Design and Permitting for the Closure of a Fly Ash Reservoir
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Our client owns a power plant in Ohio that generates electricity from two 1,300-MW coal-fired units.

In anticipation of upcoming federal coal combustion residuals (CCRs) regulations, they decided to initiate closure design for a 300-acre fly ash pond, recognizing the many years it would take to design, permit, and close the pond. They retained Geosyntec to help them in this effort in part due to our experience designing and successfully permitting an adjacent residual waste landfill which will partially cover this fly ash pond and act as the cover. Their objective for the project was to overcome the numerous technical challenges to provide a cost-effective cover and lower the height of the existing high hazard dam to the maximum extent practicable.  

Geosyntec's Scope of Services

Geosyntec began by completing a subsurface investigation to characterize the site. We then designed the new cover that integrated the subsurface conditions and stormwater modeling to mitigate downstream impacts and to mimic the natural ground topography.  Specialty geomembrane puncture testing allowed the protective soil to contain large rocks (4 inch) reducing the amount of soil screening.

Because acid mine drainage (AMD) from around the reservoir reduces the pH of the water and is currently treated in the reservoir, our closure design included characterizing AMD chemistry and flow rate, completing the AMD collection system for use during closure construction, and treatment system designs for post-closure conditions when the pond will no longer exist.

We used our engineering capabilities to lower a high hazard dam as the pond is lowered and fly ash is covered with a final cover.  We completed the 100% design of the lowered dam and conducted hydraulic modeling of the modified dam to design the spillway and assess potential impacts on downstream and upstream conditions.  Significant technical issues included developing a regulatory strategy to manage TSS during pond closure and addressing AMD to meet the current NPDES discharge permit limits. We completed hydrologic and hydraulic modeling to estimate flow through the spillway at different stages of the closure and completed a permit-level design for automated suspended solids monitoring and controls and prepared a groundwater monitoring plan to meet the requirements of the federal CCR rule. We also prepared construction bid documents for the pond closure and modification of the dam and are providing engineering support during construction.

Notable Accomplishments

Geosyntec was able to provide a more efficient grading design that saved significant cost by reducing the amount of volume to be relocated and reducing work over wet ash. We also obtained regulatory approval for using large size (4 inch) rocks over the geomembrane and surface cushion for the cover protective layer through additional puncture testing, saving hundreds of thousands of dollars by avoiding extensive sieving. 

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Project Summary

  • Location: Southern Ohio
  • Client: Confidential Electric Generating Company
  • Project Practice Areas: Waste Management, Environmental Management
  • Type of Facility: Coal-fired plant
  • Services Provided: CCR Rule groundwater monitoring plan, Ash pond closure design, NPDES permit modification, Stormwater/hydraulic modeling, Subsurface site investigation, Cover engineering analyses and design, Acid mine drainage treatment/management design, High hazard dam modification, Construction bid documents, Construction and operations planning, Regulatory liaison
  • Type of Work: Coal ash pond closure
  • Governing Regulation: Federal and state