MEMORANDUM

TO: Steam Electric Rulemaking Record

FROM: TJ Finseth, ERG

Danielle Lewis, ERG

Elizabeth Sabol, ERG

DATE: January 30, 2013

SUBJECT: Methodology for Estimating Impoundment Age, Capacity, and Reductions in 

Capacity Used to Estimate Benefits Associated with Avoided Impoundment 

Failures

Steam electric power plants manage combustion residuals such as flue gas desulfurization (FGD) 

solids, fly ash, and bottom ash using either wet or dry handling practices. For plants that use wet 

handling systems, the combustion residuals are typically sluiced to one or more surface 

impoundments (i.e., settling ponds), where the solids settle out of the water. In addition to solids, 

these impoundments typically contain water with high concentrations of pollutants originating 

from the combustion residuals, including dissolved metals.

Although the impoundments are subject to state regulations, events such as the failure of a 

combustion residual impoundment at the Tennessee Valley Authority’s Kingston Fossil Plant in 

2008, demonstrate the potential severity and cost of structural failures. In that case, over 5 

million cubic yards of combustion residual was accidentally released, resulting in cleanup costs 

and other costs and damages exceeding $1 billion. Although the probabilities of catastrophic 

releases such as the Kingston’s spill may be low, the high costs associated with these events 

mean that even very small risks entail significant costs to society. In addition, other smaller 

unintentional releases (e.g., spills) from surface impoundments may also result in substantial 

damages and incur large cleanup and other costs.

The operational changes prompted by the proposed effluent limitations guidelines and standards 

(ELGs) may cause some plant owners to reduce their reliance on impoundments to handle 

combustion residuals. These changes would affect the future probability of impoundment failures 

and, as a result, accidental, and sometimes catastrophic releases, of combustion residual solids 

and wastewater. Benefits arising from the reduced risk of impoundment failures include avoided 

cleanup costs, environmental damage, and litigation costs.

EPA quantified and monetized these benefits based on predictions about how future 

impoundment structural failures and other accidental releases from impoundments would be 

reduced by the proposed ELGs, and the commensurate avoided costs of spill cleanup, natural 

resource damages, and litigation expenses. The details of this analysis are described in Chapter 7

of the Benefit and Cost Analysis for the Proposed Effluent Limitations Guidelines and Standards 

for the Steam Electric Power Generating Point Source Category.
Memorandum

January 30, 2013

Page 2

The analysis used to estimate the benefits associated with these avoided impoundment failures

takes into account the impoundment age, capacity (expressed as volume, in cubic feet), and the 

estimated reductions in impoundment capacity. Therefore, ERG used data reported in responses

to the Questionnaire for the Steam Electric Power Generating Effluent Guidelines (Steam 

Electric Survey) to obtain these characteristics from each of the 1,070 coal combustion residual

(CCR) impoundments considered in the scope of the Steam Electric ELGs.

1

ERG also estimated

how the future impoundment capacity for each impoundment would be reduced under each of 

the five regulatory options. For more information on the regulatory options considered as part of 

the rulemaking, see Section 8 of the Technical Development Document for the Proposed Effluent 

Limitations Guidelines and Standards for the Steam Electric Power Generating Point Source 

Category.

1.0 DETERMINING SURFACE IMPOUNDMENT AGE

ERG used data from Part A of the Steam Electric Survey to determine the age of CCR surface 

impoundments. Question A3-2, Table A-4, requested information on the initial year the 

impoundment was brought online for both active and inactive impoundments. ERG was able to 

obtain the year brought online for 1,059 of the 1,070 CCR impoundments from the Steam 

Electric Survey.

For the remaining 11 impoundments, ERG used the data from the 1,059 impoundments to 

calculate an average installation year associated with active impoundments and an average

installation year associated with inactive impoundments.

ERG calculated the impoundment age by subtracting the year the impoundment was brought 

online from 2009. ERG used 2009 as the basis for the age because the data collected in the 

Steam Electric Survey represents 2009.

2.0 DETERMINING SURFACE IMPOUNDMENT CAPACITY

ERG determined the capacity, or volume, for all 1,070 CCR surface impoundments using data 

from Part D of the Steam Electric Survey. Table D-5 requested information on the original 

volume (in cubic feet) of the surface impoundment. From Table D-5, ERG was able to determine 

the volume for 893 impoundments.

For the remaining 177 surface impoundments, ERG estimated the volume of the impoundments 

using one of the following three prioritized estimation methods:

1) For three of these impoundments, ERG determined the impoundment volume using the 

original surface area (in square feet), top elevation (in feet), and bottom elevation (in 

feet). ERG calculated the volume of the three impoundments using the following 

equation:

Volume (ft3) = Surface Area (ft2) × [Top Elevation (ft) – Bottom Elevation (ft)]

 

1 Using information reported for the Steam Electric Survey, EPA determined that in 2009 there were 1,070 surface 

impoundments containing coal combustion residuals and that had not been retired and closed. If surface 

impoundments containing petroleum coke residuals are also included, the total number of surface impoundments 

containing either coal or petroleum coke combustion residuals is 1,071.
Memorandum

January 30, 2013

Page 3

2) For 111 of these impoundments, ERG estimated the impoundment volume using an 

average volume per flow rate into an impoundment (in cubic feet per gallons per day

(gpd)) corresponding to status (active or inactive). Using Steam Electric Survey data for 

all combustion residual impoundments that reported both baseline flow rates into the 

impoundment and impoundment volume, ERG calculated an average volume per flow 

rate for each impoundment for which data were available and then averaged those ratios 

for all the impoundments to calculate a volume per flow factor, which was 600 ft3/gpd.

