When faced with a variety of options for coal ash pond closure, use a holistic approach rather than choosing what seems to be the quickest or cheapest on paper.
Closure and remediation solutions for coal ash ponds, or impoundments, vary greatly. Assuming the U.S. Environmental Protection Agency (EPA) promulgates the expected proposed Subtitle D regulations, technical options are plentiful, and the vast majority will be viable for most sites. Among the choices are capping, dewatering and/or stabilizing, consolidating into a new landfill, disposing off site, converting to wetlands, or any combination of these options.
There is no one-size-fits-all option, so it’s important to familiarize yourself with the advantages and disadvantages of each. This is especially important if the planning design and construction activities occur at an open and active facility. A successful closure approach accounts for the size of the pond, dewatering requirements and methods, final site use, integration of any remediation efforts, constructability, site layout, and long-term costs.
Pond Size Matters
To better accommodate materials handling, staging, and water management, divide larger ponds into working subsections. A large pond that is closed in place can be incrementally covered so that the closed areas shed precipitation as storm water rather than as contact water that must be collected and treated. Because storm water discharge is normally handled through gravity flow, this can substantially reduce water management costs over a lengthy construction period.
Consolidation of pond solids into a reduced footprint reduces cost. Because the surface area is smaller, the cost for constructing the permanent final cover will be reduced. Similarly, the cost for long-term maintenance of the cover system proportionally drops (Figure 1).
|1. In-place closure. This schematic cross-section shows the three stages of in-place consolidation and closure for ash pond solids. Courtesy: TRC Companies Inc.|
Dewatering Requirements and Methods
Water removal from an ash pond is critical to constructability, project economics, and long-term performance. Without proper dewatering of saturated ash materials before removal, excavations risk caving and cut slope collapse, creating safety hazards and potentially costly repairs.
During closure, dewatering should be conducted to limit disturbance and re-suspension of a pond’s solids, especially when construction activities take place in the immediate vicinity of the discharge structure. In some cases this may require the use of a temporary settlement structure prior to discharge.
Dewatering is also important for controlling closure costs. The cost per ton of solids for hauling ash by truck for off-site disposal is substantially higher if the ash moisture content is high, which usually translates into higher disposal costs, ranging from hundreds of thousands to millions of dollars, depending on the size of the project. Moisture content of the ash solids will likewise have a financial impact on hauling to an on-site disposal cell.
Because dewatering factors into the success of a pond closure, planners need to allot sufficient time for solids to drain. Sumps, well points, and dewatering ditches have a limited radius of influence, so as ash is drained and stabilized, dewatering must progressively advance. Drainage properties of the ash may dictate that dewatering be initiated weeks, if not months, in advance of inaugural activities. Seasonal surface drying and precipitation also affect dewatering (Figure 2).
|2. Schedule time for dewatering. Ash pond dewatering can take weeks or months. Here the task is handled using portable pumps. Courtesy: Barry Culp/TRC|
Final Pond Site Use
The end use of the pond site factors into the choice of closure options. If the overall property will be retained for future power generation or similar use, options for site use include:
■ Converting the former pond into a lined landfill.
■ Developing a wetland treatment system for leachate or process waters.
■ Reclaiming the area for plant expansion, parking, or a material laydown yard.
■ Converting a portion of the pond for use in ongoing or future wastewater management.
■ Constructing a solar panel farm.
■ Converting the site for recreational use.
Each option has specific development requirements and constraints. For example, if plans include building a structure that is sensitive to foundation settlement, the closure approach for the pond should include either removal of some or all ash materials or its compaction to the standard of an earthen structural fill.
In other instances, it’s acceptable to leave partially stabilized ash in place and bridge compressible materials with geosynthetic and compacted fill with advance accommodations made for anticipated settlement. This option works well for siting a truck-parking area, where surface settlement is easily tolerated through minor grading or addition of fill to level depressions or “bird baths.” Temporary structures, such as a lightweight mobile trailer, can be periodically leveled with screw jacks.
For sites with groundwater impacts, removal of the pond from service can alter groundwater flow and gradients. Active monitoring, and implementation of groundwater treatment regimens, can be employed to address affected groundwater.
In fact, in some instances the pond can become an integral part of site remediation through its conversion to a wetlands treatment system. In conditions where the pond bottom elevation intercepts the groundwater, closure of the pond may provide opportunities for the construction of intersecting extraction, barrier, or reaction systems.
Coal ash presents some unique challenges as a construction material potentially affecting site safety and constructability. Coal ash itself is classified as nonhazardous. However, because unintended health effects always depend upon the duration of exposure and the concentration of the chemical constituents present in coal ash (that is, crystalline silica and heavy metal content), appropriate health and safety planning should be done prior to construction activities.
Respiratory exposure of site workers to coal ash may need to be mitigated by means of dust control or respirators. Additionally, the potential for dermal contact with the material should be considered. The pH of coal ash will vary from alkaline to acidic, depending on the source of the coal and facility operations.
