REVIEW AND ANALYSIS 

OF 

NAVAJO AQUIFER MATERIAL DAMAGE CRITERIA

FOR

PEABODY WESTERN COAL COMPANY'S 

KAYENTA MINE COMPLEX

 

THE OFFICE OF SURFACE MINING RECLAMATION AND ENFORCEMENT

PROGRAM SUPPORT DIVISION

DENVER, COLORADO

APRIL 2023
OSMRE N-aquifer Material Damage Assessment Report Page i 

OSMRE Project Code: NN.AZ.0001E.0026

CONTENTS

 Page

1.0 Introduction 1 

2.0 Geologic Setting and Well Completions 2 

3.0 Analysis of PWCC and USGS Hydrologic Data Monitoring Reports 9 

 3.1 Aquifer Response to Pumping 9 

 3.1.1 PWCC Groundwater Flow Model 9 

 3.1.2 USGS Observation Wells Located Outside the Lease Area 10 

 3.1.3 PWCC Observation Wells Located within the Lease Area 12 

 3.2 Confined N-aquifer Water Quantity Review and Analysis 14 

 3.3 Confined N-aquifer Water Quality Review and Analysis 23 

 

4.0 Summary and Finding 28 

5.0 References Cited 30 
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Figures 

Figure 1. N-aquifer Monitoring Area and Coal Lease Area (Mason, 2022).

Figure 2. Rock Formations and Major Aquifer Zones of the Black Mesa Area (Mason, 2022).

Figure 3. PWCC Lease Boundary and N-aquifer Well Locations.

Figure 4. 2019 N-Aquifer Monitoring Locations Change from Pre-stress Period (Mason, 2022).

Figure 5. 3D Flow Model Simulated Water Level Response: BM-2, BM-5, BM-6 (BLM, 2017). 

Figure 6. Black Mesa Observation Well BM-6 Water Level Measurements (USGS, 2023).

Figure 7. Black Mesa Observation Well BM-5 Water Level Measurements. (USGS, 2023).

Figure 8. Observation Wells NAVOBS3 and NAV6/6OBS Response (PWCC, 2022, Table 20). 

Figure 9. Historical Total N-aquifer Withdrawal 1965 – 2019 (Mason, 2022). 

Figure 10. 2019 N-aquifer Pumping Centers and Volume (Mason, 2022).

Figure 11. Arsenic Concentrations in PWCC N-aquifer Wells (2000-2021) (PWCC, 2022). 

Figure 12. Boron Concentrations in PWCC N-aquifer Wells (2000-2021) (PWCC, 2022).

Figure 13. Chloride Concentrations in PWCC N-aquifer Wells (2000-2021) (PWCC, 2022).

Figure 14. Selenium Concentrations in PWCC N-aquifer Wells (2000-2021) (PWCC, 2022).

Figure 15. Sulfate Concentrations in PWCC N-aquifer Wells (2000-2021) (PWCC, 2022).

Figure 16. TDS Concentrations in PWCC N-aquifer Wells (2000-2021) (PWCC, 2022).
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Tables

Table 1. PWCC N-aquifer Wells Screened Aquifer Zone (feet) (PWCC, 2023, Ch.15).

Table 2. USGS Monitored Wells Screened in Confined N-aquifer. (Mason, 2022). 

Table 3. Confined N-aquifer Water Systems Water Use (OSMRE, 2016). 

Table 4. Lift and Material Damage Numeric Water Level (OSMRE, 2016). 

Table 5. Material Damage Numeric Water Level for Windmill Wells (OSMRE, 2016). 

Table 6. N-aquifer Pumping Volumes (acre-feet) 1965 – 2019 (Mason, 2022). 

Table 7. 2019 Water Level Comparison to Material Damage Levels (Mason, 2022). 
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OSMRE Project Code: NN.AZ.0001E.0026

THE OFFICE OF SURFACE MINING RECLAMATION AND ENFORCEMENT

REVIEW AND ANALYSIS OF

PWCC AND USGS HYDROLOGIC DATA MONITORING REPORTS

1.0 INTRODUCTION

In 1991, the Office of Surface Mining Reclamation and Enforcement (OSMRE) entered into a settlement

with the Hopi Tribe called an “Agreement Concerning Review of Certain Data” (hereinafter, the 

“Agreement”). The Agreement requires OSMRE to review and analyze annual hydrological reports 

prepared by Peabody Western Coal Company (PWCC) and the United States Geological Survey (USGS). 

During these reviews, OSMRE must relate the data in the reports to the material damage criteria for the 

Navajo aquifer system (N-aquifer) found in the current cumulative hydrologic impact assessment (CHIA) 

of the PWCC Black Mesa Kayenta Mine Complex (OSMRE, 1989, 2008, 2011a, 2016). The Agreement 

was established when the 1989 CHIA was effective. Since implementation of the 1989 CHIA, the CHIA 

document was revised in 2008, 2011, and 2016. Material damage criteria for the N-aquifer were 

established and modified during the revision process. As part of the Agreement, OSMRE has agreed to 

review (1) hydrologic monitoring data on the N-aquifer that PWCC collected pursuant to permit AZ0001C (and subsequent AZ-0001D, AZ-0001E, and AZ-0001F permits), and (2) water-quality and -

quantity data collected by the USGS in ongoing investigations and published reports. The Navajo Tribal 

Authority (NTUA), PWCC, the Hopi Tribe, the Western Navajo, Chinle, and Hopi Agencies of the Bureau 

of Indian Affairs (BIA) have assisted in the collection and submittal of hydrologic data (Mason, 2022).

This report contains the results of OSMRE’s review and analysis of the following reports:

• PWCC 2020 Annual Hydrological Data Report (PWCC-20HDR) (PWCC, 2021)

• PWCC 2021 Annual Hydrological Data Report (PWCC-21HDR) (PWCC, 2022)

• Groundwater, Surface-Water, and Water-Chemistry Data, Black Mesa Area, Northeastern Arizona 

– 2016-2018” (USGS 2016-2018 Report) (Mason, 2021)

• Groundwater, Surface-Water, and Water-Chemistry Data, Black Mesa Area, Northeastern Arizona 

– 2018-2019” (USGS 2018-2019 Report) (Mason, 2022)

The PWCC hydrological data reports describe monitoring PWCC completed during calendar years 2020

and 2021 and includes summary data from previous years. This evaluation also considers the USGS 2016-

2018 Report and USGS 2018-2019 Report, which provide the results of groundwater, surface-water, and 

water-chemistry monitoring in the Black Mesa area from November 2016 to December 2019. 

The following OSMRE analysis evaluates the hydrologic impacts of PWCC's wellfield pumping. In 2014, 

PWCC’s wellfield consisted of eight production wells: NAV2, NAV3, NAV4, NAV5, NAV6, NAV7, 

NAV8 and NAV9. In 2015, well NAV5 was reclaimed and abandoned, and wells NAV3 and NAV9 were 

idled. Additionally, in 2016, well NAV4 was reclaimed and abandoned, and well NAV7 sealed from 

zones previously open to the D-aquifer and idled for water quality sampling. OSMRE compared N-aquifer 

monitoring data contained in the PWCC and the USGS reports with baseline conditions presented in the 

Black Mesa Kayenta Complex CHIA. The CHIA for the PWCC Black Mesa Kayenta Complex was 

completed in 1989 and updated in 2008, 2011, and 2016. The material damage criteria and assessment 

approaches have been modified during the updates, and the material damage criteria in the 2016 CHIA are 

currently in effect. The criteria were established to evaluate potential impacts to water quality and quantity 

related to existing and foreseeable uses. This report will not repeat information contained in the CHIA, 

except for purpose of comparison of information provided in the PWCC and USGS data reports. For a full 
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OSMRE Project Code: NN.AZ.0001E.0026

discussion of the definition of baseline conditions, identification of hydrologic concerns, development of 

material damage criteria and the analysis of mining related impacts for the N-aquifer, please see the 

PWCC Kayenta Complex CHIA (OSMRE, 2016).

