by Jeff Wynn and Mark Gettings, U.S. Geological Survey
|| Background || Earlier hydrological and geological studies || The airborne electromagnetic survey and interpretation || Conclusions || References ||
In 1877 the U. S. Army established Fort Huachuca in southeastern Arizona (figure 1), in part to command the new southern frontier of the country but also to protect the Tombstone and Bisbee gold and copper mining districts. Shortly afterwards, the town of Sierra Vista sprang up outside the Fort's eastern gate. Water for the new settlers and soldiers came from wells dug by hand, and the groundwater was recharged by the late summer to early winter monsoon rains. About 20 miles to the east, the San Pedro River flowed northward from the Mexican border. During the 1920's agriculture sprang up along the river, and together with mines on the Mexican side of the border began to divert water from the San Pedro River. Gauging station records show that by the 1950's, surface water flow on the San Pedro River dropped to less than half its pre-1920's levels.
The San Pedro River is an area rich in biological diversity, and lies on a well-known migratory corridor for North American birds that winter in Central America; for example, 15 different species of hummingbird have been identified at nearby Ramsey Canyon. In 1988 the U.S. Congress established the San Pedro River Riparian Conservation Area along parts of the river adjacent to Fort Huachuca. Currently, a controversy exists between environmental groups, the U.S. Army, and the City of Sierra Vista about whether their withdrawal of water has impacted the surface flow on the river and the viability of the Conservation Area. Hydraulic modeling and well-production records (Corell, and others, 1996) suggest that recharge of the aquifer beneath Fort Hucahuca is about 7,000 acre-feet per year, while the total annual water withdrawal may be as high as 11,000 acre-feet. Most of those who have studied the issue agree that there is a significant ground-water withdrawal deficit in the vicinity of the Fort and Sierra Vista. Some isotopic evidence acquired during the early 1990's, however (D. Pool, U.S. Geological Survey Water Resources Division, Tucson, Arizona, oral communication, 1997), has suggested that most of the water in the San Pedro River comes from the Tombstone Hills to the east, rather than from the Huachuca Mountains behind the Fort on the west of the river. There also appears to have been major withdrawal of water from the river by mines on the Mexican side of the border (Corell and others, 1996).
Within this framework of conflicting claims and needs, the Environmental and Natural Resources Division of the U.S. Army Garrison at Fort Huachuca asked the U.S. Geological Survey (USGS) to provide basic data to evaluate the impact of the Fort Huachuca and Sierra Vista well-fields on the Upper San Pedro River aquifer. An airborne electromagnetic (EM) survey was conducted to map conductive bodies (including faults and water-saturated sediments) underground; survey parameters were optimized to obtain maximum depth of penetration. The San Pedro airborne EM survey was carried out by Geoterrex, Ltd., under the supervision of the USGS in March 1997. The interpretation presented here is based on partial data released to the USGS as of early May, 1997. The available data include seven uncalibrated mathematical inversions, Conductivity Depth Transforms ("CDTs"), of the 20-channel airborne EM data; a merged aeromagnetic map acquired during the survey; and a graphical representation of the flightline locations. The USGS also received 6 grids representing the horizontal and vertical components of EM channels 2, 6, and 10 (these are the shallow, intermediate, and deep penetration channels). Structures as deep as 400 meters may have been mapped as a result of this survey.
A number of studies of the hydrology of the Upper San Pedro River drainage have been conducted over the years (e.g., Brown and others, 1966), with the most recent hydrologic model reported by Corell and others (1996). Previous work and hydrologic models all assumed a fairly simple bathtub-like basin structure. A USGS regional geology and tectonics map was completed by Drewes (1980) that was later supplemented by a more local geologic map by Moore (1993). This latter map outlines a newly-discovered volcanic center that includes the Tombstone Caldera, which lies partly within the area of the airborne geophysical survey. Recognition of the caldera has resulted in a reinterpretation of the basin's basement structure. Reports by Gettings and Houser (1995) and Gettings and Gettings (1996) provide an approximate map of the sedimentary-rock-to-crystalline-rock contact underlying the Upper San Pedro River drainage. In some cases, the contact between the sedimentary rocks and underlying crystalline rocks is as deep as a kilometer (for instance beneath Huachuca City). In other cases, the contact is as shallow as 200 meters (for instance beneath parts of Sierra Vista).
The limits of the survey are latitudes 31o25'N and 31o 45'N and longitudes 110o 05'W and 110o 25'W. The survey system was a 20-channel, 3-axis airborne EM device utilizing two towed detectors. The object and intent of the survey was to map the shallow aquifer (assumed to be relatively more conductive) in the Upper San Pedro River drainage, to identify cones of depression (zones of lower conductivity) around existing well-fields, and to determine what, if any, impact these well-fields and any other structures we could find might have on the water flowing into the San Pedro Riparian Conservation Area.
