Approval and Promulgation of Implementation Plans; State of New Mexico; Grant County 

Sulfur Dioxide Limited Maintenance Plan

Technical Support Document

June 2014

OVERVIEW

The EPA designated a portion of Grant County, New Mexico as a nonattainment area for 

the 1971 sulfur dioxide (SO2) national ambient air quality standard (NAAQS) on September 11, 

1978 (43 FR 40412). The area that was designated nonattainment is located within the Air 

Quality Control Region (AQCR) No. 012, and consists of a 3.5 mile radius surrounding the 

former Kennecott Copper Corporation (now called the Chino Mines-Hurley Facility) and land 

above 6470 feet Mean Sea Level within an 8 mile radius of the of the Hurley Smelter, which is 

part of the Chino Mines-Hurley Facility in Hurley, New Mexico. New Mexico determined that 

emissions from the Hurley Smelter were the only known source of SO2 in the county and had 

caused the violations of the SO2 NAAQS that resulted in the area being designated 

nonattainment. 

New Mexico’s request for redesignation of the Grant County nonattainment area to 

attainment and the first 10-year maintenance State Implementation Plan (SIP) were approved by 

EPA on September 18, 2003 (68 FR 54672). The maintenance plan demonstrated that the area 

was expected to stay in attainment of the 1971 SO2 NAAQS for the initial maintenance period 

through 2015. The Clean Air Act, Section 175A requires that eight years after redesignation, the 

State must submit an additional revision of the SIP for maintaining the 1971 SO2 NAAQS for 

the second 10-year period. On November 1, 2013, New Mexico submitted a Limited 

Maintenance Plan for the Grant County SO2 maintenance area to fulfill the Act’s requirement to 

submit a revision to the SIP for maintaining the standard for the second 10-year period. The 

Limited Maintenance Plan SIP revision demonstrates that the area is expected to stay in 

attainment of the 1971 SO2 NAAQS through 2025. The revision also requests to discontinue the 

only SO2 monitor in the Grant Country maintenance area (the Hurley monitor, AQS ID 35-017-

0003-42401-1) in light of the negligible SO2 concentrations measured at the monitor, and to 

implement an alternative SO2 monitoring methodology in its place.

The notice of public comment period and public hearing was published in the Silver City 

Daily Press and Independent on July 31, 2013. The public was also notified by posting the 

announcement in the New Mexico Register on July 31, 2013. NMED hosted an informational 

open house on the proposed SIP revision on September 4, 2013. A public hearing was held on 

October 8, 2013. The only written comments received by NMED during the public comment 

period were those from EPA Region 6. 

CRITERIA FOR LIMITED MAINTENANCE PLAN OPTION

Since there are few specific content requirements defined in section 175A of the Act for 

maintenance plans, EPA has exercised its discretion in determining the required content. EPA 
also has a Limited Maintenance Plan policy in effect for areas that can demonstrate consistent air 

quality at or below 85% of the NAAQS for certain criteria pollutants, and our past precedent 

with respect other criteria pollutants is useful in assessing whether such an option is justifiable 

and appropriate in the case for the SO2 NAAQS. More specifically, EPA has developed 

guidance memoranda on a Limited Maintenance Plan approach specific to the ozone, particulate 

matter with an aerodynamic diameter less than 10 microns (PM10), and carbon monoxide 

NAAQS1:

 “Limited Maintenance Plan Option for Nonclassifiable Ozone Nonattainment Areas” 

from Sally L. Shaver, Office of Air Quality Planning and Standards, dated November 16, 

1994.

 “Limited Maintenance Plan Option for Nonclassifiable CO Nonattainment Areas” from

Joseph Paisie, Office of Air Quality Planning and Standards, dated October 6, 1995.

 “Limited Maintenance Plan Option for Moderate PM10 Nonattainment Areas” from 

Lydia Wegman, Office of Air Quality Planning and Standards, dated August 9, 2001.

EPA has not issued guidance specific to the SO2 NAAQS, but we nevertheless have the 

authority to exercise our reasonable discretion and conclude that a limited maintenance plan 

option would be justifiable and appropriate in this case for the SO2 NAAQS. New Mexico has 

opted to develop a Limited Maintenance Plan for the Grant County SO2 maintenance area to 

fulfill the second 10-year maintenance plan required by the Act. Taking the approach presented 

in past guidance memoranda, a limited maintenance plan is available as an option to States who 

have demonstrated that the design values for SO2 in the maintenance area are at, or below, 85 

percent of the NAAQS. As considered here, we believe it is sensible to use an 85 percent 

benchmark for the 24-hour SO2 NAAQS, which corresponds to 0.119 parts per million (ppm). . 

