Ash pond

An ash pond is an engineered structure for the disposal of bottom ash and fly ash. The wet disposal of ash into ash ponds is the most common ash disposal method,^[1] but other methods include dry disposal in landfills. Dry-handled ash is often recycled into useful building materials. Wet disposal has been preferred due to economic reasons, but increasing environmental concerns regarding leachate from ponds has decreased the popularity of wet disposal.^[1] The wet method consists of constructing a large "pond" and filling it with fly ash slurry, allowing the water to drain and evaporate from the fly ash over time.^[2] Ash ponds are generally formed using a ring embankment to enclose the disposal site. The embankments are designed using similar design parameters as embankment dams, including zoned construction with clay cores. The design process is primarily focused on handling seepage and ensuring slope stability.

Leachate from fly ash can contain heavy metals in excess of allowable RCRA standard.^[3] The flow of water through the fly ash and into ground water is controlled by using low-permeability clay layers and cutoff trenches/walls. Low-permeability clays have permeability on the order of 10⁻⁷ cm/s. Vertical flows through the foundation are controlled by siting fly ash ponds on areas of thick clay or rock layers that provide suitably low permeability through the base of the pond. Areas with high sub-surface permeability can be improved by importing suitable clay. Horizontal flows through the embankment are controlled using clay zones within the embankment. Cut off trenches and cut off walls are used to connect the embankment clay zones and the foundation clay layers. Cut off trenches are trenches that are dug into the selected low-permeability sub-surface layer and backfilled with clay to key the embankment clay zone into the sub-surface. Cut off trenches are generally used when the low permeability foundation layer(s) are near surface. Cut off walls are similar to cut off trenches, but are generally much deeper and narrower, and use either slurry or grout instead of clay.

Due to few federal and state regulations concerning ash ponds, most do not use geomembranes, leachate collection systems, or other flow controls often found in municipal solid waste landfills.^[2] Following a failure that caused the Tennessee Valley Authority’s Kingston Fossil Plant coal fly ash slurry spill, the Environmental Protection Agency began overseeing the regulation of all ash ponds in order to establish national fly ash pond standards.

References

Ponds, pools and puddles
Ponds	Ash pond Balancing lake Ballast pond Beel Cooling pond Detention pond Dew pond Evaporation pond Facultative lagoon Garden pond Ice pond Immersion pond Kettle pond Log pond Melt pond Mill pond Raceway pond Retention pond Sag pond Salt evaporation pond Sediment pond Settling pond Solar pond Stabilization pond Stepwell Stew pond Stormwater management pond Polishing pond Tailings Tarn Treatment pond Waste pond
Pools	Anchialine pool Brine pool Natural pool Plunge pool Reflecting pool Spent fuel pool Stream pool Swimming pool Tide pool Vernal pool
Puddles	Bird bath Coffee ring effect Puddle Puddles on a surface Seep puddle
Biota	Beaver dam Duck pond Fish pond Goldfish pond Koi pond
Ecosystems	Freshwater ecosystem Lake ecosystem Lentic ecosystem Aquatic ecosystem
Related topics	Aerated pond Bakki shower Big fish–little pond Body of water Full pond Hydric soil Phytotelma Water aeration Pond of Abundance Pond liner Ponding Puddle (M C Escher) Spring Swimming hole Water garden Water Lilies (Monet) Water well