach, rather, it provides a relative comparison of each variable to support an overall decision. Figure 1 provides an example of such a tool to evaluate the impacts (beneficial or adverse) of the two options, and the relative difference in impact between the closure options for each pathway.

Closure Options – 1

Example decision tool summary comparing relative impacts of closure in place (CIP) and closure by removal (CBR) for a hypothetical site.

Results of applying this framework will vary depending on site-specific variables, such as site size, hydrogeology, landfill availability, and proximity of the community. However, some generalizations can be drawn based on experience derived from using the decision tool:

  • Groundwater and Surface Water Quality: If groundwater or surface water quality was impacted by a facility prior to closure, CIP and CBR will have beneficial impacts; they are both effective if the base of CCR is above the water table and there are no lateral flows into the facility (non-intersecting groundwater table). CIP is generally less effective than CBR if there is a continuing source of groundwater flow through the CCR after capping (intersecting groundwater table). Because surface water impacts at closed facilities are derived from groundwater, the same relationship holds
  • Air Quality, GSR, Worker Safety, and Community Safety: CBR is generally expected to have greater adverse impacts on these pathways than CIP because the CBR construction project is larger scale for longer duration, generates more truck traffic, and uses more energy than CIP.

The Tennessee Valley Authority (TVA) used this decision tool to support its environmental impact statements for closure of ten CCR surface impoundments.

Groundwater Monitoring under the CCR Rule – 2

The evaluations indicated that CBR would have greater adverse impact than CIP on air, safety, and GSR measures for nine of the ten impoundments (one impoundment could not be evaluated), while the relative beneficial impacts of CIP and CBR on groundwater and surface water quality were site-specific and variable. This comprehensive evaluation was factored into TVA’s overall determination that CIP was the preferred closure option for the ten impoundments.

The rest of this article focuses on methods for closure in place.

Closing in Place

The CCR Rule contains general requirements for closure and cap design. However, closing CCR ponds is more complex than typical landfills. Ponds can vary significantly in many aspects and each closure requires careful attention to site-specific attributes that can impact the final design, such as: type of pond (e.g., valley fill vs. raised berm), pond size and percent filled, geotechnical characteristics of CCRs and surrounding containment structures, water management, post-closure stability and land use, proximity to surrounding communities, existing groundwater impacts, and equipment and material availability.

The final closure design and completion schedule can vary considerably according to the complexities introduced by these factors; closure of some large ponds could take as long as 5 to 15 years.

Site Investigation and Engineering Design

Some decades old ponds may not have complete information regarding pond dimensions, geotechnical characteristics of the ponded CCRs and berm materials, and surrounding hydrogeology. This information is needed to ensure pond stability during and after closure, as well as to identify and mitigate any actual or potential environmental releases.

Typical geotechnical investigations inside the pond include obtaining data on CCR stratigraphy, grain size, Atterberg limits, undisturbed samples for consolidation and strength testing, porewater pressures, hydraulic conductivity, and in situ testing using cone penetrometer equipment and piezometers. Hydrogeologic information includes stratigraphy, groundwater levels, groundwater quality, and aquifer characteristics. If groundwater quality impacts are documented, corrective actions can be built into the closure design.

Adaptation of Potential Failure Mode Analysis (PFMA) has been used to aid in the identification of potential problem areas that need to be addressed prior to closure. The PFMA process was developed by the Federal Energy Regulatory Commission for hydropower dams as a risk reduction tool. PFMA is not required for CCR ponds but can be used to facilitate a more focused site investigation and risk mitigation program.

Pond Dewatering and Surface Stabilization

The CCR Rule requires removal of free water by drainage or solidification, and stabilization sufficient to support the cap. Free standing water is typically removed via gravity drainage (decanting) or pumping, at rates that avoid excessive entrainment of suspended solids, and do not cause instability of the CCR, which can threaten the structural integrity of interior dikes or the perimeter containment system, or cause overtopping due to displacement by the CCR. Discharge of any free water or porewater collected during dewatering may be subject to National Pollutant Discharge Elimination System regulation under the Clean Water Act.

Following removal of free water, the CCRs are typically at or near saturation and have low shear strength. Varying methods of porewater removal and surface stabilization are generally employed to improve the stability of the surficial deposits and their ability to safely support construction equipment. Porewater removal methods include shallow well point systems, enhanced gravity drainage using a network of open trenches or drain tiles, or filtration dewatering with geotextile tubes. Stabilization is usually accomplished by incorporating a bridging or blending layer. A bridging layer (e.g., soil, bottom ash, or high strength geotextile/geogrid) distributes the construction load over a larger area of the soft CCRs, thereby increasing the factor of safety (FS) for bearing capacity. A blending layer (e.g., soils, fly ash, quicklime) serves to lower the CCR moisture content, thereby increasing bearing capacity.

Final Cover System and Post-Closure Care

Operating ponds have a relatively flat surface; consequently they will require some reshaping to achieve suitable grades to shed runoff during the post closure period. Many closure designs use CCR material to achieve final grades, either by redistributing CCR already in the pond, often reducing the pond footprint, or by consolidating CCR from other smaller ponds near the site. If sufficient CCRs are not available, soil is often used to achieve grades. For large sites in particular this can be a significant construction activity, requiring excavating, trucking, and placement of more than a million cubic yards of fill material.

