U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 064-99

March 1999

Hydrogeology

Three rock formations that are 

hydraulically connected compose the N 

aquifer—the Navajo Sandstone, the 

Kayenta Formation, and the Lukachukai 

Member 1 of the Wingate Sandstone. 

The N aquifer is unconfined in parts of 

the study area that generally correspond 

to areas of ground-water recharge (fig. 2; 

Eychaner, 1983). In the Black Mesa 

area, the N aquifer is separated from the 

overlying D aquifer by a leaky confining rock layer that is predominantly 

siltstone and massive mudstone (fig. 3).

In 1968, Peabody Coal Company 

began strip-mining operations on 

land leased from the Navajo and Hopi 

Tribes on Black Mesa (fig. 1). Of the 

11 to 13 million tons of coal that are 

extracted each year, an average of 

about 5 million tons are transported 

as slurry by a 273-mile-long pipeline 

from the coal-lease area west to the 

Mohave Generating Station near 

Laughlin, Nevada. Transporting the 

coal in slurry form consumes, on 

average, about 3,800 acre-ft of water 

annually. The slurry water is provided 

through a network of 8 wells that tap 

the confined parts of the D and N 

aquifers underlying Black Mesa. Most 

of the slurry water is pumped from 

the confined part of the N aquifer 

which also is the primary source of 

water for municipal users within the 

5,400-square-mile Black Mesa area.

The Navajo Nation and Hopi Tribe 

became concerned about the longterm effects of industrial withdrawals 

from the N aquifer on the availability 

and quality of water supplies for 

domestic and municipal purposes. 

These concerns led to an ongoing 

investigation of the water resources of 

the Black Mesa area begun in 1971 by 

the U.S. Geological Survey (USGS) 

in cooperation with the Arizona 

Department of Water Resources 

(ADWR) and the Bureau of Indian 

Affairs (BIA).

Monitoring the Effects of Ground-Water

Withdrawals from the N Aquifer in the

Black Mesa Area, Northeastern Arizona

Page

37° 111°30' 111° 30' 110° 109°30'

30'

36°

35°30'

A

A'

UTAH

ARIZONA

COCONINO CO.

NAVAJO CO.

Laguna

Cre

Na

ek

vajo Creek

Little

Co lorado

i R

ver

Din

bi ne

to

sh a W

HOPI

INDIAN

RESERVATION

COAL-LEASE

AREA

TRACE OF SECTION

RESERVATION

INDIAN

NAVAJO

ca

Polac

sh a W

NAVAJO

INDIAN

RESERVATION

NAVAJO

IND. RES.

NAVAJO CO.

APACHE CO.

Modified from Brown

and Eychaner (1988)

MATHEMATICAL-MODEL

BOUNDARY

0

0

25 KILOMETERS

25 MILES

Ganado

sh a W

sh a W

bi ai Or

Mo

ko en

pi

Kayenta

Hotevilla

Tuba

City

Chinle

Data-Collection Activities

Data-collection activities include 

continuous and periodic measurements 

of ground water and surface water at 

about 100 sites in the Black Mesa area 

(fig. 1). Ground-water data from wells 

completed in the N aquifer include 

annual pumpage from municipal and 

industrial well systems, annual water 

levels from selected municipal and 

stock wells, water levels from continuous-record observation wells, 

and water chemistry from selected 

municipal and industrial well sites. 

Discharge and water-chemistry data are 

collected from selected springs that 

issue from the N aquifer. 

 Surface-water data are collected at 

streamflow-gaging stations and include 

continuous-stage data and bimonthly 

stage and discharge measurements. 

Miscellaneous discharge data also have 

been collected at selected sites during 

seepage investigations.

Figure 1. Study area and data-collection network.

BLACK

MESA

BOUNDARY

EXPLANATION

GROUND -WATER

 DATA SITE

SURFACE-WATER

 DATA SITE

SPRING DATA SITE

Prepared in cooperation with the

Arizona Department of Water Resources and Bureau of Indian Affairs

1Of former usage, formally abandoned by Dubiel (1989).
USGS Fact Sheet

February 1999 FS-064-99

For more

information

contact:

District Chief

U.S. Geological Survey

520 N. Park Ave., Suite 221

Tucson, Arizona 85719-5035

(520) 670-6671

http://az.water.usgs.gov

— Gregory R. Littin 

Na+HCO3

Ca+HCO3

MATHEMATICAL-MODEL

BOUNDARY

Page

111° 110°

36°

37°

UTAH

ARIZONA

COALLEASE

AREA

BLACK MESA

BOUNDARY

0

0

10 KILOMETERS

10 MILES

EXPLANATION

RECHARGE

 AREA

LEAKY CONFINED

 CONDITIONS

F i g u r e 2. Ground-water chemistry—calcium bicarbonate (Ca+HCO3) 

water changes to a sodium bicarbonate 

(Na+HCO3) water.

