1 Date: June 28, 2010

Subject: Minutes of the June 17, 20 10 Conference Call with Representatives from Environmental Groups
EPA Contract No. EP

- D

- 06

- 118; EPA Work Assignment No. 4

- 06 RTI Project No. 0210426.004.006

From: Chuck French OAQPS/SPPD/MMG (D243

- 02) U.S. Environmental Protection Agency Research Triangle Park, NC 27711

To: Docket ID No. EPA

- HQ

- OAR

- 2010

- 0239

I

. Introduction

A conference call with representatives from several environmental groups was held o n June 1 7 , 20 10 . The purpose of the meet ing was to discuss their potential comments to the National Emission Standards for Hazardous Air Pollutants: Gold Mine Ore Processing and Production Area Source Category and Addition to Source Category List for Standards; Proposed
Rule. This memorandum s ummarizes the major topics discussed during the meeting

.

II

. Attendees

Kendra Zamzow, Center for Science in Public Participation Brook Brisson, Clean Water and Mining Program, Northern Alaska Environmental Center Bonnie Gestring, Earthworks Dave Chambers, Ce nter for Science in Public Participation John Hadder, G reat B asin R esource Watch Joel Reynolds, Solutions Statistical Consulting Glenn C. Miller, Department of Natural Resources and Environmental Science, University of Nevada, Reno

Chuck French, U.S. EPA, OAQPS

Jeremy Aldworth, RTI International Marvin Branscome, RTI International Gabrielle Raymond, RTI International

III

. Discussion

The major topics discu ssed during the meeting included the methodology for calculating Maximum Achievable Control Technology ( M ACT ) standard s , the ore pretreatment affected

source , and the carbon processes affected source

. The EPA provided explanations and2 clarifications on topics brought up by the teleconference participants. Several people wanted
clarification on EPA’s basis f or us ing the 99% U pper P rediction L imit calculation for the MACT
standard and using data that had variations within the runs

. The participants questioned why EPA

group ed roasters and autoclaves together in the ore pretreatment group. They mentioned that a
new propos ed mine in Alaska, Donlin Creek, could emit 3,200 lbs o f mercury under the proposed
standard. For the alternative percent removal option for new sources in the carbon processes affected s ource , the y want EPA to look at the M arigold Mine’s perc ent removal rather than
R ound M ountain’s.

Attachment 1 is a summary of the statistical review of the MACT floor analysis submitted by Mr. Reynolds prior to the conference call. Attachment 2 contains topics for discussion for the meeting submitted by Kend ra Zamzow prior to the conference call.Attachment 1Solutions Statistical Consulting 14 June 2010

SolutionsConsulting@ak.net 1 Statistical Review of:

Development of the MACT floors and MACT for the proposed NESHAP for gold mine ore processing and production

Metals & Minerals Group Sector Polices and Programs Division
Of fice of Air Quality Planning and Standards U.S. EPA, RTP, NC. April 14, 2010.

S UMMARY The formulas presented in the document improperly base the MACT floor on a 99%
confidence interval for the population mean emission rate rather than a tolerance interva l for, say, the 99 th percentile of the emission rate distribution. Further, unless the replicate observations (‘runs’) are based on the same amount of input (concentrate, ore) each time,
the unit

- specific and source

- specific estimates of mean emissions ra te and run

- to

- run variation (‘process variation’) in emissions rates should be based on ratio estimators.

S UMMARY OF P ROPOSED S TANDARD According to the document under review:

**•** ‘... the maximum achievable control technology (MACT) standard for existing
affe cted sources must be at least as stringent as the average emissions limitation achieved by sources in the MACT pool

**•** For new sources, MACT standards must be at least as stringent as the c ontrol
level ac hieved in practice by the best controlled similar [existing] affected
source.’ , ’ where the pool is (i) the best performing 12 percent of existing affected sources for which the Administrator has data for categories with 30 or more sources

, or (ii) the best performing 5 sources for source categories with fewer than 30 sources.
Th e level of minimum stringency is called the MACT floor

.