Because of its grain-size distribution, some ash deposits demonstrate thixotropic behavior. The ash materials appear to be competent, solid materials that maintain a steep “angle of repose” as they are excavated. However, after being subjected to vibration, such as during transport in the bed of a truck, the physical properties change dramatically as the ash liquefies and becomes a fluid that is impossible to stack, grade, or cover (Figure 3).
|3. Liquefaction of ash pond solids after transport to disposal area. Courtesy: Barry Culp/TRC|
Constructability will be influenced by the ability to safely excavate and stack ash in a stable configuration. Equipment operating on a weak, unstable surface can also become bogged down and disabled, or even engulfed. Therefore, stability is an important consideration for excavation safety, equipment access, material stacking and handling, and ash fill construction (Figure 4).
|4. Placement and spreading of ash hauled to an on-site landfill. Courtesy: Barry Culp/TRC|
Stability of ash solids within a pond will be controlled by the shear strength of the ash materials. This shear strength is influenced by factors that include particle size distribution, moisture content, and density. Weaker ash materials tend to be finely graded, saturated, and loosely compacted, whereas ash that’s coarse, dry, and dense is stronger. Ash material that demonstrates high shear strength can be stacked at steeper slopes than materials with a low strength. Steeper slopes allow a given volume of ash to be stacked within a smaller footprint.
There is a significant economic benefit for designing a disposal cell with a smaller footprint for higher-strength ash containment because construction and long-term maintenance costs are lower.
Site Configuration Constraints
Usually, the “as-built” conditions do not match the originally designed configuration. If the pond has been periodically dredged or excavated, it may be deeper than the design assumed. Through progressive excavation, the perimeter berms may eventually be narrower or steeper than originally designed. Material excavation, hauling, and placement activities typically represent the majority of project costs associated with ash pond closure. If the actual shape is even a foot or two deeper than assumed, expect the costs to increase dramatically, especially with larger ponds.
Production costs increase when the cycle time of hauling vehicles is lengthened, where excavating equipment access is limited, or where the double handling of the ash is necessary. These conditions are usually imposed by site constraints such as the presence of adjacent wetlands, utility infrastructure, or historical and cultural resources that dictate the alignment of temporary haul roads. In some cases, a narrow perimeter berm may limit two-way truck traffic. Site entrance locations also can be dictated by a governing transportation authority.
A minimum vertical separation between overhead electrical lines and ground surface is typical and may limit the placement and stacking height of ash beneath power lines. Storm water from the closed pond footprint and its impact on receiving streams may also become an important post-closure consideration.
The long-term, post-closure maintenance of either a pond or solid waste disposal cell generally requires monitoring for cover integrity, surface drainage, and performance. The better closure designs minimize post-closure costs involving repairs of cover erosion, restoration of vegetative planting, correction of surface settlement, and maintenance of drainage structures. Minimal dependence upon mechanical pumping for conveyance of fluids is almost always a plus. Gravity’s switch is always in the “on” position.
Trade-offs and competing priorities introduce complexities into even a simple pond closure. Comparing a pond closure in a suburban setting with one in a rural environ illustrates the differences that site conditions pose.
A pond closure near residences is more likely to have constricted site access imposed by fences, utilities, and adjacent structures than a pond closure in an unpopulated area. This may constrain the type and size of construction equipment that can safely operate.
In a suburban area, increased awareness of construction equipment noise by residents proximate to the site may limit operating hours. Restricting the workflow to five weekdays and limiting the hours extends the schedule and therefore escalates closure costs.
Hauling ash materials for off-site disposal/reuse or importing fill materials from other borrow sources for in-place closure is a key factor in a suburban area closure site. Heavy truck traffic on suburban streets or roads may be more limited in their “turn-around time” than on roads in a rural area. In these conditions, an in-place closure may be better suited than a “clean closure” with its removal of all pond solids, as total haul volumes (and corresponding truck traffic) are usually lower.
Real estate values for property occupied by a pond in a suburban location may be higher than the value of pond property in a rural setting. An operating plant at a suburban site may be property constrained, and the facility could potentially use a closed pond area for vehicle parking, plant expansion, material staging, and so forth. These considerations may drive the final closure approach toward conditions that are more structurally robust, requiring stabilization/removal of ash solids, a reinforced cover, or other accommodations beyond a typical closure.
Comprehensive Analysis Should Guide Decisions
Technical options for ash pond closures are project-specific, depending upon the pond location, site conditions, and ash properties. However, thoughtful and comprehensive evaluation of the wide variety of available solutions, even under some of the limitations of pending regulations, can yield significant dividends in terms of both short-term and long-term cost savings. ■
— Mark Johnson, PG (mjohnson@ trcsolutions.com) is a senior client services manager and Kent Nilsson, PE (firstname.lastname@example.org) is a senior engineer at TRC Companies Inc.