Open-File Report 2006–1058
U.S. GEOLOGICAL SURVEY
Open-File Report 2006–1058
In 2004–05, the Black Mesa monitoring program included metering and estimating ground-water withdrawals, measuring depth to ground water, measuring discharge in streams and springs, and collecting and analyzing water samples from wells and springs. Annual ground-water withdrawals are gathered from 28 well systems within the NTUA, BIA, and Hopi municipal systems and the PWCC industrial well field. Annual measurements of discharge were made at 4 springs, and annual measurements of ground-water levels were made at 33 wells. Spring discharges and ground-water levels were measured between January and May 2005. Ground-water samples were collected from 11 wells and 4 springs in March–May 2005 and analyzed for chemical constituents. Continuous recorders at the 6 observation wells have been upgraded for telemetry, and the water-level data from these wells are available on the World Wide Web (http://waterdata.usgs.gov/az/nwis/rt). Identification information for the 50 wells used for water-level measurements and water-quality sampling is shown in table 2.
Withdrawals from the N aquifer are separated into three categories: (1) industrial withdrawals from the confined area, (2) municipal withdrawals from the confined area, and (3) municipal withdrawals from the unconfined areas (table 1 and fig. 3). The industrial category includes eight wells in the well field of PWCC in northern Black Mesa (fig. 4). The BIA, NTUA, and Hopi Tribe operate about 70 municipal wells that are combined into 28 well systems (fig. 4). Withdrawals from the N aquifer were compiled primarily on the basis of metered data (tables 1 and 3).
Withdrawals from wells equipped with windmills are not measured in this monitoring program. About 270 windmills in the Black Mesa area withdraw water from the D and N aquifers, and estimated total withdrawals by the windmills are about 65 acre-ft/yr (HSIGeoTrans, Inc. and Waterstone Environmental Hydrology and Engineering, Inc., 1999). This amount is less than 1 percent of the total annual withdrawal from the N aquifer.
In 2004, the total ground-water withdrawal from the N aquifer was about 7,210 acre-ft (table 1), which is less than a 1-percent decrease from the total withdrawal in 2003. Withdrawals for municipal use from the confined area totaled 1,240 acre-ft, which is about an 8-percent decrease from 2003. Withdrawals for municipal use from the unconfined areas totaled 1,600 acre-ft, which is about an 11-percent increase from 2003. Withdrawals for industrial use totaled 4,370 acre-ft, which is a 2-percent decrease from 2003, and withdrawals for municipal use totaled 2,840 acre-ft, which is a 2-percent increase from 2003.
Withdrawals from the N aquifer have been increasing since the 1970s; however, the percentages of withdrawals for industrial and municipal uses have varied during this time (tables 1 and 4, fig. 3). For 1965–2004, industrial withdrawals were 63 percent of the total withdrawals and municipal withdrawals were 37 percent. From 1965 to 1972, total withdrawals increased from 70 to 8,760 acre-ft, industrial withdrawals were 74 percent of total withdrawals, and municipal withdrawals were 28 percent of total withdrawals. From 1973 to 1984, total withdrawals were 66,470 acre-ft, industrial withdrawals were 70 percent of total withdrawals, and municipal withdrawals were 30 percent of total withdrawals. In 1985, total withdrawals were 4,720 acre-ft, industrial withdrawals were 53 percent of total withdrawals, and municipal withdrawals were 47 percent of total withdrawals. From 1986 to 2003, total withdrawals were 123,640 acre-ft, industrial withdrawals were 60 percent of total withdrawals, and municipal withdrawals were 40 percent of total withdrawals. In 2004, total withdrawals were 7,210 acre-ft, industrial withdrawals were 61 percent of total withdrawals, and municipal withdrawals were 39 percent of total withdrawals.
Ground water in the N aquifer is under confined conditions in the central part of the study area and under unconfined or water-table conditions around the periphery (fig. 5). The ground water generally moves radially from the recharge areas near Shonto to the southwest towards Tuba City, to the south towards the Hopi Reservation, and to the east towards Rough Rock and Dennehotso (Eychaner, 1983).
Ground-water levels are measured once a year and compared with levels from previous years to determine changes over time. Only water levels from municipal and stock wells that were not considered recently pumped, influenced by nearby pumping, or blocked or obstructed were used for comparison. During February to April 2005, water levels in 33 of the 34 wells that are used for annual measurements met these criteria (table 5). Six of the 33 wells are the continuous-recording observation wells, and water levels were measured manually in these wells 4 times between June 2004 and June 2005. Twenty-five of the 33 water levels measured in 2005 were compared with water levels for the same wells measured in 2004. Water levels in the remaining 8 wells could not be compared because of obstructions, effects of pumping, or other conditions that prevented an accurate water level to be measured in 2004 and (or) 2005.