In the CDT in figure 5, we interpret the red-purple shallow subhorizontal conductor on the upper right side of the figure to be the shallow San Pedro aquifer; among other things, the upper margin shallows to the east in agreement with well-data. On the left side of figure 5, a depression in the water table can be seen. It is unclear how much of this depression is caused by the Fort Huachuca/Sierra Vista well-fields and how much is due to pre-existing listric faulting on the Huachuca Mountains front-range; this figure certainly suggests a cone-of-depression around the well-field, but other CDTs show an extended depression continuing many kilometers south of where pumping is taking place. A rising shoulder to the east (right side) of the depressed water table appears due to recharge from waste ponds close to this location. A deeper conductor (the subhorizontal, reddish-to-slightly-purple ellipse below the shallow conductor) can be seen on the right side of figure 5; this apparently is a thick clay body recognized in wells and encountered also in ground-based electrical geophysical soundings (Don Pool, U. S. Geological Survey, Tucson, Arizona, written communication). White incursions on the bottom of the CDT are caused by man-made conductors that overload the dynamic range of the EM system response. In effect, the CDT algorithm cannot deal with this interference and does not try to calculate a conductivity-versus-depth here.
The magnetic data and the individual channel information (neither shown here) when combined with seven of these CDTs allow us to construct a map of the shallow aquifer (figure 6).
Figure 6 shows the interpreted shallow Upper San Pedro aquifer as a light blue semi-transparent overlay, superimposed on the geologic map of Drewes (1980). The heavy red band in the northeast part of the map is the San Pedro River Riparian Conservation Area. Blue arrows show where there is apparently a hydraulic connection between isolated parts of the shallow aquifer, beneath the depressed water table ("DWT" in the figure). Where there is no hydraulic connection, the word "Breach" has been inserted. Wells and depth to water table in feet are shown as dark blue numbers where available.
A semi-transparent narrow pink band on the right (the "Deep Conductor" on the northeast) side of the map is a narrow conductive body extending to great depth. It appears to be a water-filled conductive fault zone and coincides closely with the mapped western margin of the Tombstone Caldera (Moore, 1993). The semi-transparent brown body south and west of it is interpreted to be the clay body that was encountered in wells; parts of it may serve as a structural barrier between the aquifer beneath Fort Huachuca and the San Pedro River. Several lithologic boundaries or facies changes in shallow underlying rocks can be seen in the EM data. These buried lithologic boundaries may influence water movement and are shown as blue dashed lines in the figure. The Sawmill Canyon fault coincides with a right-lateral offset in this contact and is shown as a pink dashed-dot line. The Nicksville Fault also shows up in the EM data, but the geophysically-interpreted fault is offset nearly 500 meters southwest of where it is mapped on the Drewes (1980) map. The EM structure may in fact be another parallel fault.
It appears that the airborne EM survey permits us to accurately map the shallow Upper San Pedro River aquifer east and north of Fort Huachuca in southeastern Arizona. Depressions in the water table can be clearly seen in the CDT cross-sections, and hydraulic connectivity can be inferred in most cases. The Tombstone Caldera margin appears as a deep, through-going, narrow conductive body in the northeast portion of the surveyed area. Deep clay bodies and facies changes can also be seen. The maximum depth of penetration may be as much as 400 meters in this survey. Subsequent hydrologic models can now take advantage of the greater subsurface information this survey makes available to all parties. Ideally, this should lead to resolution of conflicts in water-usage in this area of southeastern Arizona.
Brown, S.G.;Davidson, E.S.; Kister, L.R.; and Thomsen, B.W., 1966, Water Resources of Fort Huachuca Military Reservation, southeastern Arizona: U.S. Geological Survey Water-Supply paper 1819-D, 57 p.
Corell, Steven W.; Corkhill, Frank; Lovvik, Daryl; and Putman, Frank, 1996, A groundwater flow model of the Sierra Vista Subwatershed of the Upper San Pedro Basin - southeastern Arizona: Arizona Department of Water Resources Modeling Report No. 10, 107 p.
Drewes , Harald, 1980, Tectonic Map of Southeast Arizona: U.S. Geological Survey MI-Map 1109, 2 plates with text.
Gettings, M.E.; and Houser, B.B., 1995, Preliminary results of modeling the gravity anomaly field in the upper San Pedro basin, southeastern Arizona: U.S. Geological Survey Open-File Report 95-76, 8 p.
Gettings, P.E.; and Gettings, M.E., 1996, Modeling of a magnetic and gravity anomaly profile from the Dragoon Mountains to Sierra Vista, southeastern Arizona: U.S. Geological Survey Open-File Report 96-288, 13 p.
Moore, R.B., 1993, Geologic Map of the Tombstone Volcanic Center, Cochise County, Arizona: U.S. Geological Survey MI-Map 2420, 1 plate with text.
Wynn, J.C., 1999, A description, comparison to other data, and interpretation of the electromagnetic data from the 1997 airborne E.M. survey, Fort Huachuca vicinity, Cochise County, Arizona, in Gettings, M.E., Bultman, M.W., and Wynn, J.C., An interpretation of the 1997 Airborne ElectroMagnetic (AEM) survey, Fort Huachuca vicinity, Cochise County, Arizona: U.S. Geological Survey Open-File Report 99-7-B.
Wynn, J.C., Pool, Don, Bultman, Mark, Gettings, Mark, and Lemieux, Jean, 2000, Airborne EM as a 3-D aquifer-mapping tool: Proceedings Volume, SAGEEP-2000 Conference, 20-24 February 2000, Arlington, VA., pp. 93-100.
Wynn, J.C., 2000, Mapping an aquifer in three dimensions - analysis and interpretation of airborne geophysical surveys of the Upper San Pedro River Basin, Cochise, County, southeastern Arizona: US Geological Survey Open-File Report 00-517, 46 pages, 27 figures.