EPA Creates Alternate Liner Demonstration Process for CCR Surface Impoundments

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Coal Combustion Residuals (CCR),
Surface Impoundment Liners,
Fate and Transport Modeling


US EPA Creates Alternate Liner Demonstration Process for CCR Surface Impoundments

On October 15, 2020, the United States Environmental Protection Agency (US EPA) released a pre-publication version of its final Holistic Approach to Closure Part B rule, which creates an alternate liner demonstration (ALD) process for coal combustion residual (CCR) surface impoundments that are classified as “unlined.” In the Holistic Approach to Closure Part A rule, the classification of clay-lined surface impoundments was changed from “lined” to “unlined” to comply with the 2018 District of Columbia Circuit Court ruling. Under current standards, a surface impoundment is considered lined only if it consists of a composite liner containing a minimum 30 mil geomembrane (or a minimum 60 mil high-density polyethylene geomembrane) underlain by 2 feet of compacted soil with a conductivity no greater than 10-7 cm/s, or an alternative composite liner that meets the same performance standards. Unlined surface impoundments are required to stop receiving CCRs and initiate closure or retrofit by April 11, 2021.


US EPA acknowledges in the new Part B rule that some surface impoundment liner systems, whether engineered or natural, that do not meet the federal liner standards can still be protective of human health and the environment. The ALD process promulgated in the final rule allows utilities to submit to US EPA a site-specific demonstration that these types of “unlined” surface impoundments are protective of human health and the environment.


A successful ALD requires two lines of evidence to demonstrate that “operation of the surface impoundment will not result in groundwater concentrations above relevant GWPS [groundwater protection standards] at the waste boundary” (final rule pre-publication, p. 50). The first line of evidence requires thorough characterization of the site hydrogeology and the second line of evidence requires an assessment of the potential for infiltration from the bottom of the impoundment. Data collected for both lines of evidence must be incorporated into a final demonstration, which will require fate and transport modeling to evaluate groundwater concentrations downgradient of the impoundment. The rule (p. 78) states that complex sites may require the use of a probabilistic fate and transport model in order to account for the full range of operational characteristics and geological conditions. Deterministic models may be used at less complex, more homogenous sites.

US EPA established a two-step process for ALDs. Utilities are first required to submit an initial application to demonstrate that they meet minimum requirements. The deadline for submittal of the initial application is November 30, 2020. US EPA will review the application and determine whether a facility is allowed to proceed to the more comprehensive ALD. Facilities have until November 30, 2021, to submit the comprehensive demonstration.

Gradient is well positioned to help companies prepare their ALDs in a cost-effective manner. Gradient was the principal investigator for the Electric Power Research Institute (EPRI) study “Model Evaluation of the Relative Performance of Alternative Liners,” which was cited by US EPA in the Part B rule as the example of the type of fate and transport modeling that can be performed as part of a comprehensive ALD evaluation. Other specific areas of Gradient’s CCR expertise include:

  • Fate and transport evaluations, including expertise in groundwater and surface water modeling; impoundment liner performance; and both probabilistic and deterministic modeling approaches;
  • Geochemical transformation evaluations for constituents in groundwater, surface water, and at the groundwater-surface water interface;
  • Closure alternatives assessments; and
  • Assessments of potential human health and ecological risks.

If you have any questions about the ALD requirements or Gradient’s capabilities, please visit our website or contact us. 

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