A'

A

Ground-Water Withdrawal

Water is withdrawn from the unconfined part of the N aquifer for municipal 

use and from the confined part of the 

aquifer for industrial and municipal use. 

Eight wells at the coal-lease area are for 

industrial use. The BIA, Navajo Tribal 

Utility Authority, and Hopi Tribe operate 

about 70 municipal wells throughout the 

Black Mesa area.

The present and future effects of 

ground-water withdrawals on water levels 

are analyzed using a finite-difference 

ground-water flow model developed by 

Eychaner (1983) and recalibrated by 

Brown and Eychaner (1988). Measured 

water levels are routinely compared to 

simulated water levels to test the 

reliability of the model as a predictive 

tool and to estimate how much of the 

water-level decline is the result of 

industrial or municipal pumping.

Surface-Water Discharge

 Outflow from the N aquifer occurs 

mainly as surface flow in Moenkopi 

Wash and Laguna Creek and as springs 

near the boundaries of the aquifer (Davis 

and others, 1963). Stage and discharge 

data are collected at the continuousrecord gaging stations at Moenkopi, 

Dinnebito, and Polacca Washes, and at 

Laguna Creek. Continuous-record gaging 

stations at Dinnebito and Polacca Washes 

monitor outflow from two prominent 

seepage areas along the western and 

southern boundaries of the aquifer. 

Discharge data are collected at selected 

springs and miscellaneous measurement 

sites along Moenkopi Wash and Laguna 

Creek.

Water Chemistry

In general, water in the N aquifer is a 

calcium bicarbonate type in the 

upgradient or recharge part of the study 

area north and northwest of Black Mesa 

and a sodium bicarbonate type elsewhere 

throughout Black Mesa and surrounding 

areas (fig. 2). As water moves from the 

recharge area under Black Mesa toward 

discharge areas to the west, southwest, 

and northeast, ion exchange along the 

flow path generally converts calciumbicarbonate type water to sodiumbicarbonate type water (Wickham, 1992). 

Water from the N aquifer is analyzed 

for selected chemical constituents to 

determine if declining hydraulic heads are 

inducing vertical leakage from the 

overlying D aquifer (fig. 3). On the 

average, the concentration of dissolved 

solids in water from the D aquifer is 

about 7 times greater than that of water 

from the N aquifer, concentration of 

chloride ions is about 11 times greater, 

and the concentration of sulfate ions is 

about 30 times greater. Any increase in 

the leakage rate as a result of pumping 

from the N aquifer probably would 

become evident as an increase in 

concentrations of dissolved solids, 

chloride, and sulfate in the most heavily 

pumped wells (Eychaner, 1983).

A

8,000

FEET

A'

Kayenta

COAL-LEASE

AREA

N AQUIFER

7,000

6,000

5,000

4,000

3,000

2,000

Polacca Wash

Dinnebito Wash

Moenkopi Wash

Laguna Creek

D AQUIFER

N AQUIFER PIEZOMETRIC SURFACE

VERTICAL SCALE GREATLY EXAGGERATED

DATUM IS SEA LEVEL

CONFINING LAYER

Figure 3. Conceptual hydrogeologic profile. 0

0

10 KILOMETERS

10 MILES

CONFINING LAYER

References Cited

Brown, J.G., and Eychaner, J.H., 1988, 

Simulation of five ground-water withdrawal 

projections for the Black Mesa area, Navajo 

and Hopi Indian Reservations, Arizona: U.S. 

Geological Survey Water-Resources 

Investigations Report 88– 4000, 51 p.

Davis, G.E., Hardt, W.F., Thompson, L.K., and 

Cooley, M.E., 1963, Records of ground-water 

supplies, part 1, of Geohydrologic data in the 

Navajo and Hopi Indian Reservations, Arizona, 

New Mexico, and Utah: Arizona State Land 

Department Water-Resources Report 12–A, 

159 p.

Dubiel, R.F., 1989, Sedimentology and revised 

nomenclature for the upper part of the upper 

Triassic Chinle Formation and the lower 

Jurassic Wingate Sandstone, northwestern 

New Mexico and northeastern Arizona, in

Anderson, O.J., Lucas, S.G., Love, D.W., and 

Cather, S.M., eds., Southeastern Colorado 

Plateau: New Mexico Geological Society 

Fortieth Annual Field Conference, September 

28–October 1, 1989, p. 213–223.

Eychaner, J.H., 1983, Geohydrology and effects of 

water use in the Black Mesa area, Navajo and 

Hopi Indian Reservations, Arizona: U.S. 

Geological Survey Water-Supply Paper 2201, 

26 p.

Wickham, Matthew, 1992, The geochemistry of 

surface water and groundwater interactions for 

selected Black Mesa drainages, Little Colorado 

River basin, Arizona: Tucson, University of 

Arizona unpublished master's thesis, 249 p.