**•** Due to variability in emissions resulting from variation in operating conditions, the floor should be set ‘...at a level that a best performing source can expect to meet “every d ay and under all operating conditions”’.

**•** These objectives are interpreted as leading to the following formula for the
achieved emission limit:

x **+** t d f v T

, where



### x is the average of the (average) total emission rates from each of the best performing sources in the MACT pool,

t df is the ‘99 th percentile of the t distribution with df degrees of freedom, and v T is the total variance for the top performing affected sources.Solutions Statistical Consulting 14 June 2010

SolutionsConsulting@ak.net 2 **•** Each of the terms is broken down into component formulas built up from unit

specific estimat es of (i) mean emissions rates and (ii) variation in emissions rates,

to source

- specific estimates of (i) mean emissions rates and (ii) variation in emissions rates, to the top

- level terms given above.

M AJOR C RITICISMS
1

. Consider the best performing source. Setting the floor ‘...at a level that the best performing source can expect to meet “every day and under all operating conditions”’ requires estimating the distribution of possible emissions rates from that source’s production process (say hourly emission rates, though this temporal scale choice is a decision that needs to be specified). This distribution reflects the process variation in emissions rates arising from variation in the production conditions.
The floor is to be set at a threshold just excee ding the maximum possible emission rate. Unfortunately, this is an impossible standard to quantify. It requires assuming a specific mathematical distribution as an adequate description of the emissions rate distribution. Even then, most common feasible

ma thematical distributions have infinite upper thresholds , hence the floor would logically be set to ∞ , an operationally useless threshold

. 2

. A reasonable approach would be to interpret the language of the standard as requiring a threshold that is only exceeded in ‘standard operating conditions’ 1% or 5% or x% of the time. This requires a decision as to what percentile of the
emissions rate distribution to use as the threshold (e.g., 99%? 95%? 90%?)

. Call the percentile p

. 3

. Then, for that source , the standard requires using the sample data to estimate the unknown value of the p th percentile of the emissio ns rate distribution. Further, you want to account for the uncertainty in this estimate by providing an interval estimate. Specifically, what you want is a tolerance interval (Vardeman 1992). A tolerance interval ‘brackets a portion of the full populat ion distribution, such as the smallest 99% of the population [of emissions rates from this source]. Thus a one

- sided upper 99% tolerance interval with 95% confidence is a threshold expected [with 95% confidence] to exceeded by only 1% (=100%

- 99%) of obser vations from the ...’ emissions rate process (Reynolds and Braman 2009).

4

. Rather than the required tolerance interval, which estimates an upper limit on the feasible observation values, the proposal provides a form of 99% confidence
interval on the mean sourc e

- specific emission rate (the first formula above). These are very distinct interval estimates estimating very different things.

5

. For a new source, the MACT threshold would simply be the upper limit of the pth percentile tolerance interval from the best performing source.Solutions Statistical Consulting 14 June 2010

SolutionsConsulting@ak.net 3 6

. Assuming one did employ an appropriate tolerance interval approach, the language of the written objectives is ambiguous as to how to accommodate both
the source

- specific tolerance interval component and the desire to average

across t he best performing sources.

Question 1: is ‘best performing’ defined with respect to the average emission rate from a source, or with respect to the tolerance interval on the emission rates of that source (which would account for differences in both mean emissions rate and process variation in the emissions rate at a source)?

Question 2: How should one ‘average’ the emission rates from the best

performing sources? Option 1: take the 5 th best performing source and use its source

- specific
tolerance interval as the floor. Option 2: calculate tolerance intervals for each source in the MACT pool, take
the 5 th smallest upper limit as the floor. Option 3: calculate tolerance intervals for each source in the MACT pool, take the average of the upper limits. Option 4: pool the process variation information from across sources (equivalent to assuming that all sources share the same shaped distribution of emission rates but that is simply centered at different mean rates), use as the basis for a
tolerance interval cent ered on the 5 th smallest mean emission rate. Option 2 seems closest to the intention of the description. Option 4, if the assumption is tenable, would provide the most precise estimate of the desired
threshold.

