Memorandum

TO: Portland Cement NESHAP Docket

From Keith Barnett, US EPA

Subject: Intra-quarry Variability Estimate for Mercury

DATE: July 21, 2010

A kiln is tied to its quarry. In assessing kilns’ performance for mercury, we have 30 

days of limestone data - that means data from the parts of the quarry which were mined and 

utilized during this 30 day period. Variability of inputs beyond this 30-day period can be and 

was estimated by EPA using statistical extrapolations. EPA also received information from 

commenters concerning intra-quarry mercury variability, and EPA has studied other means 

(beyond statistical extrapolations), of assessing intra-quarry mercury variability based on the 

data presented by the commenters. Specifically, EPA evaluated whether mercury variability in 

the limestone quarries of two of the best performing kilns could be further informed by the 

mercury variability of nearby limestone quarries which are part of the same geologic limestone 

formation.

Kilns #s 1248 and 1259 are among the best performing kilns for mercury. They share a 

common quarry. Kilns #1239, 1376, 1688 and 1690 also quarry limestone from the same 

limestone geologic formation as kilns #s 1248 and 1259. All six of these kilns are located in or 

near Midlothian, Texas. Midlothian sits atop the Austin Escarpment, a thick limestone

(calcium carbonate, CaCO3) formation inclined to form a ridge that runs Northeast to 

Southwest across Northern Texas. The Austin Escarpment was formed during the Cretaceous 

Period during which the ocean rose and fell in multiple episodes. This resulted in the deposit of 

multiple layers of limestone deposits interspersed with sand and clay layers having 

compositions following the environment in which they formed: shallow marine, deltaic, and 

coastal. The limestone layers of the Austin escarpment is thought to have formed in clear, 

warm, shallow marine waters, from the accumulation of shell and coral debris. The Austin 

escarpment, also called the Austin chalk formation, is an unaltered and unmetamorphosed 

limestone deposit1

, so the mercury present is likely a relic of the mercury content of the 

environment during which it formed. That is to say that mercury content of the ocean during the 

Cretaceous Period was influenced by volcanic activity, seafloor spreading, and other geological 

processes. The marine organisms living in that environment accumulated mercury in proportion 

to the level in the environment and their lifespan. As these marine organisms died and added to 

the sediment deposition of what is now the Austin chalk (limestone) formation, the mercury 

from those organisms is incorporated into that deposit. Not surprisingly, the level of mercury in 

this unaltered sedimentary formation is very low. 

 1 Dallas Paleontological Society. Surface Geology of Dallas and Tarrant Counties, Texas. Phil Scoggins,. 2010. 10 

pages. http://www.dallaspaleo.org/details/surface_geology.htm. 
The variations in mercury concentrations within an unaltered sedimentary limestone 

deposit such as the Austin chalk are thought to be primarily from variations in the duration of 

the shallow marine environment and the mercury levels of the oceans during the Cretaceous 

period in which these deposits were formed. So although different samples of limestone from 

this formation may vary in mercury content from one sample to another, they should all be 

within a low range. In addition, one would not expect a geogenic (that is, original to the rock) 

high mercury level to occur within this particular limestone deposit. 

Based on this information we believe that the mercury variability – the variability, not 

the level -- of all three quarries is identical. Therefore, mercury data from the three quarries can 

be used to estimate the quarry mercury variability in any one of these quarries (specifically the 

quarry serving the two MACT floor pool kilns - 1248 and 1589) over time. The limestone is 

influenced by the time when it is formed. Since the amount of mercury being deposited in any 

point in time will vary, you would expect that the mercury level in these quarries varies

depending on the level of the strata they are currently mining. Therefore, one way to estimate 

this variance is to look at these three quarries and determine if their mercury levels vary, and if 

so, the extent of that variability.

For each facility we have the following information:

Table 1. Quarry Mercury Information for Midlothian

As noted, kilns 1248 and 1589 are floor kilns (due to controls). Kilns 1248 and 1589 are the 

same facility; kilns 1376, 1688, and 1690 are the same facility. The distance between these 

facilities ranges from 2 to 9 miles. (See attached Google Earth map; the placemarks denote kiln 

locations.)

The three averages range from 0.059 to 0.021. The low ends of the ranges are similar, 

but the high end of the ranges varies, with 1239 being the highest and 1248 being the lowest. In 

order to estimate the variability based in this information, we calculated an average of the three 

quarries, and the average relative standard deviation. This resulted in an estimated variance of 

0.38. This variance would be added to the calculated variance based in daily emissions to 

incorporate this variance over time.

We have additional information for a second location where there are two facilities 

(three kilns). There facilities are next to each other.
Table 2. Quarry Mercury Data for Mason City

Kilns 1246 and 1316 mercury levels were almost all below the detection limit. 

Apparently the detection limit for kiln 1356 was lower, so it is more difficult to discern the 

differences here. For this reason, data from this location was not used in this analysis. A 

Google Earth map of this location is attached. The untitled placemarks denote kiln locations.

In order to incorporate this information into the floor calculation we calculate the 

relative standard deviation of the three quarries as shown in Table 3.

Table 3. Calculation of the Relative Standard Deviation for the Three Midlothian 

Quarries.

Quarry

Limestone Concentration

(ppm)

1 0.059397

2 0.020893

3 0.027433

Average Hg Content of three Quarries 0.035908

Std Deviation of Averages 0.020603

Relative Standard Deviation 0.573779

.
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Midlothian is located close to the highest elevation in Ellis County atop the Austin Escarpment, a thick 

chalk ridge that runs in a northeast to southwest direction. The Austin Escarpment through Midlothian 

represents a 600-year potential limestone reserve that has attracted three cement plant operations.