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BECKHAM, LISA

From: Lee, Anita

Sent: Friday, June 13, 2014 11:19 AM

To: Lusk, Joel

Cc: McKaughan, Colleen; Lyons, Ann; Richard.Grimes@aps.com

Subject: FW: Response to Rain pH question from UFW

Attachments: Copy of pH_calcs_precip.xlsx

Hi Joel, 

Per the voice mail I just left for you, attached is the pH analysis from APS and AECOM in response to question 4 of your 

email dated 5/29/14. As you see in Richard’s email below, the analysis AECOM did was conservative for three major 

reasons, and the result was an estimated pH of 4.6, which is close to your estimate of approximately 5. 

As I mentioned in my voice mail to you, I will be out of the office on work-related travel next M-W and out on leave ThFri. If you have any questions on the pH analysis or the BA, please contact Richard Grimes directly (and please cc me if 

connect via email). If you have other process-related questions, please contact Colleen McKaughan or Ann Lyons (cc’ed 

above). 

Thank you, and have a great weekend! 

Anita 

Anita Lee, PhD 

Environmental Scientist 

US EPA, Air Division, Planning Office (Air-2) 

75 Hawthorne Street 

San Francisco, CA 94105 

(415) 972-3958 

From: Richard.Grimes@aps.com [mailto:Richard.Grimes@aps.com] 

Sent: Friday, June 13, 2014 9:04 AM 

To: Lee, Anita 

Subject: Response to Rain pH question from UFW 

Recent data from the National Atmospheric Deposition Program (NADP) 2012 summary 

(http://nadp.sws.uiuc.edu/lib/data/2012as.pdf) displays an annual average pH of 5.5 for precipitation in the northwestern 

part of New Mexico (to the east of the project area). A conservative estimate of rain pH could be made by using a pH of 

5.5 as a starting point and adding the hydronium ion concentration from modeling to the annual rainfall without 

considering any buffering from particulates. If we assume that the average annual precipitation for the Farmington, NM, 

area is 8.59 inches per year* (21.8 cm/yr) with a starting precipitation pH of 5.5, then add the total hydronium ion 

equivalents deposited on the soil (worst case of 0.0047 hydronium ion equivalents equivalents/m2/yr based on modeled 

deposition), the resulting pH suppression would be 0.9 (or a change in pH value by -0.9) for a precipitation pH of 4.6. 

This is conservative because: 

· It does not account for buffering that occurs in the precipitation. Water droplets in the air, especially in the west, form 

around and collect dust particles that are often composed of calcium compounds and other buffering agents before 

coming to the earth as precipitation. This causes pH buffering resulting in a higher pH. 

· This calculation includes the deposition of all acidic deposition form Four Corners Power Plant not just the increase in 

H2SO4 emissions resulting from the installation of SCR. 
2

· This calculation assumes all acid deposition is wet deposition. In reality, only a fraction of the acid deposition is wet 

deposition due to limited rainfall in the area. Therefore the impact on precipitation pH would be much less. 

 

Given that the total amount of acid-forming compounds emitted from the FCPP will be much less in the future due to the 

shutdown of Units 1, 2, and 3, the future impacts from acidic compounds deposited in the region will be less than historic 

conditions. The small amount of acid-forming compounds coming from FCPP would not result in anything close to 

precipitation of pH 3.0 reported by Neufeld to cause foliar damage to plants. 

 

* http://www.usclimatedata.com/climate/farmington/new-mexico/united-states/usnm0109

 

 

 

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