In accordance with 30 CFR 816.41, a coal mine operator must meet the following four hydrologic 

performance standards: (1) minimize disturbance of the hydrologic balance within the permit and adjacent 

areas, (2) prevent material damage to the hydrologic balance outside the permit area, (3) assure the 

protection or replacement of water rights, and (4) support approved post mining land uses in accordance 

with the terms and conditions of the approved permit. The CHIA establishes material damage criteria for 

N-aquifer hydrologic impacts that may result due to coal mining activities of the Kayenta Mine Complex. 

2.0 GEOLOGIC SETTING AND WELL COMPLETIONS

The PWCC Kayenta Mine Complex is in the Black Mesa area in northeastern Arizona (Figure 1). The 

mesa is a prominent feature with cliff height more than 1,200 feet on the north and northeast sides and is 

incised by several major drainages. Figure 2 is a generalized stratigraphic column showing rock 

formations and major aquifer zones of the Black Mesa area.

The N-aquifer system in the Black Mesa area consists of the Wingate Sandstone, the Kayenta Formation, 

and the Navajo Sandstone. The N-aquifer system is approximately 1,100 feet thick beneath the lease area. 

The Carmel Formation overlies the N-aquifer system and is approximately 140 feet thick beneath the lease

area. The D-aquifer system is approximately 1,040 feet thick beneath the lease area and overlies the 

Carmel Formation and consists of the Entrada Sandstone, the Morrison Formation and the Dakota 

Sandstone. The Carmel Formation, a shaly siltstone and massive mudstone, confines the N-aquifer from 

the D-aquifer and generally is not considered a water-bearing unit or a part of the D-aquifer. However, the 

Carmel Formation may produce water from limited zones. The Mancos Shale is a regionally extensive 

formation several hundred feet thick beneath the lease area that restricts vertical flow between the PWCC 

mining operations in the Mesa Verde Group and the D- and N-aquifer systems where PWCC withdrawals 

groundwater. Both the D- and N-aquifer systems are confined by the Mancos Shale in the vicinity of the 

Kayenta Mine Complex and in much of the Black Mesa area. A confined aquifer is bounded above and 

below by beds of low hydraulic conductivity, and its potentiometric head is measured as the level to which 

water rises above the top of the aquifer in a well installed in the aquifer. Whereas, an unconfined aquifer is 

not overlain by a confining bed and its hydrologic head is represented by the water table level.

AZ-0001F is the current permit issued to PWCC to mine at the Kayenta Mine Complex (PWCC 2023). 

The permit application package gives detailed geologic and hydrologic information on the lease area

(PWCC, 2023, Ch 15). PWCC monitors the N-aquifer quality and water levels within the lease area at 

production wells NAV2, NAV4, NAV5 (abandoned January 2015), NAV7, NAV8, NAV9, and nonpumping observation wells NAV3P and NAV6P. Wells NAV2 through NAV6 were drilled in 1967-1968, 

NAV7 was drilled in 1972, NAV8 was drilled in 1980 and NAV9 was drilled in 1983. NAV3P and 

NAV6P are abandoned well bores that have been converted to N-aquifer observation wells and used for 

monitoring purposes instead of production wells NAV3 and NAV6 since 1995. NAV1 was a test well 

drilled to examine all the different water-bearing units as part of the slurry pipeline feasibility study 

(Stetson, 1966). Figure 3 shows the locations of PWCC’s N-aquifer production wells within the lease

area. Construction and completion information and lithologic logs for each of the N-aquifer wells are

included in the permit application package (PWCC, 2023, Ch.15). 

Table 1 presents a tabulation of the length of completed well intervals in geologic formations. All PWCC 

production wells penetrate the geologic formations of the N-aquifer. However, each well is completed in 

different water-bearing zones of the N-aquifer. Of particular interest are production wells NAV2, NAV6 

and NAV8, which may have different water quality characteristics due to differences in well completion 
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compared to other PWCC production wells. The annular space around a portion of the blank casing 

adjacent to the D-aquifer in NAV2 is not grout sealed. Therefore, D-aquifer water has the potential to 

migrate into the well bore at NAV2 and potentially cause local degradation of the N-aquifer water quality. 

NAV6 is the only well that is not at least partially completed in the overlying Carmel Formation. NAV8 is 

the only well not drilled past the Navajo Sandstone into the underlying Kayenta Formation and Wingate 

Sandstone.

The permit application package describes PWCC's monitoring program in detail (PWCC, 2023, Ch. 16). 

PWCC monitors the N-aquifer in the lease area for water quality, water level elevation, and pumping 

volume. For active pumping wells, field measurements are taken for pH, specific conductivity, total 

dissolved solids (TDS), salinity, and temperature quarterly, and a sample collected from each well for

laboratory analysis of a larger suite of chemical parameters. For idled production wells, water quality 

samples are collected once every five years prior to permit renewal. Water-level data are collected using a 

combination of data loggers and an airline bubbler system. Well production is measured using totalizing 

flow meters. The pumpage values are reported quarterly to OSMRE, USGS, Hopi Tribe, and Navajo 

Nation and are included in the annual hydrology monitoring report.

Groundwater levels from 34 wells installed in the N-aquifer were measured for the USGS 2018-2019

Report (Figure 4). Six of the wells measured for groundwater level are continuously monitored USGS 

observation wells dedicated for monitoring the N-aquifer and not water supply wells. The six dedicated 

USGS observation monitoring wells are identified on Figure 4 as: BM1, BM2, BM3, BM4, BM5, and 

BM6. Withdrawal volume information from the N-aquifer is compiled primarily on the basis of metered 

data from individual wells operated by BIA, NTUA, and Hopi Tribe. In 2019, groundwater quality was 

measured at zero wells and four springs. The four monitored spring locations are Pasture Canyon Spring, 

Moenkopi School Spring, Burro Spring, and Unnamed Spring near Dennehotso. 
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Figure 1. N-aquifer Monitoring Area and Coal Lease Area (Mason, 2022).
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Figure 2. Rock Formations and Major Aquifer Zones of the Black Mesa Area (Mason, 2022).
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Figure 3. PWCC Lease Boundary and PWCC N-Aquifer Well Locations.
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Figure 4. 2019 N-aquifer Monitoring Locations Change from Pre-stress Period (Mason, 2022).
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Table 1. PWCC Pumping Wells Screened Aquifer Zone (feet) (PWCC, 2023, Ch. 15). 

NAV 

Well 

Number

D-Aquifer

Screened Interval (feet)

N-Aquifer

Screened Interval (feet)

Morrison Entrada Carmel Navajo Kayenta Wingate Chinle

2(a) 0 0(a) 27 735 150 194 0 

3/3P 0 0 10 690 170 268 0 

4(c) 26 160 150 700 60 308 0 

5(b) 203 155 150 725 155 229 0 

6/6P 0 0 0 684 160 294 18

7(d) 0 122 150 690 165 206 0 

8 0 0 163 787 0 0 0 

9 0 0 4 710 150 245 0 

(a) Well Number 2 is not completed in the D-aquifer; however, the annular space around its blank casing adjacent to 

the D-aquifer is not grout sealed. Therefore, D-aquifer water has the potential to migrate into the well bore.