The wells used for water-level measurements are distributed throughout the study area (fig. 5). All but one of the wells are completed in the N aquifer; however, the characteristics of the wells vary considerably. Well 6H-55 was previously thought to be completed in the N aquifer but is actually completed in the D aquifer. Construction dates range from 1934 to 1993, depths range from 107 ft near Dennehotso, Arizona, to 3,636 ft near PWCC, and depths to the top of the N aquifer range from 0 near Tuba City, Arizona, to 2,205 ft near Forest Lake, Arizona (table 6).
From 2004 to 2005, water levels declined in 14 of the 25 wells for which comparisons could be made (table 5). The median water-level change in the 25 wells was -0.4 ft (table 7). From 2004 to 2005, water levels declined in 6 of the 13 wells in the unconfined areas of the N aquifer. The median water-level change was -0.1 ft. Water-level changes in the unconfined areas ranged from -2.8 ft at Tuba City NTUA 1 to 29.3 ft at 9Y-95 in Rough Rock. In the confined area, water levels declined in 8 of 12 wells from 2004 to 2005. The median water-level change was -1.2 ft. Water-level changes in the confined area ranged from -4.9 ft at Kits’iili NTUA 2 to 11.8 ft at Keams Canyon PM2 (tables 5 and 7).
Annual median water-level changes for the water-level network wells from 1983 to 2005 are shown in figure 6. Annual median changes before 1983 are not shown because there were insufficient water-level data to compute median values. For wells in the confined area, the annual median water-level change was -1.7 ft, and there is no appreciable trend in the annual median water-level changes from 1983 to 2005. For wells in unconfined areas, the annual median water-level change was 0.2 ft, and there is no appreciable trend in the annual median water-level changes from 1983 to 2005.
From the prestress period (prior to 1965) to 2005, the median water-level change in 33 wells was -9.0 ft (table 7). Water levels in 16 wells in the unconfined areas had a median change of -0.6 ft. Water-level changes in the unconfined areas ranged from -33.2 ft at Tuba City NTUA 1 to +15.0 ft at 9Y-95 in Rough Rock (table 5). Water levels in 17 wells in the confined area had a median change of -32.0 ft (table 7). Water-level changes in the confined area ranged from -193.3 ft at Keams Canyon PM2 to 14.0 ft at 3K-311 (fig. 5 and table 5).
The areal distribution of water-level changes from the prestress period to 2005 is shown in figure 5. Hydrographs of water levels in the annual well network show the time trends of changes since the 1950s, 1960s, or 1970s (fig. 7). In most of the unconfined areas, water levels have changed only slightly. In the Tuba City area, however, water levels in three wells have declined about 30 ft (fig. 5). Water levels have declined in most of the confined area; however, the magnitudes of declines are varied. Larger declines are near the municipal pumping centers (wells Piñon PM6, Keams Canyon PM2) or near the industrial pumping centers (BM6). Smaller declines occur away from the pumping centers (wells 10T-258, 8K-443, 10R-111, BM4; fig. 5).
Hydrographs for the Black Mesa continuous-record observation wells show continuous water-level since the early 1970s (fig. 8). Water levels in the two wells in the unconfined areas (BM1 and BM4) have had small seasonal or year-to-year variation since 1972. Water levels in wells BM2, BM3, BM5, and BM6 in the confined area have consistently declined since the early to mid-1960s (fig. 8).
Ground water in the N aquifer discharges from many springs around the margins of Black Mesa, and four of these springs are monitored for discharge. Three springs are in the western or southwestern part of the Black Mesa area, and one is in the northeastern part (fig. 9). Discharges from Moenkopi School Spring, the unnamed spring near Dennehotso, Pasture Canyon Spring, and Burro Spring are measured annually and compared to discharges from previous years to determine changes over time (fig. 10). Discharge was measured in March–April 2005 at the four springs (table 8). Measurements at Burro Spring, Moenkopi School Spring, and Pasture Canyon Spring are made volumetrically, and measurements at the unnamed spring near Dennehotso are made with a flume. The measurements may not reflect the total discharge at each site because some ground water may rise to the land surface downgradient from the measuring point.
In 2005, measured discharges were 0.2 gal/min from Burro Spring, 11.5 gal/min from Moenkopi School Spring, 21.5 gal/min from the unnamed spring near Dennehotso, and 33.3 gal/min from Pasture Canyon Spring (table 8). From 2004 to 2005, discharge stayed the same at Burro Spring, decreased by 5 percent at Moenkopi School Spring, increased by 71 percent at the unnamed spring near Dennehotso, and increased by 8 percent at Pasture Canyon Spring. For the periods of record at all four springs, the discharges have fluctuated but no increasing or decreasing trends are apparent (fig. 10).