2

See Section 3 for more information on how ERG calculated impoundment-specific flow 

rates entering the surface impoundments. ERG estimated the volume (in cubic feet) by 

multiplying the impoundment-specific flows rates, as described in Section 3, by the 

calculated volume per flow factor.

3) For 63 of these impoundments (53 active and 10 inactive impoundments), ERG 

calculated the average volume for active and inactive impoundments based on the 896 

impoundments for which ERG had impoundment volumes reported in the Steam Electric 

survey (893 impoundments) or ERG calculated the volume based on impoundment 

dimensions (3 impoundments, identified in bullet 1). These 896 impoundments account 

for 832 active and 64 inactive impoundments. The average volume for active 

impoundments was 49,400,000 ft3and the average volume for inactive impoundments 

was 48,700,000 ft3.

3.0 ESTIMATING REDUCTION IN IMPOUNDMENT CAPACITY

ERG estimated the reduction in impoundment capacity for all 1,070 CCR surface impoundments

for each of the regulatory options. Each of the five regulatory options will result in a decrease in 

the wastewater entering the impoundments either by reducing or eliminating the volume of 

wastewater sent to the impoundment. ERG estimated that the impoundment capacity would 

decrease by the percent reduction in flow into each impoundment. While the reduction in flow 

entering the pond will not necessarily decrease the size of the impoundment unless the plant 

closes portions of the impoundment after the implementation of the ELGs, the impoundment will 

receive less combustion residual solid waste and therefore, will be less likely to have a breach of 

the impoundment. Furthermore, any breach or spill that does occur would likely be less severe 

since the impoundment would contain less combustion residual solids or wastewater.

ERG used information from Part A, Section 3 of the Steam Electric Survey to determine the 

types of combustion residual waste contained in each of the 1,070 CCR impoundments. ERG 

determined which impoundments received FGD wastewater, fly ash transport water, and bottom 

ash transport water. The Steam Electric Survey did not require plants to differentiate between 

CCR leachate from impoundments versus CCR leachate from landfills. For this reason, ERG was 

unable to evaluate which impoundments contained combustion residual landfill leachate and 

therefore, leachate was not included in the analysis.

ERG then calculated a baseline flow associated with each impoundment. As described in EPA’s

Incremental Costs and Pollutant Removals for Proposed Effluent Limitation Guidelines and 

 

2 ERG used Steam Electric Survey data from 625 combustion residual impoundments that reported both baseline 

flow rates into the impoundment and the impoundment volume.
Memorandum

January 30, 2013

Page 4

Standards for the Steam Electric Generating Point Source Category Report (DCN SE01957), 

ERG estimated the amount of FGD, fly ash, and bottom ash wastewater discharged by each plant 

for use in baseline and post-compliance pollutant loadings calculations. ERG used these wastespecific baseline flow rates to calculate the total baseline flow for each impoundment. ERG 

assumed each impoundment would receive the total volume of any combustion residual waste 

generated that is contained in the impoundment. For example, if an impoundment contains FGD 

wastewater and fly ash transport water, ERG calculated the baseline flow into the impoundment 

as the volume of FGD wastewater generated plus the volume of fly ash transport water 

generated. Additionally, if the plant operates two ponds in series, ERG assumed that the flow 

entering the first pond is equivalent to the flow entering the second pond (if no wastewater is 

sent only to the second pond).

ERG then calculated the reduction in flow into the impoundment for the five regulatory options.

ERG calculated post-compliance flows for each impoundment based on the types of combustion 

residual contained in the impoundment and the effect of the regulatory option. For example, if an 

impoundment contains FGD wastewater and fly ash transport water, for Option 1, the postcompliance flow would include only the fly ash transport water and for Option 3, the postcompliance flow would be zero. ERG then calculated the flow reduction by subtracting the postcompliance flow from the baseline flow for each impoundment.

ERG calculated the percent reduction in flow using the following equation:

Percent Reduction = (Baseline Flow – Post-Compliance Flow)/ Baseline Flow

As stated previously, ERG assumed that the percent reduction in flow is equivalent to the percent 

reduction in impoundment capacity because the impoundment will no longer have to handle or 

treat this percentage of flow.

After calculating the percent reduction in flow, ERG multiplied this percent reduction by the 

baseline volume calculated in Section 2 to estimate the reduced capacity for each impoundment 

under each regulatory option. ERG calculated a reduced capacity for each regulatory option 

using the following equation:

Option Volume = Baseline Volume * Percent Reduction

4.0 RESULTS

The age, baseline capacity, and reduced capacity for each of the 1,070 CCR impoundments 

under the five regulatory options are presented in the document entitled “Estimated 

Impoundment Age, Capacity, and Reductions in Capacity” (DCN SE01826).