(b) Reclaimed and Abandoned January 2015.

(c) Reclaimed and Abandoned August 2016.

(d) D-aquifer zone sealed August 2016.
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OSMRE Project Code: NN.AZ.0001E.0026

3.0 ANALYSIS OF PWCC AND USGS HYDROLOGIC DATA MONITORING REPORTS

The following evaluates available PWCC and USGS data using N-aquifer material damage criteria

identified for water quantity and quality to determine if material damage has occurred as a result of 

PWCC’s wellfield pumping.

3.1 Aquifer Response to Pumping

On December 31, 2005, PWCC reduced pumping from approximately 4,500 ac-ft/year to approximately 

670 ac-ft/year, averaging 1,235 ac-ft/year for the 10-year period 2010-2019. The N-aquifer system is

responding to the reduction in PWCC pumping. Response to this change will first be observed at the 

PWCC pumping center, then at the more distant USGS monitoring wells that are less influenced by 

municipal or industrial pumping. OSMRE utilizes three components to assess the aquifer response to the 

reduced PWCC pumping volumes. These components are:

(1) A Groundwater Flow Model. 

(2) USGS Observation Wells Located Outside the Lease Area.

(3) PWCC Observation Wells Located Within the Lease Area.

3.1.1 PWCC Groundwater Flow Model

PWCC submitted a groundwater flow model of the D- and N-aquifers to OSMRE in 1999 for use to 

evaluate and predict the effect of PWCC pumping on the groundwater system (PWCC, 1999). OSMRE 

considered the flow model appropriate for determination of hydrologic consequences in September 2004. 

However, OSMRE made an additional request for a validation of the groundwater model at that time. In 

2005, a supplemental report to the flow model was provided to OSMRE that further evaluated the 

sensitivity of model assumptions that may influence drawdown predictions, specifically N-aquifer system 

thickness and aquifer structure. The supplemental report also validated model predictions by comparing 

simulated and measured water levels for the BM wells through 2004 (PWCC, 2005). The model validation 

was completed again in 2010 using measured water level data through 2009, and results incorporated into 

the PWCC PHC (PWCC, 2023, Ch. 18). Although some deviation from the predictions was apparent, the 

overall trends continued to remain in good agreement. In 2012, OSMRE requested that the model be 

recalibrated, as new data had been collected over a 16-year period that could be used to recalibrate and 

improve the model predictive accuracy. PWCC updated the flow model as part of a published Draft 

Environmental Impact Statement, and this model (hereinafter: the 3D Flow Model) replaces the previous 

transient groundwater flow model developed and calibrated initially during the late 1990s and maintained 

during the period between 1999 and 2014 (BLM, 2017).

Figure 5 illustrates simulated water level change from baseline in three continuously recorded USGS 

monitoring wells in the Black Mesa area. BM-2 is approximately nineteen miles to the northeast of the 

PWCC NAV pumping wells, and approximately seven miles to the southeast of the Kayenta municipal

pumping center. The proximity of BM-2 to Kayenta municipal pumping makes it difficult to differentiate 

the effects of Kayenta municipal pumping from PWCC industrial pumping solely using water level 

measurements. In 2019, Kayenta community pumped 380.4 acre-feet and PWCC pumped 672.8 acre-feet 

(Mason, 2022). However, the utility of the calibrated 3D Flow Model allows the allocation of total 

drawdown impacts to be quantified. Of the total drawdown from all pumping since baseline conditions, 

the 3D Flow Model simulated total drawdown at BM-3 near Kayenta community at 108.06 feet in 2005, 

and of the total drawdown 3.36 feet was attributed to PWCC pumping (BLM, 2017). 
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Figure 5. 3D Flow Model Simulated Water Level Response: BM-2, BM-5, BM-6 (BLM, 2017).

 

USGS monitoring well BM-5 is approximately 30 miles south of the PWCC NAV pumping wells, within 

15 miles of most Hopi Tribe municipal pumping locations, and some Navajo Nation pumping locations. In 

2019, 734.8 acre-feet of municipal pumping from the Hopi Tribe and Navajo Nation occurred in the 

confined N-aquifer within 15 miles of BM-5. Figure 5 illustrates that the continued hydrologic stress to 

the N-aquifer around the area of BM-5, coupled with water level recovery in response to reduced PWCC 

pumping, will cause water level measurements to stabilize at BM-5. The 3D Flow Model projects 

maximum drawdown occurred at BM-5 in 2010. The 3D Flow Model was also used to evaluate PWCC 

allocation of impacts at the Hopi village of Kykotsmovi located 11 miles south of BM-5. Of the total 

drawdown from all pumping since baseline conditions, the 3D Flow Model simulated a total drawdown of 

30.86 feet at Kykotsmovi PM3 in 2005, and of the total drawdown 15.19 feet was attributed to PWCC 

pumping (BLM, 2017). 

USGS monitoring well BM-6 is a good gauge for monitoring PWCC impacts to the confined N-aquifer 

system due to the long period of continuous water level measurements, the remoteness from significant 

municipal pumping, and proximity to the PWCC wellfield. BM-6 is approximately 17 miles south of the 

PWCC wellfield, and approximately 13 miles northwest of the Navajo Nation Pinon municipal pumping 

center. Pinon pumped 359.7 acre-feet in 2019 (Mason, 2022). The 3D Flow Model simulates a relatively 

quick recovery response at BM-6 from PWCC reduced pumping. Figure 5 illustrates water levels in BM-6 

are projected to stabilize at approximately year 2030 in response to PWCC reduced pumping volume and 

continued nearby municipal pumping. Another recovery curve begins in 2030, a few years after PWCC 

pumping is projected to be discontinued and municipal pumping continued (Figure 5).

3.1.2 USGS Observation Wells Located Outside the Lease Area

Model predictions are validated with field measurements. The USGS has monitored withdrawals and 

water level changes in the Black Mesa area since 1971. N-aquifer monitoring well BM-6 was installed in 

April 1977 and well BM-5 installed in April 1972. USGS monitoring wells BM-5 and BM-6 provide a 

good indication of overall regional N-aquifer response due to their location in the confined N-aquifer 

relative to pumping areas. Since real time data for these wells is collected and publicly available at 

http://waterdata.usgs.gov/az/nwis/gw, the most current data is included due to the reporting value in this 

assessment. 

See Figures 6 & 7 

for 2003-2022

comparison to 

measured data.
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Figure 6. Black Mesa Observation Well BM-6 Water Level Measurements (USGS, 2023).

Figure 6 illustrates water level measurements collected at BM-6 from 1/1/2004 through 3/1/23. Consistent 

with PWCC model predictions, the N-aquifer responded to PWCC reduced pumping, and measured water 

levels began to rise in BM-6 during December 2006. BM-6 reached a maximum depth to water of 861.2 

feet below land surface December 4, 2006, which represented a decline of 164.2 feet from the pre-stress 

water level of 697.0 feet below ground surface. In response to reduced PWCC pumping, the N-aquifer

water level at BM-6 recovered 35.1 feet since December 2006. The depth to water rose to 826.1 feet 

below land surface as of March 1, 2023, which represents a 21.4-percent recovery from the December 

2006 low. 

