Ground-water, surface-water, and water-chemistry data, Black Mesa Area, northeastern Arizona — 2000–2001, and performance and sensitivity of the 1988 USGS numerical model of the N aquifer

The N aquifer is the major source of water in the 5,400-square-mile area of Black Mesa in northeastern Arizona. Availability of water is an important issue in this area because of continued industrial and municipal use, a growing population, and precipitation of about 6 to 14 inches per year.

The monitoring program in Black Mesa has been operating since 1971 and is designed to determine the long-term effects of ground-water withdrawals from the N aquifer for industrial and municipal uses. The monitoring program includes measurements of (1) ground-water pumping, (2) ground-water levels, (3) spring discharge, (4) surface-water discharge, and (5) ground-water chemistry.

In 2000, total ground-water withdrawals were 7,740 acre-feet, industrial use was 4,490 acre-feet, and municipal use was 3,250 acre-feet. From 1999 to 2000, total withdrawals increased by 9 percent, industrial use increased by 7 percent, and municipal use increased by 12 percent.

From 1999 to 2001, water levels declined in 10 of 15 wells in the unconfined part of the aquifer, and the median change was -0.4 foot. Water levels declined in 8 of 16 wells in the confined part of the aquifer, and the median change was -0.2 foot.

From the prestress period (prior to 1965) to 2001, the median water-level change for 33 wells was -17.2 feet. Median water-level changes were -1.2 feet for 15 wells in the unconfined part of the aquifer and -31.0 feet for 18 wells in the confined part.

Discharges were measured once in 1999 and once in 2001 at four springs. Discharges decreased by 5 percent and 33 percent at two springs and increased by 3 percent and 81 percent at two springs. For about the past 10 years, discharges did not significantly change in Burro Spring, the unnamed spring near Dennehotso, and Moenkopi School Spring. The record of discharge from a consistent measuring point for Pasture Canyon Spring is too short for statistical analysis of trends.

Continuous records of surface-water discharge have been collected from July 1976 to 2000 at Moenkopi Wash, July 1996 to 2000 at Laguna Creek, June 1993 to 2000 at Dinnebito Wash, and April 1994 to 2000 at Polacca Wash. Median flows for November, December, January, and February of each water year were used as an index of ground-water discharge to those streams. There is no significant trend in the median winter flows for Moenkopi Wash from 1977 to 2000. The records for the other three streams are too short for a statistical analysis of trends. The median winter flows for Dinnebito Wash and Polacca Wash, however, appear to have decreased during the last 6 years. There is no apparent trend in the median winter flows for Laguna Creek since 1997.

In 2001, water samples were collected from 12 wells and 4 springs and analyzed for selected chemical constituents. Dissolved-solids concentrations ranged from 102 to 628 milligrams per liter. Water samples from 9 of the wells and from the 4 springs had less than 350 milligrams per liter of dissolved solids. Water-chemistry data with sufficient years of record for a statistical analysis of trends over time are available from 7 wells and 4 springs. From about the mid-1980s or early 1990s to 2001 there are no significant trends in the concentrations of dissolved solids, chloride, and sulfate in water samples from 6 of the 7 wells. The concentration of one tested constituent (dissolved solids) in samples from Rocky Ridge PM3 significantly increased from 1990 to 2001. From the late 1980s to 2001, there are no significant trends in the concentrations of dissolved solids, chloride, and sulfate in water samples from Burro Spring, the unnamed spring near Dennehotso, and Pasture Canyon Spring. From 1987 to 2001, concentrations of chloride and sulfate significantly increased in water samples from Moenkopi School Spring and concentrations of dissolved solids did not significantly change.

The performance and sensitivity of the 1988
USGS numerical model of the N aquifer were
analyzed. The overall performance of the model in
steady-state conditions is reasonable for residuals
of heads (difference between observed and
simulated steady-state heads); 80 percent of the
absolute values of residuals are less than 38 feet.
Simulated flows are about 40 percent different
than estimated flows at two of three discharge
areas; however, this comparison is only a rough
approximation of performance because the
accuracy of the estimated steady-state flows is
uncertain.

The overall performance of the model for
transient conditions is fair for residuals of changes
in head (difference between observed and
simulated changes in head from steady state to
1999); 80 percent of the absolute values of
residuals are less than 31 feet. The model is biased
in two areas. In the Tuba City area, simulated
changes in head are more negative than observed
changes in head; all six residuals are positive, and
three residuals are between 75 and 155 feet. In the
confined area of the aquifer, observed changes in
head are more negative than simulated changes in
head; 12 of the 17 residuals are negative, and
8 residuals are between -57 and -20 feet.

Analysis of model sensitivity indicates that
recharge, transmissivity, and storage coefficient
are the most important parameters for estimating
heads, changes in heads, and flows. A strong
correlation between recharge and transmissivity
and a lack of independent and reliable estimates of
recharge, transmissivity, and discharge create a
uniqueness problem in model calibration. Several
models could be constructed and calibrated with
different values of recharge or transmissivity and
still have similar fits to the observed data.
Information from recent data and studies and more
advanced modeling techniques could be used to
develop a more representative and less uncertain
model. Future data collection and studies should
focus on obtaining a better definition of recharge,
discharge, transmissivity, and storage coefficient.