There is currently only one monitor located in the Grant County maintenance area, the Hurley, 

New Mexico monitor (AQS ID 35-017-0003-42401-1). This monitor has been operating since 

1997 and EPA determined in a letter to NMED dated August 26, 2002, that the Hurley monitor 

was placed where modeling indicated the highest SO2 concentration was likely to occur. For this 

submission, the state provided data showing that the SO2 design value for the 24-hour SO2 

NAAQS (0.14 part per million (ppm)) has been 0.0 ppm for each of the five most recent years 

(2007-2011) for which certified ambient air quality data is available for the Hurley monitor. This 

data is presented in Table 1. The design values are significantly below the 85% threshold, 

making the Grant County maintenance area suitable for a Limited Maintenance Plan option. The 

area’s design value is significantly below the NAAQS, and it is reasonable to expect that there 

will not be NAAQS violations when taking account of all anticipated emissions. 

 

1 Copies of these guidance memoranda can be found in the docket for this proposed rulemaking. 
Table 1. SO2 Monitoring Data for Hurley Monitor 35-017-0003-42401-1

YEAR

24-hour SO2 NAAQS

DESIGN VALUE

(ppm)

2007 0.00

2008 0.00

2009 0.00

2010 0.00

2011 0.00

MAINTENANCE PLAN ELEMENTS

1. Attainment Emissions Inventory

The State’s submittal includes an emissions inventory to identify the level of emissions in 

the maintenance area that is sufficient to attain the NAAQS. The inventory should represent 

emissions during the same five-year period associated with air quality data used to determine 

whether the area meets the applicability requirements of the limited maintenance plan option. 

New Mexico’s Grant County Limited Maintenance Plan submittal includes an SO2 

emissions inventory for Grant County Title V sources and Minor Point sources for the years 

2007-2011 (see Table 2). These base years represent the most recent emissions inventory data 

available and are consistent with the data used to determine applicability of the limited 

maintenance plan option (i.e., design values at, or below, 85 percent of the 24-hour SO2 

NAAQS). 

Table 2. SO2 Emission Inventory for Grant County Title V Sources

YEAR ACTUAL EMISSIONS

(SO2 tpy)

2007 4.14

2008 0.82

2009 4.66

2010 5.70

2011 6.57

The former Hurley Smelter, which is the source that caused the violations of the SO2 

NAAQS that resulted in the area being designated nonattainment, was dismantled and its stacks 

removed in July 2006. The Hurley Smelter was a copper ore smelter that was part of the Chino 
Mine-Hurley Facility located in Hurley, New Mexico.

2 The former Hurley Smelter was the only 

major source of SO2 located within the maintenance area boundary. There are currently no major 

sources of SO2 located in the Grant County maintenance area. There are currently two Title V 

sources in Grant County (both located outside of the Grant County maintenance area): the Chino 

Mine and the Tyrone Mine. As shown in Table 2, the combined actual SO2 emissions for the two 

sources has been no more than 6.57 tons per year (tpy) for each of the years 2007-2011. The 

primary sources of SO2 emissions for both facilities are blasting fugitives and diesel generator 

engines. There are currently four minor point sources located within the Grant County

maintenance area with combined annual allowable SO2 emissions of 316 tpy for each of the 

years 2007-2011. However, the actual SO2 emissions generated by these sources are minimal. 

This data supports New Mexico’s conclusion that the control measures contained in the original 

attainment plan will continue to protect and maintain the 1971 SO2 NAAQS in the Grant County 

maintenance area. The New Mexico submittal supports a finding that the requirement for an 

attainment emissions inventory is satisfied.

Table 3. SO2 Emissions Inventory for Grant County Minor Point Sources

YEAR

ANNUAL ALLOWABLE

EMISSIONS

(SO2 tpy)

2007 316.30

2008 316.30

2009 316.34

2010 316.34

2011 316.34

2. Demonstration of Continued Attainment

To adequately ensure maintenance over the maintenance period, EPA has supported 

demonstrations with monitoring data that can show the area is significantly below a given 

NAAQS. In this case, the plan demonstrates the area meets air quality criteria conventionally 

applied for limited maintenance areas (i.e., design value at or below 85% of the 24-hour SO2 

NAAQS, or 0.119 parts ppm). There is no requirement to project emissions over the maintenance 

period. Instead, EPA has stated that if an area is at or below 85 percent of exceedance levels, the 

air quality along with the continued applicability of PSD requirements, any control measures 

already in the SIP, and Federal measures, should provide adequate assurance of maintenance 

over the remainder of the 10-year maintenance period. When EPA approves a limited 

maintenance plan, EPA is concluding that an emissions budget may be treated as essentially not 

constraining for the length of the maintenance period because it is unreasonable to expect that 

such an area will experience so much growth in that period that a violation of the SO2 NAAQS 

would result.