Based on USGS monitoring at well BM-5, Figure 7 illustrates a continued decline in N-aquifer water 

levels several years after PWCC reduced N-aquifer pumping. USGS monitoring well BM-5 is 

approximately 30 miles south of the PWCC wellfield and within 15 miles of most Hopi municipal 

pumping locations, and some Navajo Nation pumping locations. In 2019, 734.8 acre-feet were pumped 

from the confined N-aquifer within 15 miles of BM-5. This continued hydrologic stress to the N-aquifer 

around the area of BM-5, coupled with water level recovery in response to reduced PWCC pumping, will 

likely result in the stabilization of water level measurements at BM-5. Figure 7 indicates that a low water 

level measurement of 427.80 feet occurred on November 27, 2011. The March 1, 2023, water level 

measurement of 417.51 feet below land surface represents a 10.29-foot water level rise from the November 

2011 recorded low. 
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Figure 7. Black Mesa Observation Well BM-5 Water Level Measurements (USGS, 2023).

3.1.3 PWCC Observation Wells Located within the Lease Area

PWCC was the dominant user of water from the confined N-aquifer from 1970-2005. As such, the PWCC 

pumping activity caused the most significant drawdown in the confined N-aquifer at the PWCC wellfield. 

As a result of PWCC’s reduced pumping, aquifer response will first be apparent at the PWCC pumping 

center. Opposed to the delayed response measured at USGS wells BM-6 and BM-5, water level response 

to reduced PWCC pumping on 1/1/2006 was immediate at the PWCC NAV pumping wells within the 

PWCC lease area.

N-aquifer water level response is measured within the lease area at observation monitoring wells 

NAV3OBS and NAV6OBS (Figure 8). Observation monitoring well NAV3OBS is within the lease area 

and the PWCC well field (near idled pumping well NAV3), and NAVOBS6 is near idled pumping well 

NAV6. Note that N-aquifer response for NAVOBS6 was shifted to the response at idled pumping well 

NAV6 in 2014. Static depth to water at NAVOBS3 was 730 feet below ground surface when initially 

completed. Static depth to water at NAVOBS6 and NAV6 was 895 feet. Maximum depth to water for 

NAVOBS3 occurred in 2005, measured at 1155 feet below ground surface, or 425 feet of drawdown. 

Maximum depth to water for NAVOBS6 occurred in 2004, measured at 1344 feet below ground surface, 

or 449 feet of drawdown. Water level measurements in 202l were 881 and 1100 feet below ground surface 

at NAV3OBS (274 feet recovery) and NAV6 (244 feet recovery) respectively. Since maximum 

drawdown, static water level has recovered by 64.5% at NAV3OBS and 54.3% at NAV6. 
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NAVOBS3:

 TTTTTTTTTT RRRRRR RRRRRRRR

TTTTTTTTTT DD DDDD DDDD ∗ 100 = PPPPPPPPPPPPPP RR RRRR RR 274 ff425 ff ∗ 100 = 64.5% RR RRRR RR

NAVOBS6/NAV6:

 TTTTTTTTTT RRRRRR RRRRRRRR

TTTTTTTTTT DD DDDD DDDD ∗ 100 = PPPPPPPPPPPPPP RR RRRR RR 244 ff449 ff ∗ 100 = 54.3% RR RRRR RR

Figure 8. Observation Wells NAVOBS3 and NAV6/6OBS Response (PWCC, 2022, Table 20).
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3.2 Confined N-aquifer Water Quantity Review and Analysis 

EVALUATION CRITERION: Numeric water levels that will be physically measured for all wells 

screened in the confined area of the N-aquifer that are monitored by 

the USGS (OSMRE, 2016) 

This section discusses the development of ‘material damage’ criteria as required by the “Stipulated 

Settlement Agreement of Appellants to Nizhoni Ani et.al. February 2012 Request for Review”, IV. 

Hydrology Claims: No. 2 which states: 

OSM shall identify and adopt, as material damage criteria for the Navajo Sandstone Aquifer (“NAquifer”), numeric water levels that will be physically measured for all wells screened in the confined area 

of the N-aquifer that are monitored by the U.S. Geological Survey (USGS).

OSMRE has defined ‘material damage’ as:

Material damage to the hydrologic balance outside the permit area means any quantifiable adverse impact 

from surface coal mining and reclamation operations on the quality or quantity of surface water or 

groundwater that would preclude any existing or reasonably foreseeable use of surface water or 

groundwater outside the permit area.

Given the large size of the N-Aquifer system, its artesian head, depth, thickness and the amount of 

groundwater in storage, “precluding any existing or reasonably foreseeable use” of groundwater is an 

economic rather than physical concern. Historic mine pumping has declined from a high of about 4,500 

ac-ft in 2005 to approximately 670 ac-ft in 2019 and will be further reduced in the future until ceasing 

completely. PWCC wellfield pumping was reported at 204 ac-ft in 2020, and 162 ac-ft in 2021. The 

question is not one of physical availability but of the cost to supply water from significant depth to users at 

the land surface.

AFFORDABILITY OF WATER

Many Navajo residents within the N-Aquifer study area reside in communities that receive water service 

through the Navajo Tribal Utility Authority (NTUA). Many Hopi have water service through village water 

systems. However, some members of both tribes in the study area have no water service, relying on 

hauled water for potable use. Transportation costs are typically many times higher than the commodity 

costs of the water itself.

Poverty rates among residents in the N-Aquifer study area are high; an estimated 38 percent of all 

households live in poverty, that is, with annual household income below the Federal poverty thresholds. 

Households in poverty have fewer economic resources and adjustment options to respond to increases in 

prices. Economic data from the Census Bureau suggests that while some Navajo and Hopi households in 

the study area have incomes above the poverty level, median household and per capita incomes are still 

below the corresponding values for off-reservation areas in Coconino and Navajo counties and across 

Arizona. Consequently, increases in commodity prices, such as water costs, may result in greater 

economic hardships for individuals living on the reservations within the N-Aquifer study area (Dutton 

2016). 

Given the above economic considerations, OSMRE has determined that “precluding any existing or 

reasonably foreseeable use” of N-Aquifer groundwater at the Kayenta Mine Complex is defined as 

increasing the cost of pumping water by more than one (1) dollar per month per connection (household) as 

a result of declining water levels in community production wells due to drawdown caused by water supply 
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pumping at the Kayenta Mine Complex. Thus, the material damage numeric water levels in the confined 

area N-Aquifer community water supply wells monitored (for water levels) by the USGS are based on 

limiting the decline in water level to less than the cost of electric power to lift groundwater of 

$1/household/month for wells that supply potable water to communities. 

Some of the USGS monitored wells are windmills and primarily supply water for livestock use. Water 

level change in these wells does not have an economic impact unless the water level falls below the ability 

of the windmill to lift water to the surface. This level is the depth of the windmill’s drop pipe installed in 

the well. If the water level is drawn down below the bottom of the drop pipe, the drop pipe and cylinder 

must be removed, lengthened and re-installed. 

Review of the USGS monitored wells on Black Mesa indicate that 14 wells typically meet the criterion of 

being ‘screened in the confined N-aquifer’ and ‘monitored for water level by the USGS’. The USGS BMseries monitoring wells were excluded since the use at these locations are for observation; therefore, the 

preclusion of use is not applicable. These wells are identified in Table 2, with key well data. Location of 

the wells are shown on Figure 4. 

Table 2. USGS Monitored Wells Screened in Confined N-Aquifer (Mason, 2022).