Surface-water discharge in the study area is a measurement of ground-water discharge to streams and direct runoff of rainfall or snowmelt. Ground water discharges to some channel reaches at a fairly constant rate throughout the year; however, the amount of discharge that results in surface flow is affected by seasonal fluctuations in water uptake by plants and in evapotranspiration (Thomas, 2002a). In contrast, the amount of rainfall or snowmelt runoff varies widely throughout the year. In the winter and spring, the amount and timing of snowmelt runoff are a result of the temporal variation in snow accumulation, air temperatures, and rate of snowmelt. Although most rainfall runoff is in the summer, runoff can occur throughout the year. The amount and timing of rainfall runoff depend on the intensity and duration of thunderstorms in the summer and cyclonic storms in the fall, winter, and spring.
Continuous surface-water discharge data have been collected at selected streams each year since the monitoring program began in 1971 to provide information about ground-water discharge and runoff from rainfall and snowmelt. In this study, the total discharge in streams is roughly separated into ground-water discharge and runoff so that the temporal trends in ground-water discharge can be monitored.
In 2004, discharge data were collected at five continuous-recording streamflow-gaging stations (tables 9–13). Data collection at these stations began in July 1976 (Moenkopi Wash, 09401260), July 1996 (Laguna Creek, 09379180), June 1993 (Dinnebito Wash, 09401110), April 1994 (Polacca Wash, 09400568), and August 2004 (Pasture Canyon Spring, 09401256; fig. 9 and table 14). Discharge data from August through December 2004 for Pasture Canyon Spring is published in this report; however, they are not sufficient to enable evaluation of possible trends or provide comparisons to data for previous years. The annual average discharges at the other four gaging stations vary considerably during the stations’ periods of record (fig. 11A), and no long-term trends are apparent except for Moenkopi Wash. Discharge of Moenkopi Wash shows a decreasing trend from 1983 to 1993 and an increasing trend from 1995 to 2003.
The ground-water discharge component of total flow at the four streamflow-gaging stations was estimated by computing the median flow for four winter months—November, December, January, and February (fig. 11B). The 120 consecutive daily mean flows for those four months were used to compute the median flow. Ground-water discharge is assumed to be constant throughout the year, and the median winter flow is assumed to represent this constant annual ground-water discharge. Most flow during the winter is ground-water discharge; rainfall and snowmelt runoff are minimal. Most of the precipitation in the winter falls as snow, and the cold temperatures prevent appreciable snowmelt. Also, evapotranspiration is at a minimum during the winter. The median flow for November, December, January, and February, rather than the average flow, is used to estimate ground-water discharge because the median is less affected by occasional winter runoff.
The median flow for November, December, January, and February is an index of ground-water discharge rather than an absolute estimate of discharge. A more rigorous and accurate estimate would involve detailed evaluations of streamflow hydrographs, flows into and out of bank storage, gain and loss of streamflow as it moves down the stream channel, and interaction of ground water in the N aquifer with ground water in the shallow alluvial aquifers in the stream valleys. The median winter flow, however, is useful as a consistent index for evaluating possible time trends in ground-water discharge.
Median winter flows were calculated for the 2004 water year; thus, daily mean flows for November and December 2003 (Truini and others, 2005) were combined with daily mean flows for January and February 2004. These median winter flows were 2.65 ft3/s for Moenkopi Wash, 0.56 ft3/s for Laguna Creek, 0.34 ft3/s for Dinnebito Wash, and 0.13 ft3/s for Polacca Wash (fig. 11B). In 2004, flows for Moenkopi Wash and Laguna Creek both decreased, and flows for Polacca and Dinnebito Washes both increased (fig. 11B). Since 1995, the median flows for Moenkopi Wash, Dinnebito Wash, and Polacca Wash have generally decreased. Median flow values for Laguna Creek are available only since 1997 but no increasing or decreasing trend is apparent. Annual precipitation at Betatakin, about 15 mi west of Kayenta (fig. 1), has been less than average for 6 of the 9 years since 1995 (fig. 11C). Precipitation data for 2003 are incomplete. Precipitation was above average for calendar year 2004 (19.8 in.; fig. 11C).
Water samples are collected from selected wells and springs each year of the Black Mesa monitoring program. Field measurements are made and water samples are analyzed for major ions, nutrients, iron, boron, and arsenic. Samples typically are collected from 12 wells and 4 springs in each year of the program—from the same 8 wells every year and from the other 4 wells on a rotational basis. In 2005, samples could not be collected at Pĩnon PM1 because of well maintenance and repairs. Since 1989, samples have been collected from the same 4 springs. Long-term data for specific conductance, total dissolved solids, chloride, and sulfate for the wells and springs sampled each year are shown in the report published for that year. Historical data for other constituents for all the wells and springs are available from the USGS water-quality database (http://waterdata.usgs.gov/az/nwis/qw) or can be found in the past monitoring reports that are cited in the “Previous Investigations” section of this report.