The CCR rule includes the following requirements for the final cover system, intended to minimize infiltration and erosion:

  • Permeability less than or equal to the bottom liner or natural subsoils, and in no case greater than 1×10-5 cm/s;
  • Minimum 18 inch soil infiltration (barrier) layer;
  • Minimum 6 inch soil erosion layer capable of sustaining vegetation;
  • Accommodation for settling and subsidence.

The rule does not specify the components of the barrier layer, but the rule preamble suggests that composite barrier layers (compacted clay and a geomembrane) have shown better long-term performance than compacted clay alone. Alternative covers that meet the aforementioned requirements can also be proposed.

The CCR Rule contains requirements for a 30-year post-closure care, including maintenance of the cover and groundwater monitoring. The post-closure care period can extend beyond 30 years if the facility is in assessment monitoring for groundwater impacts (see below).

Emergency Action Plan

The CCR Rule requires operating ponds with high or significant hazard potential ratings to have an Emergency Action Plan (EAP). The hazard potential rating system was developed by the Federal Emergency Management Agency for dam safety, and pertains to the potential adverse consequences of a dam failure, not the probability of a failure occurring. A significant hazard potential indicates potential for economic loss and environmental damage, but no probable loss of human life. A high hazard potential indicates potential for probable loss of life in the event of a failure.

EAPs must contain, among other things, inundation maps that show estimated extent of release from a CCR pond in the event of a failure, to assess the likely downstream impacts. EAPs for conventional dams assume water is impounded behind the dam for estimating the extent of inundation and potentially impacted facilities. Ash ponds impound a mixture of water and saturated fly ash during active operation, which could result in rapid movement of free water and liquefied ash during a release. However, some states also require EAPs for closed CCR ponds, which do not impound free water. EPRI is working with the Army Corp of Engineers Engineer Research and Development Center (ERDC) to develop physical and numerical models to better simulate the dynamics of ash flows from closed ponds in the event of a failure, which will enable improved predictions of the potential extent of inundation as a function of ash saturation.

Groundwater Monitoring

Depending on if and how states implement the CCR Rule, ponds may be required to monitor groundwater under federal and state rules. Since the monitoring requirements of the CCR Rule do not necessarily align with state groundwater monitoring requirements, some facilities may have multiple groundwater monitoring programs using different monitoring wells, different constituent lists, and different sample frequencies.

Establishment of a groundwater monitoring program is site-specific, based on: 1) whether monitoring is performed for the CCR Rule, a state program, or both; 2) the configuration of the site, including proximity to surface water; 3) the proximity of other potential sources of contamination; and 4) site hydrogeology.

All groundwater monitoring programs lead to one of two end points: 1) no release to groundwater is detected in which case groundwater monitoring continues for a specific period, which is usually specified in the governing regulations; or 2) a release to groundwater has been detected, which can trigger additional monitoring and can lead to establishment of a remediation or corrective action program. Figure 2 illustrates the life-cycle for groundwater monitoring under the CCR Rule which has three distinct phases of groundwater monitoring:

  • Detection monitoring is the first stage of groundwater monitoring, based on seven indicator constituents sampled in monitoring wells as close to the waste boundary as feasible. If a statistically significant increase (SSI) in concentration relative to background is observed at any one monitoring well for any single constituent, and the SSI cannot be attributed to a source other than the CCR facility, then assessment monitoring is triggered.
  • Assessment monitoring expands on the list of constituents, using the same monitoring well system as for detection monitoring. Concentrations for assessment monitoring constituents are compared to a groundwater protection standard (GWPS), which is either the federal maximum contamination level (MCL) or background concentration if background is higher than the MCL or if a constituent does not have a MCL. If any assessment monitoring constituent at any monitoring well has a concentration at a statistically significant level (SSL) relative to the GWPS, and the SSL cannot be attributed to a source other than the CCR unit, then corrective action is triggered. If there are no SSLs for the assessment monitoring constituents, then the facility remains in assessment monitoring until concentrations return to background levels for two consecutive sample events, at which time detection monitoring continues (unless the time is greater than 30 years post closure).
  • Corrective action monitoring is performed for constituents with concentrations determined to have SSLs, using monitoring wells installed within the area of corrective action, which are not necessarily the same as the detection/assessment monitoring well system. Assessment monitoring continues during corrective action monitoring. Corrective action continues until concentrations in the corrective action monitoring wells decrease to levels lower than the GWPS for three consecutive years.

Groundwater monitoring continues for the 30-year post-closure care period, assuming that the facility is in the detection monitoring phase. Groundwater monitoring continues indefinitely if the facility is in assessment monitoring.

Summary

CCR pond closures are expected to continue and accelerate over the next decade in response to the recent promulgation of the CCR and ELG rules, along with the decommissioning of many older coal-fired power plants. Pond closures can be quite complex due to the variability among ponds and the unique geotechnical characteristics of wet, fine-grained CCRs. Community and worker safety are of paramount importance in the closure design and construction activities. Detailed knowledge of site-specific engineering and environmental characteristics of the ponded material, containment structures, and hydrogeologic conditions is critical to ensuring a safe closure and long-term performance of the facility.

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