Well Name 

Surface 

Elevation 

(ft msl)

2019

Depth to 

Water 

(ft bls)

Depth Top of 

N-Aquifer 

(ft bls)

Well Type 2 

Forest Lake NTUA1 (4T-523) 6,654 1,155.6 NR1 P

Keams Canyon PM2 5,809 477.5 900 P 

Kitsiili NTUA2 6,780 1,336.8 2,205 P

Kykotsmovi PM1 5,657 207.1 880 Pa

Kykotsmovi PM3 5,618 248.8 840 P

Pinon PM6 6,397 907.8 1,870 Pa

Howell Mesa (3K-311) 5,855 450.2 615 W

Marsh Pass (8T-522) 6,040 131.9 480 W

Kayenta West (8T-541) 5,885 284.0 700 W

Rough Rock (10R-119) 5,775 260.9 310 W

Rough Rock (10T-258) 5,903 309.9 460 W

Rough Rock (10R-111) 5,757 191.8 210 W

Sweetwater Mesa (8K-443) 6,024 548.1 590 W

White Mesa Arch (1K-214) 5,771 218.8 250 W

1. NR – Not reported 

2. P – Community Production Well 

Pa –Community Production Well Abandoned 

 W – Windmill Livestock Well

MATERIAL DAMAGE NUMERIC WATER LEVEL IDENTIFICATION

To set numerical water levels in the above wells to satisfy the material damage criteria, wells are divided 

into two categories: 1) community water supply wells, and 2) windmill equipped stock watering wells.
OSMRE N-aquifer Material Damage Assessment Report Page 16

OSMRE Project Code: NN.AZ.0001E.0026

Community Water Supply Wells

As described above, the material damage water level is set based on the depth to water below land surface 

(ft bls) that would result in increasing the household cost of water by no more than $1 per month. This 

depth to water, or lift, is computed as described below.

The cost of pumping groundwater is given by the following equation (Campbell 1973):

CC CC /HH HH = (GG GG) xx (LLLLLL ) XX (0.746) xx (EEEE CC CC $/KKKK − h )

(3960) xx (PP PPPP EE ) xx ( EE )

Using typical Arizona well values for the following parameters: 

• Pump efficiency (75 percent)

• Motor efficiency (90 percent)

The above equation with typical Arizona values was solved for lift, (in ft); the equation in this form is:

LLLLLL = /h

$ KKKK − h ∗ GG GG ∗ 3.7411EE − 04

To calculate the lift, it is necessary to estimate the usage, in gpm, per household. This was done by 

dividing the reported annual water system usage by the number of connections (households) served by the 

system. The systems with their annual withdrawal, number of households and use per household are given 

in Table 3. 

Table 3. Confined N-Aquifer Water Systems Water Use (OSMRE, 2016).

Water System

2011 

Withdrawal 

(gpm)

Number of 

Households

2011 Use 

Per Day Per 

Household 

(GPD)1

Forest Lake 9.6 49 282

Keams Canyon 36.6 142 371

Kits’illi 13.1 74 254

Kykotsmovi 41.4 250 239

Pinon 208.5 1427 210

1. GPD – gallons per day

The estimated use per household numbers for these systems average 271 gallons per day, which is more 

than twice the 108 average for all NTUA systems (NTUA 2015). The reasons for this are uncertain; 

however, since lift is inversely proportional to use (pumping rate), these values suggest the analysis is 

conservative.

The 2011 cost of electric power from NTUA is $0.07 KW-hr. To account for potential increases in power 

cost during the life of the mine the cost per kilowatt hour was increased by 30 percent to $0.091 KW-hr for 

this analysis. Since lift is inversely proportional to power cost, using a higher power cost results in a lower 

lift to meet the $1 dollar per month threshold, adding conservatism to the analysis.

Substituting per household use (in gpm) into the lift equation and converting $/hour to $/month gives the 

additional lift to increase the cost of water by one dollar ($1) per household per month. Results of this 
OSMRE N-aquifer Material Damage Assessment Report Page 17

OSMRE Project Code: NN.AZ.0001E.0026

calculation for the water system supply wells included in Table 2 are given in Table 4. It is not known, in 

detail, which wells supply which households. Therefore, for purposes of assigning material damage water 

levels to each identified USGS monitored community water supply well, the same numeric water level 

value is assigned to all wells within the area of the relevant water system. 

Table 4. Lift and Material Damage Numeric Water Level (OSMRE, 2016).

Well Name Lift (ft)1

2012 

Depth to 

Water 

(ft bls)

Material 

Damage 

Depth to 

Water 

(ft bls)

Material 

Damage 

Water 

Elevation 

(ft msl)

Forest Lake NTUA1 (4T-523) 274 1,174 1,448 5,206

Keams Canyon PM2 205 498 703 5,106

Kits’iili NTUA2 306 1,336 1,642 5,138

Kykotsmovi PM1 310 212 522 5,135

Kykotsmovi PM3 310 251 561 5,057

Pinon PM6 368 917 1,286 5,112

1. Lift resulting in a cost of $1/per month per household

Windmill Wells

As noted in Table 2, eight (8) of the USGS monitored confined N-Aquifer wells are windmills primarily 

used for livestock watering. For those monitored wells, a different basis for the numeric material damage 

water level is applied. 

Once water levels in a windmill well fall below the bottom of the drop pipe, the windmill can no longer lift 

water to the surface; the drop pipe must be extended (deepened) for the windmill to continue to function. 

OSMRE has therefore set the material damage numeric water level equal to the depth of the windmill drop 

pipe. Unfortunately, the depth of the drop pipe was not available for two (2) of the windmill wells; Howell 

Mesa (3K-311) and White Mesa Arch (1K-214). For these windmills, the material damage water level was 

set at the top of the N-aquifer. On this basis, the material damage numeric water level for USGS 

monitored wells with windmills are given in Table 5.

Table 5. Material Damage Numeric Water Level for Windmill Wells (OSMRE, 2016).

Well Name

Top of 

NAquifer 

(ft bls)

Well 

Depth

(ft bls)

2012 

Depth 

to 

Water 

(ft bls)

Depth 

of Drop 

Pipe1

 

(ft bls)

Material 

Damage 

Depth to 

Water 

(ft bls)

Material 

Damage 

Water 

Elevation 

(ft msl)

Howell Mesa (3K-311) 615 745 444 615(1) 615 5,240

Marsh Pass (8T-522) 480 933 128 189 189 5,851

Kayenta West (8T-541) 700 890 298 420 420 5,465

Rough Rock (10R-119 310 360 298 336 336 5,439

Rough Rock (10T-258) 460 670 257 336 336 5,567

Rough Rock (10R-111) 210 360 312 262 262 5,495

Sweetwater Mesa (8K-443) 590 720 199 588 588 5,436

White Mesa Arch (1K-214) 250 356 545 250(1) 250 5,521

(1) Drop pipe depth not available – Material Damage Water Elevation is Top of N-aquifer.
OSMRE N-aquifer Material Damage Assessment Report Page 18

OSMRE Project Code: NN.AZ.0001E.0026

Numeric Water Levels are Conservative

The material damage numeric water levels given in Tables 4 and 5 are conservative since they do not 

differentiate between water level drawdown due to mine pumping and non-mine (community and 

windmill) pumping. The effects of all pumping are reflected in any change in water level. The threshold 

of economic impact ($1 dollar per month analysis) is protective of the most vulnerable population (those at 

or below the poverty level) and the cost of electric power is increased 30 percent, decreasing the material

damage numeric depth to water value (shallower water level).

Differentiation of Change in Water Level Due to Pumping 

As noted above, the material damage water levels do not differentiate between water level drawdown 

between PWCC mine-related pumping and that due to community, windmill and other pumping. Since 

OSMRE is responsible for regulating PWCC’s activities and has no regulatory authority over community 

or other pumping, if a material damage level is reached in a given well, the pumping contributing to the 

drawdown must be assigned to PWCC and others. This will be accomplished by monitoring changes to 

pumping volumes by PWCC, the communities and any other withdrawals (including future industrial or 

other uses). Active windmills pump at generally consistent (and low) rates and can be excluded from the 

analysis. Changes in pumping rates can be input to the PWCC Black Mesa Groundwater Flow Model to 

estimate the relative change in water level at the well under consideration due to each pumping source. 

