MEMORANDUM 

TO: Steam Electric Rulemaking Record 

FROM: TJ Finseth, ERG 

DATE: April 19, 2013 

SUBJECT: Sensitivity Analysis Evaluating the Effect of Nondetect Results in the Ash 

Loadings Model 

ERG is providing technical support to the U.S. Environmental Protection Agency’s Office of 

Water, Engineering and Analysis Division (EAD) for development of effluent limitations 

guidelines and standards (ELG) for the Steam Electric Power Generating Industry. EPA, with 

the support of ERG, evaluated wastewater flow rates and pollutant concentration data from 

survey responses, site visits, sampling episodes, trade associations, and previous rulemakings to 

estimate pollutant discharge loadings associated with the discharge of fly ash and/or bottom ash 

transport water. In addition, EPA estimated the corresponding pollutant removals associated 

with installing and operating dry or closed-loop ash handling technologies and practices. ERG 

developed a computerized loadings model to calculate plant-specific pollutant discharge loadings 

for all power plants reported to be discharging fly ash and/or bottom ash transport water. 

The purpose of this memorandum is to describe the sensitivity analysis that ERG performed to 

evaluate the effect of nondetect results used in the ash loadings model. 

 

1.0 ASH LOADINGS DATA SOURCES

As described in Section 10.2.2 of EPA’s Incremental Costs and Pollutant Removals for 

Proposed Effluent Limitation Guidelines and Standards for the Steam Electric Generating Point 

Source Category (DCN SE01957 and SE03581), EPA used the following data sources to 

characterize the effluent stream from ash impoundments at steam electric power plants: 

 EPA ash impoundment sampling data from the detailed study (DCN SE00003); 

 Electric Power Research Institute (EPRI) Power Plant Integrated Systems-Chemical 

Emissions Study (PISCES) Reports (DCNs SE01818-SE01823); 

 Permit application data, as provided by member companies of the Utility Water Act 

Group (UWAG) (DCNs SE02918 and SE02918A1); 

 Development Document for Final Effluent Limitations Guidelines, New Source 

Performance Standards, and Pretreatment Standards for the Steam Electric Point 

Source Category, EPA 440-1-82-029, November 1982 (1982 TDD) (DCN SE02933); 

and 

 The Steam Electric Technical Questionnaire Database (DCN SE01958 and SE03580). 
Memorandum 

April 19, 2013 

Page 2 

Additional details regarding each of these five data sources are included in Section 3 of EPA’s 

Incremental Costs and Pollutant Removals for Proposed Effluent Limitation Guidelines and 

Standards for the Steam Electric Generating Point Source Category. 

As stated in Section 10.2.2 of EPA’s Incremental Costs and Pollutant Removals for Proposed 

Effluent Limitation Guidelines and Standards for the Steam Electric Generating Point Source 

Category, EPA reviewed the data representing ash pond effluent streams and, as appropriate, 

made some substitutions to the data sets. For data collected during the detailed study, EPA set 

nondetected values1 equal to one-half the quantitation limit (EPA required their analytical 

laboratories to report the sample-specific quantitation limit). For the other data sets (i.e., EPRI, 

1982 TDD, and Form 2C), EPA could not confirm whether the results for nondetected values 

were presented as less than the quantitation limit or less than the detection limit (or some other 

value); therefore, EPA set nondetected values equal to the value provided in the data source (e.g., 

<2 μg/L became 2 μg/L). 

ERG performed a sensitivity analysis to evaluate how the baseline ash loadings would change if 

EPA handled nondetected values reported in the EPRI, 1982 TDD, and Form 2C data differently. 

ERG did not evaluate changes to the ash loadings based on the treatment of the nondetected 

values in the dataset associated with EPA’s ash impoundment detailed study sampling data 

2.0 METHODOLOGY FOR SENSITIVITY ANALYSIS

To evaluate the change in the baseline ash loadings based on the treatment of the nondetected 

values reported in the EPRI, 1982 TDD, and Form 2C data, ERG calculated the baseline loadings 

based on the following three scenarios: 

 Use the reported value for all nondetected values (i.e., methodology used for 

proposed rule); 

 Use one-half the reported value for all nondetected values; and 

 Use zero for all nondetected values. 

To perform these calculations, ERG created two new versions of the Steam Electric Pollutant 

Loadings Model for Ash Transport Water (DCN SE02899). In one version Steam Electric 

Pollutant Loadings Model for Ash Transport Water Sensitivity Analysis – One-Half Nondetected 

Values (DCN SE03882.A2), ERG changed all the nondetected values associated with the EPRI, 

1982 TDD, and Form 2C data equal to one-half the reported value. In the second version Steam 

Electric Pollutant Loadings Model for Ash Transport Water Sensitivity Analysis – Zero 

Nondetected Values (DCN SE03882.A3), ERG changed all the nondetected values associated 

with the EPRI, 1982 TDD, and Form 2C data equal to zero. These sensitivity analysis models 

are included as Attachments 2 and 3 to this memorandum. After updating the nondetected values, 

ERG ran the loadings models to calculate the baseline pollutant loadings associated with fly ash 

and bottom ash transport water. Other than the changes to the nondetected pollutant 

concentrations, no other changes were made to the models. 

 

1

 To simplify the discussion, for the purpose of this memorandum, the term “non-detected value” is used to refer to 

both values measured below the quantitation limit and those values measured below the detection limit. 
Memorandum 

April 19, 2013 

Page 3 

3.0 SENSITIVITY ANALYSIS RESULTS

Table 1 presents the results of the sensitivity analysis. The results of the sensitivity analysis at 

the pollutant level can be found in Attachment 1 to this memorandum. As part of this sensitivity 

analysis, ERG also looked at the change to the baseline ash loadings if sulfide was excluded 

from the ash loadings. Table 2 presents the results of the sensitivity analysis if sulfide was 

excluded from the ash loadings. 
Memorandum 

April 19, 2013 Page 4 

Table 1. Fly Ash and Bottom Ash Baseline Loadings Sensitivity Analysis Results (Including 

Waste Stream 

Proposal Methodology: 

Nondetected Values Treated as 

Reported Value 

Nondetected Values Treated as One-Half Reported 

Value Nondetected Valu

Loading 

(Pounds/yr) TWPE/yr 

Loading 

(Pounds/yr) TWPE/yr 

Percent Change 

in TWPE 

Compared to 

Proposal 

Loading 

(Pounds/yr) TW

Analysis Including Sulfide Fly Ash 927,079,982 2,521,986 925,164,080 1,729,929 -31% 923,248,177 Bottom Ash 2,997,297,020 2,716,059 2,993,891,100 1,721,910 -37% 2,990,485,179 

Table 2. Fly Ash and Bottom Ash Baseline Loadings Sensitivity Analysis Results (Excluding 

Waste Stream 

Proposal Methodology: 

Nondetected Values Treated as 

Reported Value 

Nondetected Values Treated as One-Half Reported 

Value Nondetected Valu

Loading 

(Pounds/yr) TWPE/yr 

Loading 

(Pounds/yr) TWPE/yr 

Percent Change 

in TWPE 

Compared to 

Proposal 

Loading 

(Pounds/yr) TW

Analysis Excluding Sulfide Fly Ash 926,531,845 986,408 924,857,563 871,238 -12% 923,183,281 Bottom Ash 2,996,730,172 1,128,065 2,993,602,350 912,993 -19% 2,990,474,527