The N aquifer is an extensive aquifer and the primary source of groundwater in the 5,400-square-mile Black Mesa area in northeastern Arizona. Availability of water is an important issue in northeastern Arizona because of continued water requirements for industrial and municipal use by a growing population and because of low precipitation in the arid climate of the Black Mesa area, which is typically about 6 to 14 inches per year.
The U.S. Geological Survey water-monitoring program in the Black Mesa area began in 1971 and provides information about the long-term effects of groundwater withdrawals from the N aquifer for industrial and municipal uses. This report presents results of data collected as part of the monitoring program in the Black Mesa area from January 2007 to September 2008. The monitoring program includes measurements of (1) groundwater withdrawals, (2) groundwater levels, (3) spring discharge, (4) surface-water discharge, and (5) groundwater chemistry.
In 2007, total groundwater withdrawals were 4,270 acre-feet, industrial withdrawals were 1,170 acre-ft, and municipal withdrawals were 3,100 acre-ft. Total withdrawals during 2007 were about 41 percent less than total withdrawals in 2005. From 2006 to 2007, however, total withdrawals increased by 4 percent, industrial withdrawals decreased by approximately 2 percent, and total municipal withdrawals increased by 7 percent.
From 2007 to 2008, annually measured water levels in the Black Mesa area declined in 6 of 11 wells measured in the unconfined areas of the N aquifer, and the median change was -0.2 feet. Water levels declined in 9 of 18 wells measured in the confined area of the aquifer. The median change for the confined area of the aquifer was -0.2 feet. From the prestress period (prior to 1965) to 2008, the median water-level change for 33 wells in both the confined and unconfined area was -12.9 feet. Median water-level changes were -1.0 feet for 15 wells measured in the unconfined areas and -33.2 feet for 18 wells measured in the confined area.
Spring flow was measured at two springs in 2008. Flow decreased at both Moenkopi School Spring and Pasture Canyon Spring from previous years. Flow fluctuated during the period of record, but a decreasing trend was apparent.
Continuous records of surface-water discharge in the Black Mesa area were collected from streamflow-gaging stations at the following sites: Moenkopi Wash at Moenkopi 09401260 (1976 to 2007), Dinnebito Wash near Sand Springs 09401110 (1993 to 2007), Polacca Wash near Second Mesa 09400568 (1994 to 2007), and Pasture Canyon Springs 09401265 (August 2004 to 2007). Median winter flows (November through February) of each water year were used as an index of the amount of groundwater discharge at the above-named sites. For the period of record of each streamflow-gaging station, the median winter flows have generally remained constant, which suggests no change in groundwater. The period of record is too short to determine if there is a trend at Pasture Canyon Spring.
In 2008, water samples collected from 6 wells and 2 springs in the Black Mesa area were analyzed for selected chemical constituents and the results compared with previous analyses. Concentrations of dissolved solids, chloride, and sulfate have varied at all 6 wells for the period of record, but neither increasing nor decreasing trends over time were found. Dissolved-solids, chloride, and sulfate concentrations increased at Moenkopi School Spring during the more than 12 years of record at that site. Concentrations of dissolved solids, chloride, and sulfate at Pasture Canyon Spring have not varied much since the early 1980s, and there is no trend in those data.