The amount of drawdown due to PWCC can be computed and a determination made if the material 

damage numeric water level has been exceeded due to PWCC withdrawals. 

MATERIAL DAMAGE NUMERIC WATER LEVEL REVIEW

N-aquifer withdrawals and changes in N-aquifer potentiomentic head level are monitored to evaluate the

confined N-aquifer quantity material damage criterion. Between the calendar years 1996 and 2005, PWCC 

pumped a total of 4,013 to 4,640 acre-feet per year from all its production wells, averaging 4,336 acre-feet 

per year for the same period (Mason, 2022). The Bureau of Indian Affairs, Navajo Tribal Authority, and 

Hopi Tribe operate about 70 municipal water supply wells. Also, there are about 270 windmills in the 

Black Mesa Area that are used to withdraw water from the D- and N-aquifers. Figure 10 illustrates water 

withdrawn from the N-aquifer by industrial (PWCC) and municipal users through 2019, as estimated on 

the basis of USGS information (Mason, 2022). Municipal withdrawals are shown as confined and 

unconfined sources. A total of 3,070 acre-feet were pumped from the N-aquifer in 2019 (Mason, 2022). 

Figure 11 illustrates the locations of the confined and unconfined pumping centers and quantities of water 

pumped at each pumping center in 2019. Figure 4 illustrates monitored well locations and water level 

changes from the 1965 pre-stress period.

Industrial use accounted for 21.8-percent of the total N-aquifer use in 2019, and 33.3-percent of the total 

use in the confined N-aquifer (Mason, 2022). PWCC pumping was reduced on December 31, 2005, when 

use of the coal slurry pipeline to the Mojave Generating Station ceased. Table 6 summarizes annual 

withdrawal from the N-aquifer since 1965, separated by industrial confined pumped quantity, municipal 

confined pumped quantity, and municipal unconfined pumped quantity. 
OSMRE N

-aquifer Material Damage Assessment Report Page 19

OSMRE Project Code: NN.AZ.0001E.0026
OSMRE N-aquifer Material Damage Assessment Report Page 20

OSMRE Project Code: NN.AZ.0001E.0026

Figure 10. 2019 N-aquifer Pumping Centers and Volume (Mason, 2022).
OSMRE N-aquifer Material Damage Assessment Report Page 21

OSMRE Project Code: NN.AZ.0001E.0026

Table 6. N-Aquifer Pumping Volumes (acre-feet) 1965 – 2019 (Mason, 2022).

 

Year Industrial Confined Municipal Confined Municipal Unconfined Total

1965 0 50 20 70

1966 0 110 30 140

1967 0 120 50 170

1968 100 150 100 350

1969 40 200 100 340

1970 740 280 150 1170

1971 1900 340 150 2390

1972 3680 370 250 4300

1973 3520 530 300 4350

1974 3830 580 360 4770

1975 3500 600 510 4610

1976 4180 690 640 5510

1977 4090 750 730 5570

1978 3000 830 930 4760

1979 3500 860 930 5290

1980 3540 910 880 5330

1981 4010 960 1000 5970

1982 4740 870 960 6570

1983 4460 1360 1280 7100

1984 4170 1070 1400 6640

1985 2520 1040 1160 4720

1986 4480 970 1260 6710

1987 3830 1130 1280 6240

1988 4090 1250 1310 6650

1989 3450 1070 1400 5920

1990 3430 1170 1210 5810

1991 4020 1140 1300 6460

1992 3820 1180 1410 6410

1993 3700 1250 1570 6520

1994 4080 1210 1600 6890

1995 4340 1220 1510 7070

1996 4010 1380 1650 7040

1997 4130 1380 1580 7090

1998 4030 1440 1590 7060

1999 4210 1420 1480 7110

2000 4490 1610 1640 7740

2001 4530 1490 1660 7680

2002 4640 1500 1860 8000

2003 4450 1350 1440 7240

2004 4370 1240 1600 7210

2005 4480 1280 1570 7330

2006 1200 1300 1600 4100

2007 1170 1460 1640 4270

2008 1210 1430 1560 4200

2009 1390 1440 1400 4230

2010 1170 1450 1420 4040

2011 1390 1460 1630 4480

2012 1370 1380 1260 4010

2013 1460 1410 1110 3980

2014 1580 1280 1310 4170

2015 1340 1370 1260 3970

2016 1090 1380 1070 3540

2017 1110 1330 1270 3710

2018 1170 1370 1130 3670

2019 670 1340 1060 3540

Italicized Pumping Volumes are Estimated
OSMRE N-aquifer Material Damage Assessment Report Page 22

OSMRE Project Code: NN.AZ.0001E.0026

Table 7 presents water levels measured by the USGS at selected community wells and windmills in the 

confined N-aquifer monitored by the USGS in 2019. The pre-stress N-aquifer water levels of these 

selected wells were compared to the current water levels. The feet from each material damage water level 

are also presented based on 2019 water level measurements published by the USGS. As indicated in Table 

7, water levels remain above established material damage water levels in all wells in the confined Naquifer monitored by the USGS. Therefore, based on available information, OSMRE concludes that 

material damage to the hydrologic balance of the confined N-aquifer, attributable to mining, has not 

occurred based on the 2016 CHIA criterion for N-aquifer quantity. 

Table 7. 2019 Water Level Comparison to Material Damage Levels (Mason, 2022) 

Location Well ID Units

Pre-Stress Water Level 

(feet bgs)

Material Damage Water Level 

(feet bgs)

2019 Water Level 

(feet bgs)

Feet from Pre-Stress 

Water Level

Feet from material 

damage level

Forest Lake 4T-523 Feet 1096 1448 1155.6 -59.6 292.4

Pinon PM6 Feet 743.6 1285 907.8 -164.2 377.2

Keams Canyon PM2 Feet 292.5 703 477.5 -185.0 225.5

Kits'iili NTUA 2 Feet 1297.9 1642 1336.8 -38.9 305.2

Kykotsmovi PM1 Feet 220 522 207.1 12.9 314.9

Kykotsmovi PM3 Feet 210 561 248.8 -38.8 312.2

Howell Mesa 3K-311 Feet 463 615 450.2 12.8 164.8

Kayenta West 8T-541 Feet 230 420 284.0 -54.0 136

Marsh Pass 8T-522 Feet 125.5 189 131.9 -6.4 57.1

Rough Rock 10R-119 Feet 256.6 336 260.9 -4.3 75.1

Rough Rock 10T-258 Feet 301 336 309.9 -8.9 26.1

Rough Rock 10R-111 Feet 170 262 191.8 -21.8 70.2

Sweetwater Mesa 8K-443 Feet 529.4 588 548.1 -18.7 39.9

White Mesa Arch 1K-214 Feet 188 250 218.8 -30.8 31.2
OSMRE N-aquifer Material Damage Assessment Report Page 23

OSMRE Project Code: NN.AZ.0001E.0026

3.3 Confined N-aquifer Water Quality Review and Analysis

EVALUATION CRITERION: Safe Drinking Water Act standards for arsenic, 

boron, chloride, selenium, sulfate, and TDS 

water quality constituents at PWCC N-aquifer 

supply wells.

The next evaluation criterion addresses the potential degradation of N-aquifer water quality. 

