Ash pond

An ash pond, also called a coal ash basin or surface impoundment, is an engineered structure used at fossil fuel power stations for the disposal of two types of coal combustion products: bottom ash and fly ash. The pond is used as a landfill to prevent the release of ash into the atmosphere. Although the use of ash ponds in combination with air pollution controls (such as wet scrubbers) decreases the amount of airborne pollutants, the structures pose serious health risks for the surrounding environment.
Ash ponds use gravity to settle out large particulates (measured as total suspended solids) from power plant wastewater. This technology does not treat dissolved pollutants. The ponds generally have not been built as lined landfills, and therefore chemicals in the ash can leach into groundwater and surface waters, accumulating in the biomass of the system.
In the United States, federal design standards for ash ponds were strengthened in 2015, although various provisions of the new regulations are on hold as of 2020, pending ongoing litigation.
Pond design.
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.
Failure of a pond's earthen embankment can cause ash spills on adjacent land and rivers, with serious environmental damage, as evidenced in the 2008 Kingston Fossil Plant spill in Tennessee and the 2014 Dan River coal ash spill in North Carolina.
Disposal methods.
The wet disposal of ash into ash ponds is the most common ash disposal method, 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. 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.
Leachate from fly ash can contain heavy metals in excess of allowable U.S. standards under the Resource Conservation and Recovery Act (RCRA). 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.
Health and environmental impacts.
Not all substances present in coal will burn, and hence the non-combustible material is present in more concentrated amounts in coal ash than in coal itself. Substances commonly found in coal ash include arsenic, barium, beryllium, boron, cadmium, nickel, lead, mercury, molybdenum, selenium and thallium. Elevated levels of radioactivity may also be present. Many of these substances, especially heavy metals, can have negative effects on humans when ingested. Because of biomagnification, the concentration of unwanted chemicals in animals can increase up a food chain (similarly to mercury in tuna). Coal ash, a product of combustion, concentrates these elements and can contaminate groundwater or surface waters if there are leaks from an ash pond.
In the United States, coal ash is a major component of the nation's industrial waste stream. As of 2012 approximately 60 percent of US coal ash was disposed in surface impoundments and landfills. The US had 310 active on-site landfills in 2012, averaging more than 120 acres in size with an average depth of over 40 feet, and more than 735 active on-site surface impoundments, averaging more than 50 acres in size with an average depth of 20 feet. In 2017, of fly ash, and of bottom ash were generated.
A 2019 report by the Environmental Integrity Project states that for U.S. coal-fired plants with available monitoring data, 91 percent of them have contaminated groundwater with "unsafe levels of toxic pollutants."
An environmental justice advisor to the US Environmental Protection Agency (EPA) has stated that the increased toxic exposures from ash ponds will have disproportionate adverse health effects on low-income and minority communities.
Regulation in different countries.
United States.
In the United States, due to few federal and state regulations concerning ash ponds, most power plants do not use geomembranes, leachate collection systems, or other flow controls often found in municipal solid waste landfills. In 1980 the U.S. Congress defined coal ash as a "special waste" that would not be regulated under the stringent hazardous waste permitting requirements of RCRA. In 2000, EPA stated that coal fly ash did not need to be regulated as a hazardous waste.
Following a 2008 failure that caused the Tennessee Valley Authority's Kingston Fossil Plant coal fly ash slurry spill, EPA began developing regulations that would apply to all ash ponds in the US.
EPA published a Coal Combustion Residuals (CCR) regulation in 2015. The agency continued to classify coal ash as non-hazardous (thereby avoiding strict permitting requirements under ), but with new restrictions:
Some of the provisions in the 2015 CCR regulation were challenged in litigation, and the United States Court of Appeals for the District of Columbia Circuit remanded certain portions of the regulation to EPA for further rulemaking.
EPA also published an effluent guidelines (wastewater) regulation in 2015 pursuant to the Clean Water Act. The regulation limits discharges of toxic metals from power plants, including ash ponds and other wastestreams. The Agency estimated that the regulation would reduce the industry's metals discharges of 1.4 billion pounds annually. In October 2020 EPA published a final rule that reverses some provisions of the 2015 wastewater regulation. The revised regulation extends the compliance deadline for some power plants, and exempts some others completely from the more stringent 2015 requirements. The 2020 rule has been challenged in litigation.
EPA published a proposed RCRA rule on August 14, 2019 that would use location-based criteria, rather than a numerical threshold (i.e. impoundment or landfill size) that would require an operator to demonstrate minimal environmental impact so that a site could remain in operation.
Following a court remand, EPA published its "CCR Part A" final rule on August 28, 2020 requiring all unlined ash ponds to retrofit with liners or close by April 11, 2021. Some facilities may apply to obtain additional time—up to 2028—to find alternatives for managing ash wastes before closing their surface impoundments. Further litigation on the CCR regulation is pending as of 2021.
Netherlands.
Ash ponds are not allowed in the Netherlands, as they are a type of landfill. Instead, all coal ash is recycled in the Netherlands.
Remediation.
Remediation options include "capping, dewatering and/or stabilizing, consolidating into a new landfill, disposing off site, converting to wetlands, or any combination of these options." 
There are some initiatives, such as a 2015 initiative by Duke Energy, to excavate existing ash ponds to reduce the environmental effects of coal-burning power facilities on the surrounding environment.