 

2 The Chino Mine-Hurley Facility modified its Title V and New Source Review (NSR) permits to remove all 

equipment and stacks associated with the former Hurley Smelter. A copy of the revised permit issued on May 23, 

2007, is found in Appendix B of the State’s submittal.
As discussed above, the state provided data showing that the SO2 design value for the 24-

hour SO2 NAAQS (0.14 part per million (ppm)) has been 0.0 ppm for each of the five most 

recent years (2007-2011) for which certified ambient air quality data is available for the Hurley 

monitor. These values are below the 85% threshold demonstrating that a Limited Maintenance 

Plan option was a suitable decision for planning purposes. The New Mexico submittal supports a 

finding that the requirement for demonstrating continued attainment is satisfied.

3. Air Quality Monitoring

Over the course of the last 30 years, New Mexico has operated four SO2 monitors in the 

Grant County maintenance area. There have been no monitored exceedances of the 1971 SO2 

NAAQS in the Grant County maintenance area since 1979. The only SO2 monitor currently 

located in the Grant County maintenance area is the Hurley monitor (AIRS Monitor ID 35-017-

0003-42401-1), which has been in operation since 1997. As discussed above, EPA determined in 

a letter to NMED dated August 26, 2002, that the Hurley monitor was placed where modeling 

indicated the highest SO2 concentration was likely to occur. Following the July 2006 

dismantling of the Hurley Smelter, which is the source that was determined to be responsible for 

the violations of the SO2 NAAQS that resulted in the area being designated nonattainment,

monitored SO2 concentrations have been negligible for the most recent five years for which 

certified ambient air quality data is available,. The state provided data from the Hurley monitor 

showing that the SO2 design value for the 24-hour SO2 NAAQS (0.14 part per million (ppm)) 

has been 0.0 ppm for each of the five most recent years (2007-2011) for which certified ambient 

air quality data was available (see Table 1). 

A. New Mexico’s Request to Modify the SO2 Monitoring Network:

Given the limited number of SO2 emission sources, the limited amount of SO2 

emissions, and the negligible monitored SO2 concentrations in the Grant County maintenance 

area, New Mexico is requesting to discontinue SO2 air monitoring within the maintenance area 

and to implement in its place an alternative SO2 monitoring methodology that does not utilize a 

gaseous analyzer for determining compliance with the SO2 NAAQS. The alternative SO2 

monitoring method will consist of using PSD and Title V modeling and any required postconstruction monitoring for new and modified air quality permits and an annual emission review 

of all major SO2 sources located in the Grant County maintenance area. In the event that PSD or 

Title V modeling, PSD or Title V post-construction monitoring, or the annual emissions review 

of major sources within the maintenance area indicate there is a significant increase in SO2 

emissions that may cause a potential SO2 NAAQS violation, New Mexico will reinstitute a 

gaseous SO2 monitor at the Hurley, NM monitoring location (AIRS ID 35-017-0003-42401-1) 

or at a site expected to read greater SO2 levels than this site. If the monitored SO2 values after 

one year are at or below 50 percent of the 24-hour or annual SO2 NAAQS, or both, the monitor 

would again be removed and the alternative SO2 monitoring methodology reinstated. The 

process would be repeated each time PSD or Title V monitoring, PSD or Title V postconstruction monitoring, or the annual emission review of major SO2 sources within the 

maintenance area indicate a potential SO2 NAAQS violation. 
B. Does Modification of the SO2 Monitoring Network Meet the Monitoring Network 

Modification Requirements under 40 CFR section 58.14(c)?

To verify the attainment status of the area over the maintenance period, the maintenance 

plan should contain provisions for continued operation of an appropriate, EPA-approved air 

quality monitoring network, in accordance with 40 CFR part 58. Over the course of the last 30 

years, New Mexico has operated four SO2 monitors in the Grant County maintenance area. There 

have been no monitored exceedances of the 1971 SO2 NAAQS in the Grant County maintenance 

area since 1979. The Hurley monitor (AIRS Monitor ID 35-017-0003-42401-1), which has been 

in operation since 1997, is the only SO2 monitor currently operating in the Grant County 

maintenance area. EPA determined in a letter to NMED dated August 26, 2002, that the Hurley 

monitor was placed where modeling indicated the highest SO2 concentration was likely to occur. 

Monitored SO2 concentrations have been negligible for the most recent five years for which 

certified ambient air quality data is available, following the July 2006 dismantling of the Hurley 

Smelter, which was the source originally responsible for the violations of the SO2 NAAQS that 

resulted in the area being designated nonattainment. The state provided data from the Hurley 

monitor showing that the SO2 design value for the 24-hour SO2 NAAQS (0.14 part per million 

(ppm)) has been 0.0 ppm for each of the five most recent years (2007-2011) for which certified 

ambient air quality data was available. 

