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For many coal combustion residuals (CCR) sites, dewatering is not just a closure requirement. It is a constructability, safety, and long-term performance challenge.
Ash basins, FGD ponds, and evaporation ponds often include large footprints, limited surface access, unstable working conditions, and site infrastructure that cannot be disrupted. These conditions can make it difficult, inefficient, or unsafe to install traditional vertical well networks.
That is why horizontal wells installed by horizontal directional drilling (HDD) are increasingly used at CCR facilities. When properly designed and installed, horizontal wells can remove pore water, support groundwater control, and improve access to areas that may be difficult or impossible to reach from the surface.
Since 2016, Ellingson-DTD has completed more than 63,000 feet of horizontal wells in and around CCR facilities across the United States, including systems installed directly within ash impoundments.
Why Horizontal Wells Are Used at CCR Sites
Horizontal wells are often considered when the target zone is large, access is limited, or surface conditions create safety concerns.
Instead of placing equipment and personnel on saturated ash or unstable surfaces, horizontal wells can be installed from stable ground adjacent to the impoundment. This allows the well screen to be positioned beneath or within the ash while reducing the need for surface access across the basin.
For environmental engineers, this provides several practical advantages:
- Access beneath active or inaccessible ash areas
- Reduced disturbance to surface infrastructure
- Fewer surface entry points
- Longer continuous screen lengths
- Improved ability to place the intake zone near the base of the basin
- Reduced need for dense vertical well networks
Horizontal wells also do not need to be perfectly straight or perfectly horizontal. Alignments can be curved to account for basin geometry, existing infrastructure, subsurface constraints, and project objectives.
Geometry Matters for Dewatering Efficiency
One of the key advantages of a horizontal well is the ability to place the well screen where dewatering is most effective.
In CCR basins, the goal is often to remove pore water from the bottom up. By positioning a horizontal well near the base of the ash, engineers can target water that may otherwise be difficult to remove using surface-based methods or widely spaced vertical wells.
Once the ash surface becomes stable, excavation or closure activities may proceed with less interference from wellpoint systems, surface piping, or extensive vertical well networks.
This is especially important on sites where schedule, access, and safety constraints are closely linked. The less work required on unstable ash, the greater the project team’s flexibility in sequencing dewatering, excavation, and closure activities.
Long Screen Lengths Improve Coverage
Horizontal wells at CCR sites have been successfully installed to lengths of up to 3,000 feet. These long, continuous screen intervals allow a single well to cover a much larger area than a typical vertical well.
That does not mean one horizontal well always replaces an entire vertical well system. The design still depends on site hydrogeology, ash characteristics, hydraulic conductivity, target drawdown, pumping objectives, and closure sequencing.
However, the extended intake area can be a significant advantage. A horizontal well can provide contact across hundreds or thousands of feet of target material while limiting the number of surface access points required.
For large ash impoundments, FGD ponds, and evaporation ponds, that coverage can make horizontal wells an efficient option for both dewatering and groundwater control.
What About Well Performance?
One common concern is whether horizontal wells can perform reliably without a conventional gravel filter pack.
That concern is understandable. Filter pack design is an important part of many vertical groundwater wells. But field experience and groundwater hydraulics show that horizontal wells can still provide reliable long-term performance when the system is designed correctly.
The key factors are screen length, pumping rate, and entrance velocity.
Horizontal CCR wells often have screen lengths that are 10 to 100 times longer than those of a typical vertical well. Because flow is distributed across a much larger intake area, the entrance velocity into the screen can be significantly lower, even when the total pumping rate from the horizontal well is higher than that of a single vertical well.
Lower entrance velocity matters because it reduces turbulence, limits fines migration, and helps reduce screen encrustation and erosion. Water well industry references commonly recommend low entrance velocities to support long-term performance. Depending on the application, recommended entrance velocities are often 1 ft/sec or less, with 0.1 ft/sec commonly referenced from Johnson Screens.
For horizontal CCR wells, calculated entrance velocities are often orders of magnitude below these recommended thresholds due to the long, continuous screen lengths.
Maintenance Is Still Part of the System
Horizontal wells are not maintenance-free. No groundwater well is.
If a well is pumped too aggressively, it can create negative internal pressure, allowing ash particles and fines to enter the well screen. That can lead to plugging. This issue is not unique to horizontal wells. Vertical wells, including those with filter packs, can also produce formation fines under pressure and require periodic maintenance.
For horizontal CCR wells, rehabilitation commonly includes removing the pump and jetting or flushing the well screen and riser to remove accumulated fines. These activities are typically straightforward and can be performed by drilling contractors or trained site personnel.
The right expectation is not that a horizontal well will never require maintenance. The right expectation is that maintenance should be considered part of the long-term operational plan, just as it would be for a conventional vertical well system.
Horizontal Wells Should Be Evaluated by Design, Not Assumption
Horizontal wells are sometimes dismissed because they are perceived as more likely to plug or less reliable than vertical wells. That assumption overlooks the most important design factors.
Performance should be evaluated based on:
- Site hydrogeology
- Ash characteristics
- Well alignment and screen placement
- Screen length
- Pumping rate
- Entrance velocity
- Construction quality
- Maintenance planning
- Dewatering and closure objectives
When those factors are properly addressed, horizontal wells can be a reliable and efficient tool for CCR dewatering and groundwater control.
The Bottom Line
Horizontal wells do work for CCR dewatering.
They are not a one-size-fits-all solution, but they are a proven option for sites where access, safety, basin geometry, and dewatering efficiency are critical considerations.
For environmental engineers evaluating closure strategies, horizontal wells should be considered when the project requires long screen intervals, limited surface disturbance, access beneath ash or infrastructure, and a dewatering approach that supports safer, more efficient closure activities.