Since PWCC pumping increases the pre-mining leakage potential between the D aquifer and Naquifer, the degradation of N-aquifer quality due to mine related pumping is a monitored 

hydrologic concern. The USGS predicted that any increase in leakage from the D aquifer would 

first appear as increased total TDS (Eychaner, 1983). The USGS (Eychaner, 1983) also identified 

increased chloride and sulfate concentrations as important indicators of increased D aquifer 

leakage. Therefore, the USGS and PWCC have compiled and evaluated TDS, chloride, and 

sulfate concentrations in N-aquifer wells since the early 1980’s. To date, “the USGS Black Mesa 

monitoring program has not detected any significant changes in the major-ion water chemistry of 

the N-aquifer that are related to induced leakage” (Thomas, 2002) (Truini and Longsworth, 2003).

OSMRE has received and reviewed N-aquifer production well water quality for several decades 

for TDS, sulfate, and chloride in addition to many other water quality parameters. Settlement

Condition (5) associated with Docket No. DV-2012-3-R states:

In the updated Kayenta Mine Complex CHIA, OSM shall add, as part of the N-aquifer

water quality material damage criteria at PWCC N-aquifer wells, numeric water quality

parameters including but not limited to arsenic, selenium, and boron that will be

evaluated through laboratory analysis. OSM shall modify the monitoring plan to require

monitoring for such parameters at the PWCC N-aquifer wells and with the same

frequency as other N-aquifer water quality material damage parameters. OSM shall not

establish material damage criteria for any parameter in excess of U.S. Safe Drinking

Water Act standards or current concentration of that parameter, whichever is higher, at

the PWCC N-aquifer wells. If the U.S. Safe Drinking Water Act does not establish a

standard for a particular parameter, OSM shall not establish a material damage criterion

in excess of Hopi or Navajo Nation livestock watering standards or the current

concentration of that parameter, whichever is higher.

PWCC N-aquifer wells are monitored for a suite of water quality parameters at a frequency 

provided in the approved permit Hydrologic Monitoring Program (PWCC, v.11, ch.16, Table 12). 

Since 2015, PWCC monitors two types of N-aquifer wells: active and idle. NAV3 and NAV9 

were idled during 2015. NAV7 was rehabilitated to eliminate hydraulic communication with the 

overlying D aquifer and idled during March 2016. In agreement with the Tribes, idled monitoring 

wells will be sampled for water quality no less than every 5-years. Idled wells are scheduled for 

water quality sampling in 2024. NAV5 was permanently abandoned on January 23, 2015, in

accordance with Tribal approval. NAV4 was abandoned March 2016. NAV2, NAV6, and NAV8 

are actively pumped to support mining and reclamation operations. The following describes the 

U.S. Safe Drinking Water Act standards for arsenic, selenium, and boron.
OSMRE N-aquifer Material Damage Assessment Report Page 24

OSMRE Project Code: NN.AZ.0001E.0026

Effective January 23, 2006, the arsenic drinking water standard is 10 μg/L. Arsenic 

concentrations are evaluated through laboratory analysis and monitored at the same frequency as 

other N-aquifer water quality material damage parameters as described in the approved permit 

(PWCC, v.11, ch.16, 2023).

Selenium drinking water standard is 0.05 mg/L. Selenium concentrations are evaluated through

laboratory analysis and monitored at the same frequency as other N-aquifer water quality material 

damage parameters as described in the approved permit (PWCC, v.11, ch.16, 2023).

Boron was identified on the second published EPA contaminant candidate list (CCL 2) in 2005. 

In May 2007, the Agency published a Federal Register (FR) notice announcing and requesting 

comment on its preliminary determinations for 11 of the 51 CCL 2 contaminants. In July 2008, 

EPA published its final determination that no regulatory action is appropriate or necessary for 

boron. Hopi Tribe drinking water standard for total recoverable boron is 1400 μg/L. Navajo 

Nation domestic water supply boron standard is 630 μg/L. Boron concentrations are evaluated 

through laboratory analysis and monitored at the same frequency as other N-aquifer water quality 

material damage parameters as described in the approved permit (PWCC, v.11, ch.16, 2023).

Figures 11, 12, 13, 14, 15, and 16 illustrate concentrations at PWCC N-aquifer wells from 2000-

2022 for arsenic, boron, chloride, selenium, sulfate, and TDS, respectively. As illustrated in the 

graphs, all concentrations for these constituents have remained below all water quality 

concentration limits. Arsenic concentrations are typically measurable in all PWCC N-aquifer 

wells between 2-4 μg/L and below the 10 μg/L standard. Boron concentrations are often not 

detectable but reported as the method detection limit concentration that is far below the 630 μg/L 

standard. Chloride concentrations are measurable in all PWCC N-aquifer wells between 2-5

mg/L and below the 250 mg/L standard. Selenium concentrations are nearly always not 

detectable but reported as the method detection limit concentration of 1 μg/L that is far below the 

50 μg/L standard. NAV8 has maintained sulfate concentrations of approximately 120 mg/L, 

compared to all other NAV wells with sulfate concentrations typically less than 30 mg/L from 

2000-2019, and all below the 250 mg/L water quality standard. Lastly, all samples from PWCC 

N-aquifer wells at the Kayenta Mine Complex have maintained a TDS concentration of less than 

350 mg/L from 2000-2021 and below the 500 mg/L TDS standard.

Slight variations in water quality between the various production wells are a result of the screened

interval. For instance, as presented in Table 10, NAV8 is the only well not drilled past the Navajo

Sandstone into the underlying Kayenta Formation and Wingate Sandstone. Therefore, NAV8 has

consistently elevated TDS and sulfate concentrations when compared to the other NAV water 

supply wells. However, the use potential for the Navajo aquifer remains unchanged at all 

production wells and is suitable for domestic and livestock uses.
OSMRE N-aquifer Material Damage Assessment Report Page 25

OSMRE Project Code: NN.AZ.0001E.0026

 

 Figure 11. Arsenic Concentrations in PWCC N-aquifer Wells (2000 – 2021) (PWCC, 2022).

Figure 12. Boron Concentrations in PWCC N-aquifer Wells (2000 – 2021) (PWCC, 2022).
OSMRE N-aquifer Material Damage Assessment Report Page 26

OSMRE Project Code: NN.AZ.0001E.0026

Figure 13. Chloride Concentrations in PWCC N-aquifer Wells (2000 – 2021) (PWCC, 2022).

Figure 14. Selenium Concentrations in PWCC N-aquifer Wells (2000 – 2021) (PWCC, 2022).
OSMRE N-aquifer Material Damage Assessment Report Page 27

OSMRE Project Code: NN.AZ.0001E.0026

Figure 15. Sulfate Concentrations in PWCC N-aquifer Wells (2000 – 2021) (PWCC, 2022).

Figure 16. TDS Concentrations in PWCC N-aquifer Wells (2000 – 2021) (PWCC, 2022).
OSMRE N-aquifer Material Damage Assessment Report Page 28

OSMRE Project Code: NN.AZ.0001E.0026

4.0 SUMMARY AND FINDING

OSMRE reviewed and analyzed the information contained in the PWCC data reports and USGS 

data from 2016 to 2021, comparing the information to historical monitoring data and to the Black 

Mesa Kayenta Complex CHIA evaluation criteria. 

OSMRE utilized three components to assess the aquifer quantity response to the reduced PWCC 

pumping volumes. These components are:

• A Groundwater Flow Model. 

• USGS Observation Wells Located Outside the Lease Area.

• PWCC Observation Wells Located Within the Lease Area.