In light of the limited number of SO2 emission sources, the limited amount of SO2

emissions, and the negligible monitored SO2 concentrations in the Grant County maintenance 

area, New Mexico is requesting to discontinue SO2 air monitoring within the maintenance area, 

as currently required by its maintenance plan, and to instead implement an alternative SO2

monitoring methodology that does not utilize a gaseous analyzer for determining compliance 

with the SO2 NAAQS. The alternative SO2 monitoring method will consist of using PSD and 

Title V modeling and any required post-construction monitoring for new and modified air quality 

permits and an annual emission review of all major SO2 sources located in the Grant County 

maintenance area. In the event that PSD or Title V modeling, PSD or Title V post-construction 

monitoring, or the annual emissions review of major sources within the maintenance area 

indicate there is a significant increase in SO2 emissions that may cause a potential SO2 NAAQS 

violation, New Mexico will reinstitute a gaseous SO2 monitor at the Hurley, NM monitoring 

location (AIRS ID 35-017-0003-42401-1) or at a site expected to read greater SO2 levels than 

this site. If the monitored SO2 values after one year are at or below 50 percent of the 24-hour or 

annual SO2 NAAQS, or both, the monitor would again be removed and the alternative SO2

monitoring methodology reinstated. The process would be repeated each time PSD or Title V 

monitoring, PSD or Title V post-construction monitoring, or the annual emission review of 

major SO2 sources within the maintenance area indicate a potential SO2 NAAQS violation. 

We accordingly find that with the alternative SO2 monitoring methodology in place, the 

Hurley monitor is no longer required as part of the 24-hour SO2 maintenance plan. By our 

approval, the maintenance plan will not contain any contingency measures to be triggered by a 

monitored air quality concentration. NMED may submit a separate request for a system 

modification (including a request for discontinuation of a State or local air monitoring station) 

for EPA’s review and approval under the bases provided in 40 CFR § 58.14. We find that the 

State has satisfied the monitoring network and verification of continued attainment requirements 

for the limited maintenance plan.
4. Contingency Measures

Section 175A(d) of the CAA requires that a maintenance plan include contingency 

provisions, as necessary, to promptly correct any violation of the NAAQS that occurs after the 

area is redesignated to attainment. Under section 175A(d), contingency measures do not have to 

be fully adopted at the time of redesignation. However, the contingency plan is considered to be 

an enforceable part of the SIP and should ensure that the contingency measures are adopted 

expeditiously once they are triggered by a specified event. The general approach for contingency 

measures discussed in the limited maintenance plan guidance memoranda provide that the 

contingency provisions should identify the measures to be adopted, a schedule and procedure for 

adoption and implementation, and a specific time limit for action by the State. However, the 

General Preamble for the Implementation of Title I of the Act Amendments of 1990 (57 FR 

13498) states that SO2 provisions require special considerations. A primary reason is that SO2 

control methods are well established and understood. Therefore, contingency measures for SO2 

need only consist of a comprehensive program to identify sources of violations of the SO2 

NAAQS and to undertake an aggressive follow-up for compliance and enforcement. 

There are currently no major SO2 sources within the Grant County maintenance area. In 

the event that an SO2 source(s) moves into or within close proximity to the Grant County SO2 

maintenance area, New Mexico will ensure that such source(s) will comply with all applicable 

state and federal SO2 regulations and requirements. New Mexico is also committing to maintain 

a comprehensive compliance and enforcement program to identify sources of violation of the 

SO2 NAAQS within the maintenance area and to undertake aggressive follow up measures to 

ensure compliance with the SO2 NAAQS. In conformance with CAA section 175A(d), New 

Mexico will also implement all measures with respect to the control of air pollutants concerned 

which were contained in the SIP for the area before redesignation of the area as an attainment 

area, to the extent such measures are applicable to any sources which may exist at the time of 

any NAAQS exceedance. We believe that New Mexico’s contingency plan is adequate for 

identifying which SO2 sources are responsible for violations of the 1971 SO2 NAAQS and 

undertaking aggressive measures to ensure compliance of the SO2 NAAQS to the submittal 

supports a findingthat New Mexico’s contingency measures plan is approvable. 

CONCLUSION

New Mexico followed all applicable administrative procedures and provided the 

technical justification to demonstrate that the second 10-year plan for maintenance of the SO2 

NAAQS is adequate to protect human health and the environment in the Grant County 

maintenance area. New Mexico has also provided information to demonstrate that 

discontinuance of the Hurley monitor is in accordance with the monitoring network regulations 

for system modifications under 40 CFR section 58.14. We recommend approval of the second 

10-year plan for maintenance of the SO2 NAAQS in the Grant County maintenance area. We 

also recommend approval of New Mexico’s request to discontinue operation of the Hurley 

monitor in the Grant County maintenance area.