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MEMORANDUM

SUBJECT: Determination of Representative Detection Level (RDL) and 3 X RDL Values for 

Mercury Measured Using Sorbent Trap Technologies 

FROM: Conniesue B. Oldham, Ph.D., Group Leader 

Measurement Technology Group, AQAD 

TO: Bob Schell, Group Leader 

Measurement Policy Group, SPPD 

This memorandum describes the process that was used to develop a representative detection 

level (RDL) as well as 3 X RDL values for mercury measured using sorbent trap technology. We 

based these values on information in the record concerning the best performing electric utility 

steam generating units (EGUs) designed for coal with a heating value greater than or equal to 

8,300 BTU/lb. As explained further below, we have developed the RDL and 3XRDL values 

assuming short sampling periods so that compliance monitoring methods could provide timely 

data for operation of mercury control systems. 

General Method for Determining RDLs

As explained in the note from Peter Westlin and Raymond Merrill to SPPD Management and 

MACT Rule Writers, we determine the representative detection level (RDL) based on the 

average of the reported pollutant specific method detection levels (MDLs) for the best 

performing units (those in the floor); we consider the resulting average MDL representative or 

characteristic of acceptable performance of source emissions testing and thus an RDL.1 We use a 

multiplication factor of three with the RDL to increase the RDL pollutant concentration to a level 

where, when measured by the compliance test method, the precision of the test method 

approximates that of other EPA methods as estimated by the ASME study, nominally 10 to 20 

percent relative standard deviation.2,3 This three times the RDL (3 X RDL) value expressed in 

units of the emission standard is then compared to the MACT floor value to ensure that the 

resulting emission limit is in a range that can be measured with reasonable precision. In other 

                                                           

1

 Note from Peter Westlin and Raymond Merrill to SPPD Management and MACT Rule Writers 

“Data and procedure for handling below detection level data in analyzing various pollutant 

emissions databases for MACT and RTR emissions limits,” revised April 5, 2012. 

2

 (ReMAP): PHASE 1, Precision of Manual Stack Emission Measurements; American Society of 

Mechanical Engineers, Research Committee on Industrial and Municipal Waste, February 2001. 

3

 The factor of three used in the 3XRDL calculation is based on a scientifically accepted 

definition of level of quantitation – simply stated, the level where a test method performs with 

acceptable precision. The level of quantitation has been defined as ten times the standard 

deviation of seven replicate analyses of a sample at a concentration level close to the MDL 

(which translates to approximately three times the MDL which is defined as three times the 

standard deviation of seven replicate analyses of a sample at a concentration level close to the 

MDL (40 CFR Part 136, Appendix B)). 
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words, if the 3 X RDL value were less than the calculated floor or emissions limit (e.g., 

calculated from the UPL), we would conclude that measurement variability has been adequately 

addressed; if it were greater than the calculated floor or emissions limit, we would adjust the 

emissions limit to comport with the 3 X RDL value to address measurement variability. 

Concerns Raised By Petitioners

With regard to the mercury emission limit for new EGUs designed for coal with a heating value 

greater than or equal to 8,300 BTU/lb, some petitioners asserted that the frequency of 

measurement contemplated for the sorbent trap monitoring system option in the final rule may 

not provide sufficiently timely feedback for operation of EGUs and mercury control systems. 

The petitioners also suggest that the mercury emission limit for new EGUs designed for coal 

with a heating value greater than or equal to 8,300 BTU/lb be adjusted upward to allow for 

compliance monitoring using Hg CEMS, which would provide the frequency of feedback needed 

to operate the process and control systems. 

We understand the petitioners’ concerns regarding timely feedback on process and control 

systems. In response to these concerns, EPA evaluated the sorbent trap monitoring systems and 

assessed whether these systems could be utilized over the shorter time periods petitioners 

maintain are needed to provide more timely data.4

 In promulgating the MATS rule, we thought 

that most facilities would opt for 1 to 2 week sampling periods when operating sorbent trap 

monitoring systems, but we now understand the need for more frequent data necessitating much 

shorter sampling periods. With shorter sampling periods approaching the frequency of CEMS, 

comes a need to examine the MDL/RDL for sorbent trap-based mercury measurements which 

was more than adequate for the sampling periods originally contemplated for sorbent trap 

monitoring systems. Though we didn’t use the sorbent trap monitoring systems to collect data 

under the ICR, the technology used in the sorbent trap monitoring systems is identical to that of 

the short term method for mercury, Method 30B and, thus, it is reasonable to rely on the MDL 

information provided for the Method 30B testing conducted under the ICR to develop an RDL 

value for the sorbent trap monitoring systems. 