7

. The observations are rates constructed by dividing two measurements (lbs mercury emitted, tons ore or concentrate processed). Such data structures are most appropriately summarized using ratio estimators (Cochran 1977). The

current unit

- specific and source

- specific estimation formulas for the em ission rate mean and variance are not correct unless the input quantity (ore or concentrate) is held constant as part of the observation process.

8

. It is unclear why the higher level averages are in all cases taken to be equally weighted averages of the lower level estimates: unit

- type means from unit

specific means, source means from unit

- specific means, mean total emission rate
from source

- specific emissions rates. It seems reasonable that some of these should be weighted means: e.g., weighting the uni t

- specific mean emission rates by each unit’s total input processed (divided by the sum of input processed across units) to properly account for variation among units in the tons of material processed (and hence variation in the total emissions from eachSolutions Statistical Consulting 14 June 2010

SolutionsConsulting@ak.net 4 s ource). I.e., treat the estimation as a form of stratified sample, with each unit (or unit type or source) as a strata.

9

. This would require different variance estimators as well.

10

. In effect, it seems like the proper approach would be to use a stratified rat io

estimator.

O THER C OMMENTS
1

. The second term in the formula for v T is unclear and appears erroneous (the first term gives the pooled variance for the average total emissions across sources, assuming all units within a sources and all sources vary indepen dently). I’ve not pursued it, however, as the whole interval is in error as shown by my comments
above.

2

. The formula assumes normality of emission rates. I’ve not checked the data to see if this is a tenable assumption.

Cochran, W.G. 1977. Sampling Tec hniques (3 rd ed). Wiley, New York.

Reynolds, J.H. and N. Braman. 2009. Using tolerance intervals to assess recovery of mussel beds impacted by the Exxon Valdez oil spill. Marine Pollution
Bulletin 58, 1496

- 1504.

Vardeman, S.B. 1992. What about the oth er intervals? The American Statistician 46,
193

- 197lAttachment 2From: Kendra Zamzow <kzamzow@csp2.org>

To: Chuck French/RTP/USEPA/US@EPA

Cc: Brook Brisson < Brook@northern.org >, Bonnie Gestring

< bgestring@earthworksaction.org >, Dave Chambers

< dchambers@csp2.org >, John Hadder < john@gbrw.org >, Joel Reynolds dba SOLutions Consulting

< s olutionsconsulting@ak.net >, Glenn C Miller

< glennm@unr.edu >

Date: 06/16/2010 01:16 PM

Subject: Topics for discussion at tomorrow's teleconference, proposed Hg rulemaking

Dear Mr. French: Here are some topics I believe would benefit from some discussion
tomorrow:

1. Separation of roasters and autoclaves in the "pretreatment" category (create 2 different categorie s) 2. Definition and treatment of "outliers" in runs used to develop MACT floor and also in future runs used in monitoring 3. Removal of Twin Creeks in the development of MACT floor for ore pretreatment We ranked all of the sources for which we had valid data representing current performance based on their emissions and identified the lowest emitting of the existing affected sources, which included four facilities with ore pretreatment processes, 11 facilities for those with carbon processes, and two faci lities with the non

- carbon concentrate processes. (page 3 "Development of MACT floors, technical memo) 4. Disposal issues (location/cost/ETA for repositories, disposal of carbon filters, keeping material out of tailings ponds) 5. Donlin

Thank you for your time and consideration,

-- Kendra Zamzow, PhD Center for Science in Public Participation PO Box 54 Sutton, Alaska 99674
907.745.3882 office
907.354.3886 cell