The primary types of water in the N aquifer are calcium bicarbonate and sodium bicarbonate. Calcium bicarbonate water generally is in the recharge areas of the northern and northwestern parts of the Black Mesa area, and sodium bicarbonate water is in the area that is downgradient to the south and east (Lopes and Hoffmann, 1997). In 2005, water samples were collected from 11 wells completed in the N aquifer (figs. 9 and 12). Sample analyses indicated primarily sodium bicarbonate water except for samples from Kayenta PM2 which is in the western part of the confined area of the N aquifer and Shonto PM2, which is in the western part of the unconfined recharge area of the N aquifer (fig. 12 and table 15).
Rough Rock PM5 and Keams Canyon PM2 yielded appreciably higher dissolved-solids concentrations (639 mg/L and 601 mg/L, respectively) than did the other 9 wells (fig. 12 and table 15). Concentrations of dissolved solids in samples from the other 9 wells ranged from 122 mg/L at Peabody 6 to 361 mg/L at Second Mesa PM2 (fig. 12 and table 15).
There are some long-term trends in the chemistry of water samples from the 7 wells having more than 10 years of data (table 16 and fig. 13). Rough Rock PM5, Keams Canyon PM2, Second Mesa PM2, and Kayenta PM2 show an increasing trend in dissolved solids; Forest Lake NTUA 1 and Peabody 2 show a decreasing trend in dissolved solids, and Kykostmovi PM2 shows a steady trend (fig. 13). The chemistry of water samples from Forest Lake NTUA 1 has varied considerably between 1982 and 2005 (table 16 and fig. 13).
Constituents analyzed from the 11 well samples were compared to U.S. Environmental Protection Agency (USEPA) Primary and Secondary Drinking-Water Regulations (U.S. Environmental Protection Agency, 2002). Maximum Contaminant Levels (MCLs), which are the primary regulations, are legally enforceable standards that apply to public water systems. MCLs protect drinking-water quality by limiting the levels of specific contaminants that can adversely affect public health. Secondary Maximum Contaminant Levels (SMCLs) provide guidelines for the control of contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water. The USEPA recommends compliance with SMCLs for public water systems; however, compliance is not required.
The concentrations of most of the analyzed constituents from the 11 wells sampled in 2005 were less than MCLs and SMCLs (table 15). The pH, however, exceeded the SMCL maximum pH of 8.5 units in samples from 9 of the 11 wells. The dissolved-solids SMCL of 500 mg/L was exceeded in the sample from Rough Rock PM5 (639 mg/L) and Keams Canyon PM2 (601 mg/L). Samples from three wells, Keams Canyon PM2 (42.7 µg/L), Rough Rock PM5 (50.8 µg/L), and Second Mesa PM2 (16.3 µg/L) had arsenic concentrations that exceeded the MCL of 10 µg/L (0.01 mg/L; table 15; U.S. Environmental Protection Agency, 2002).
In 2005, water samples were collected from four springs in the study area. Burro Spring is in the southern part of the study area, the unnamed spring near Dennehotso is in the northeastern part, and Moenkopi School Spring and Pasture Canyon Spring are in the western part (fig. 9). All the springs discharge water from unconfined areas of the N aquifer. At Burro Spring, samples are collected from a metal pipe that discharges from a holding tank. At Moenkopi School Spring, samples are collected from a horizontal metal pipe that is developed into the hillside. At the unnamed spring near Dennehotso, samples are collected from a cavity dug into the sand where the water discharges from the bedrock. At Pasture Canyon Spring, samples are collected from a pipe at the end of a channel and approximately 50 feet away from the spring.
Two water types were identified from the samples from the four springs. The unnamed spring near Dennehotso and Pasture Canyon Spring yielded a calcium bicarbonate type water, and Burro Spring and Moenkopi School Spring yielded a calcium sodium bicarbonate type water (fig. 12). Samples from the unnamed spring near Dennehotso, Moenkopi School Spring, and Pasture Canyon Spring had low dissolved-solids concentrations that ranged from 114 to 212 mg/L (table 17). The sample from Burro Spring had a dissolved-solids concentration of 357 mg/L. Concentrations of all the analyzed constituents in samples from the four springs were less than current USEPA MCLs and SMCLs (U.S. Environmental Protection Agency, 2002).
No long-term trends, since the mid-1980s, are apparent in concentrations of dissolved solids, chloride, and sulfate in water samples from the unnamed spring near Dennehotso and Pasture Canyon Spring (table 18 and fig. 14A-C). Increasing trends in concentrations of dissolved solids and chloride are evident in data from Burro Spring and Moenkopi School Spring, and an increasing trend in sulfate is evident in data from Moenkopi School Spring (table 18 and figs. 14A-C).
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