A surface impoundment is a shallow pond, pit, or lagoon used to store, treat, or dispose of liquid waste. These structures can be natural depressions in the landscape, man-made excavations, or areas enclosed by dikes or berms. Industries, municipalities, and power plants use them to manage everything from industrial wastewater to coal ash slurry.
How Surface Impoundments Work
Surface impoundments hold liquid waste while natural or engineered processes break it down, settle out solids, or simply evaporate the liquid portion. Federal regulations specifically list holding pits, settling ponds, aeration lagoons, and storage ponds as common examples. The waste they contain can range from relatively benign wastewater to hazardous chemical byproducts, depending on the industry.
The basic design is straightforward: a basin lined (in modern facilities) with synthetic materials or compacted clay to prevent liquids from seeping into the surrounding soil and groundwater. Older impoundments, built before modern regulations took effect, often have no liner at all. Waste enters the impoundment through pipes or channels, and over time, solids settle to the bottom while liquids may be treated, recycled, or allowed to evaporate.
Industries That Rely on Them
Surface impoundments serve a wide range of sectors. Mining operations use them to contain process water and tailite slurry. Agricultural facilities store animal waste and irrigation runoff. Chemical manufacturers, petroleum refineries, and paper mills use them to settle out contaminants before discharging treated water. Municipal wastewater treatment plants often incorporate impoundments as part of their processing systems.
One of the most heavily regulated categories is the coal combustion residuals (CCR) surface impoundment, used at coal-fired power plants. These ponds hold the ash, sludge, and other residuals left over from burning coal. A CCR surface impoundment is defined as a depression or diked area designed to hold an accumulation of coal combustion residuals and liquids, where the unit treats, stores, or disposes of CCR.
Groundwater Contamination Risks
The biggest environmental concern with surface impoundments is groundwater contamination. Liquid waste sits in direct contact with the bottom and sides of the basin, creating constant downward pressure. If the liner fails, or if no liner exists, contaminants can migrate into the soil and eventually reach underground aquifers that supply drinking water. Even modern liner systems sometimes leak, according to the EPA.
Unlined or poorly maintained impoundments pose the highest risk. Heavy metals, organic chemicals, and other pollutants can travel through soil over years or decades, often going undetected until a monitoring well picks up contamination or nearby wells show water quality changes. The slow pace of groundwater movement means contamination discovered today may have started years earlier, and cleanup can take just as long.
Regulatory Requirements
Federal oversight of surface impoundments falls under two main frameworks. Impoundments that handle hazardous waste are regulated under the Resource Conservation and Recovery Act (RCRA), specifically under 40 CFR Part 265, Subpart K. Those handling non-hazardous solid waste, including coal ash, fall under separate provisions in 40 CFR Part 257.
For hazardous waste impoundments, the rules are strict about what can go in. Ignitable or reactive waste cannot be placed in a surface impoundment unless it is first treated or mixed so it no longer meets the hazardous classification. Incompatible wastes cannot be placed in the same impoundment. These restrictions exist to prevent chemical reactions, fires, or toxic gas releases.
Groundwater monitoring is a core requirement. Facilities must install monitoring wells at locations around the impoundment and sample them at minimum semi-annually, collecting at least four samples per well during each sampling event. Monitoring happens in phases. Detection monitoring looks for early signs that contaminants may be leaking from the unit. If a release is suspected, the facility must escalate to compliance monitoring, which tracks specific hazardous constituents against established concentration limits. If those limits are exceeded, the facility enters corrective action, meaning active cleanup.
The concentration limits used during compliance monitoring are based on background groundwater levels, federal drinking water standards, or alternative levels set by regulators for that specific site.
Coal Ash Impoundment Rules
Coal ash ponds received their own comprehensive federal regulations in April 2015, when the EPA finalized technical standards for coal combustion residuals. These rules require measures to prevent contaminants from leaking into groundwater, blowing into the air as dust, or causing catastrophic structural failure of impoundment walls.
In May 2024, the EPA expanded these rules to cover “legacy” CCR surface impoundments, meaning older, inactive ponds at shuttered power plants that had previously escaped federal oversight. Under the updated rule, owners of legacy impoundments must install groundwater monitoring systems by February 2031, begin sampling and evaluating data that same year, and initiate closure by February 2032. These deadlines give operators time to assess conditions but set firm endpoints for cleanup.
How Surface Impoundments Are Closed
When a surface impoundment reaches the end of its useful life, closure follows one of two paths: complete removal of waste or closure in place.
Complete removal, sometimes called clean closure, involves extracting all the waste material, decontaminating any areas affected by releases, and confirming through groundwater monitoring that contaminant levels do not exceed protection standards. Closure by removal is considered complete when monitoring shows no constituents above protection standards for either two consecutive monitoring events or three years. If those standards are met, the property carries no long-term deed restrictions.
Closure in place is more common for large impoundments where removal would be impractical or prohibitively expensive. The operator must drain free liquids, stabilize the remaining waste, and install a final cover system designed to minimize water infiltration and erosion. The cover must also provide slope stability to prevent shifting or slumping over time. Critically, if waste is left in place, a permanent notation must be recorded on the property deed, notifying any future buyer that the land was used as a waste impoundment. This notation stays in perpetuity.
Post-closure care follows for impoundments closed in place. The operator must maintain the cover system, continue groundwater monitoring, and address any problems that arise. For coal ash impoundments, the post-closure care plan must be prepared by August 2031 under the current rule, with monitoring and maintenance continuing for decades afterward.
Why They Remain Controversial
Surface impoundments occupy an uneasy middle ground in waste management. They are cheaper and simpler to build than enclosed treatment systems, which makes them attractive to industries generating large volumes of liquid waste. But their open design and reliance on liners that degrade over time create ongoing risks. Legacy impoundments built without modern safeguards continue to threaten groundwater near communities that may not even know the ponds exist. The 2024 expansion of federal rules to cover legacy coal ash sites reflects growing recognition that older, unregulated impoundments remain a significant source of contamination across the country.