A few petitioners raised an issue with Hg CEMS, suggesting that we adjust the limit to allow the 

use of Hg CEMS to demonstrate compliance. However, part III of the ICR required testing for 

mercury using Method 30B, a sorbent trap-based measurement method and, thus, there were no 

Hg CEMS data produced as a result of the ICR testing requirements. Though some Hg CEMS 

data for mercury were submitted under the ICR, none of the best performing units in the floor 

provided us with any Hg CEMS data. This meant that for the MACT floor, we had no CEMS 

data that we could use to establish the level at which measurement precision for the Hg CEMS 

would be in a range similar to that of other EPA test methods; therefore, we had no way to adjust 

the new source emissions limit based on the use of Hg CEMS. 

                                                           

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 We note that we continue to believe that the final Hg limit can be measured using a sorbent trap 

monitoring system, but we acknowledge the monitoring frequency concern raised in 

reconsideration petitions. 
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Determining RDLs for Sorbent Traps

We reviewed all available Method 30B sorbent trap testing and laboratory information for each 

EGU contained in the pool of best performing units designed for coal with a heating value 

greater than or equal to 8,300 BTU/lb and collected the reported method detection level for each 

EGU. As shown in Table 1 below, 24 out of 43 best performing EGUs reported MDL values. We 

calculated the arithmetic average of these available MDL values to yield the representative 

detection level (RDL). 

The calculated average RDL of 1.1 nanograms of mercury was converted into the format of a 

new source emissions limit in pounds per gigawatt-hour (lb/GWh) by making appropriate unit 

conversions, dividing by the sampling rate in liters per minute (L/min), dividing by the sampling 

collection time in minutes (min), multiplying by a representative heat rate (11,180 BTU/KWh),5

and multiplying by 1.40 (a factor correcting the oxygen concentration to 6 percent). That 

product, when multiplied by 3 (in order to obtain the 3XRDL value) and rounded up, is 3.0E-03 

lb/GWh. 

The calculation is as follows: 

3ܴܺܮܦ ൌ 3 כ ቈቂቀଵ.ଵ ௡௚ · ௠௜௡

଴.ହ ௅ ቁ ቀ ଵଵ,ଵ଼଴ ஻்௎ଷ଴ ୫୧୬ · ௄ௐ௛ቁ ሺ1.40ሻቃ ቀ଺.ଵଵாି଴଻ ௟௕ · ௅ · ௄ௐ௛௡௚ · ஻்௎ · ீௐ௛ ቁ቉, 

and, when rounded, = 3.0ܧ െ 03 ௟௕

ீௐ௛ . 

A sampling rate of 0.5 L/min was used in the calculation, and it is based on an informal survey 

of vendor information on the normal sampling rates for commercially available sorbent trap 

monitoring systems. To provide facilities with monitoring data as timely as that for Hg CEMS, 

we first considered use of a 15-minute sampling period with the sorbent trap monitoring systems. 

However, about 10 minutes are required to change out and leak check each new pair of sorbent 

tubes, leaving only 5 minutes of actual sample collection time to represent each 15-minute 

sampling period, so a 15-minute period was deemed inappropriate. We settled on a 30-minute 

sampling period (comprised of a 10-minute sorbent tube change out and a 20-minute sample 

collection period) as a reasonable compromise. 

 

                                                           

5

 This is the average heat rate of the units used to determine the RDL value. 
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Facility ID EGU MDL, nanograms 

3130 Unit 1 0.293 

3130 Unit 2 0.293 

7213 Unit 1 0.81 

10043 Boiler 1 0.81 

10143 Boiler 1 0.293 

10566 Boiler 1 0.81 

10566 Boiler 2 0.81 

10603 31 0.172 

10671 Unit 1a 2.4 

10671 Unit 1b 2.4 

10671 Unit 2a 2.4 

10671 Unit 2b 2.4 

10673 Boiler A 2.4 

10673 Boiler B 2.4 

10678 Boiler 1 2.4 

50776 Boiler 1 0.25 

50888 Boiler 1 0.81 

50974 Boiler 1 0.174 

50974 Boiler 2 0.174 

50976 B1 0.062 

54081 Unit 1 0.52 

54081 Unit 2 0.81 

54081 Unit 3 0.81 

54081 Unit 4 0.81 

Average (or RDL) 1.1 

Table 1. Mercury Sorbent Trap Method Detection Levels for Best Performing EGUs