Additionally, the development of ‘material damage’ criteria was required by the “Stipulated 

Settlement Agreement of Appellants to Nizhoni Ani et.al. February 2012 Request for Review”, 

IV. Hydrology Claims: No. 2 which states:

OSM shall identify and adopt, as material damage criteria for the Navajo Sandstone 

Aquifer (“N-Aquifer”), numeric water levels that will be physically measured for all 

wells screened in the confined area of the N-aquifer that are monitored by the U.S. 

Geological Survey (USGS).

Review of the USGS monitored wells on Black Mesa, as of 2019, indicated that 14 wells meet the 

criterion of being ‘screened in the confined N-aquifer’ and ‘monitored for water level by the 

USGS’. The USGS BM-series monitoring wells were excluded since the use at these locations 

are for observation and not supply use. The pre-stress N-aquifer water levels of these selected 

wells were compared to the current water levels. Measured water levels remain above established 

material damage water levels in all wells in the confined N-aquifer monitored by the USGS. 

Therefore, based on available information, OSMRE concludes that material damage to the 

hydrologic balance of the confined N-aquifer, attributable to mining, has not occurred based on 

the 2016 CHIA criterion for N-aquifer quantity.

OSMRE also reviews and analyzes N-aquifer water quality for potential degradation. Chloride, 

sulfate, and TDS concentrations are primary trend indicators of potential degradation. 

Additionally, Settlement Condition (5) associated with Docket No. DV-2012-3-R states:

In the updated Kayenta Mine Complex CHIA, OSM shall add, as part of the N-aquifer

water quality material damage criteria at PWCC N-aquifer wells, numeric water quality

parameters including but not limited to arsenic, selenium, and boron that will be

evaluated through laboratory analysis. OSM shall modify the monitoring plan to require

monitoring for such parameters at the PWCC N-aquifer wells and with the same

frequency as other N-aquifer water quality material damage parameters. OSM shall not

establish material damage criteria for any parameter in excess of U.S. Safe Drinking

Water Act standards or current concentration of that parameter, whichever is higher, at

the PWCC N-aquifer wells. If the U.S. Safe Drinking Water Act does not establish a

standard for a particular parameter, OSM shall not establish a material damage criterion

in excess of Hopi or Navajo Nation livestock watering standards or the current

concentration of that parameter, whichever is higher.
OSMRE N-aquifer Material Damage Assessment Report Page 29

OSMRE Project Code: NN.AZ.0001E.0026

Based on review of arsenic, boron, chloride, selenium, sulfate, and TDS water quality constituents 

against all applicable water quality standard limits indicates that the use potential for the Navajo 

aquifer remains unchanged at all production wells and is suitable for domestic and livestock uses.

OSMRE’s review and analysis of available information contained in PWCC and USGS reports 

from 2016 to 2021 indicates that material damage has not occurred to the hydrologic balance of 

the N-aquifer outside of the PWCC Black Mesa Kayenta Complex lease area as a result of mining

and reclamation.
OSMRE N-aquifer Material Damage Assessment Report Page 30

OSMRE Project Code: NN.AZ.0001E.0026

5.0 REFERENCES CITED

Bureau of Land Management (BLM), 2017. Navajo Generating Station Extension Lease Environmental 

Assessment and Signed Findings of No Significant Impact, November. 

https://www.usbr.gov/lc/phoenix/reports/NGS/nepa.html

Campbell, M.D., and Lehr, J.H., 1973. Water Well Technology. McGraw-Hill Book Company, New 

York, NY, 681 p. 

Dutton, R., 2016. Socioeconomic Setting for the Cumulative Hydrologic Impact Assessment, Internal 

Technical Memorandum, March 2016.

Energy Information Administration (EIA), 2015. Web Interface. Electricity. Arizona. Average Retail 

Price; cents/kWh. Accessed October 7, 2015. http://www.eia.gov/electricity/state/arizona/

Eychaner, J.H., 1983. Geohydrology and Effects of Water Use in the Black Mesa, Navajo, and Hopi 

Indian Reservations: U.S. Geological Survey Water Supply Paper 2201, 26 p. 

Mason, J.P., 2021, Groundwater, surface-water, and water-chemistry data, Black Mesa area, northeastern

Arizona—2016–2018: U.S. Geological Survey Open-File Report 2021–1124, 50 p., 

https://doi.org/10.3133/ofr20201124. 

Mason, J.P., 2022, Groundwater, surface-water, and water-chemistry data, Black Mesa area, northeastern 

Arizona—2018–2019: U.S. Geological Survey Open-File Report 2022–1086, 47 p., 

https://doi.org/10.3133/ofr20221086. 

Office of Surface Mining Reclamation and Enforcement (OSMRE), 1989. Cumulative Hydrologic Impact 

Assessment (CHIA) of the Peabody Western Coal Company Black Mesa Kayenta Complex, Western 

Region Operations; Denver, CO, 131 p. April. 

OSMRE, 2008. Cumulative Hydrologic Impact Assessment of the Peabody Western Coal Company Black 

Mesa Complex, Western Region Operations; Denver, CO, 101 p. November. 

OSMRE, 2011a. Cumulative Hydrologic Impact Assessment of the Peabody Western Coal Company 

Black Mesa Complex, Western Region Operations; Denver, CO, 115 p. December.

OSMRE, 2011b. Environmental Assessment: Appendix B. Kayenta Mine Permit Renewal. Navajo 

County, Arizona. U.S. Department of Interior. Western Region. August. 

OSMRE, 2016. Cumulative Hydrologic Impact Assessment of the Peabody Western Coal Company 

Kayenta Mine Complex; Office of Surface Mining Reclamation and Enforcement, Western Region, 

Denver, CO, 180 p. September. 

PWCC, 1999. A Three-Dimesional Flow Model of the D and N-aquifers, Black Mesa Basin, Arizona. V. 

I, II, and III. Prepared jointly by HIS GeoTrans and Waterstone for PWCC. Submitted to OSMRE 

Western Region Operations September, 1999. 

PWCC, 2005. A Three-Dimensional Flow Model of the D and N-aquifers, Black Mesa Basin, Arizona. 

Supplement 1. Prepared by GeoTrans, Inc. for PWCC. Submitted to OSMRE Western Region 

Operations; Denver, CO August 1, 2005 ARMS# 05-08-05-04. July. 
OSMRE N-aquifer Material Damage Assessment Report Page 31

OSMRE Project Code: NN.AZ.0001E.0026

PWCC, 2021. 2020 Hydrological Data Report. On file with the Office of Surface Mining Reclamation 

and Enforcement, One Denver Federal Center, Building 41, Lakewood, CO 80225-0065. 27 May.

PWCC, 2022. 2021 Hydrological Data Report. On file with the Office of Surface Mining Reclamation 

and Enforcement, One Denver Federal Center, Building 41, Lakewood, CO 80225-0065. 14 June.

PWCC, 2023. Permit Application Package (PAP) for Federal Permits AZ-0001 (Black Mesa Mine) and 

AZ-0002 (Kayenta Mine), Volumes 1-28, Office of Surface Mining Reclamation and Enforcement, 

Western Region Mine Plan Reference Center, Denver, Colorado. Note: the original PAP was 

submitted in 1985.

Stetson, T.M., 1966. Feasibility of Obtaining a Ground-water Supply from Black Mesa. Thomas M. 

Stetson Civil and Consulting Engineers, San Francisco.

USGS, 2023. National Water Information System: Web Interface. Arizona. Navajo County. Well 

Numbers 361225110240701 (BMOB6) and 360055110304001 (BMOB5). Date of Access: 20 March.

<http://waterdata.usgs.gov/az/nwis/current/?type=gw&group_key=county_cd >