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Hydrology of the Southeastern Arizona Planning Area - Groundwater (Western Portion)

The Southeastern Arizona Planning Area is generally characterized by alluvial basins with relatively large reserves of groundwater in gently sloping valleys separated by mountain ranges. Anderson, Freethy and Tucci (1992) divided the alluvial basins of south-central Arizona into five groups based on similar hydrologic and geologic characteristics. One of these, the “Southeast Basins”, covers most of the planning area.

The fourteen groundwater basins in the Southeastern Arizona Planning Area are discussed here in four groups according to their general location and similar hydrologic characteristics:

Western Portion

On the western side of the planning area are a group of basins that are tributary to the San Pedro and Gila rivers; Aravaipa Canyon, Donnelly Wash, Lower San Pedro and Upper San Pedro. Ggroundwater is found in stream alluvium and basin fill sediments in these basins. 

Aravaipa Canyon Basin

The sparsely populated Aravaipa Canyon Basin is characterized by a relatively flat northwest-trending valley in the southern half of the basin and an incised valley, Aravaipa Canyon that cuts through the Galiuro Mountains, in the northern half. The principal aquifers are the unconfined stream alluvium, which is the major source of groundwater, and a confined basin fill aquifer. Water level records suggest that the confined aquifer leaks into the unconfined aquifer. The thickness of the younger alluvium decreases to the south. (Holmes, 2003) Groundwater flow is similar to the surface water runoff pattern; northwest along the central axis of the valley.  Groundwater flows towards the head of Aravaipa Canyon where its flow path is geologically restricted, resulting in the perennial portion of Aravaipa Creek (Holmes, 2003). Groundwater recharge is from infiltrating precipitation and runoff and is estimated to range from 7,000 to 16,700 AFA (Table 3.1-3).  Groundwater discharge is to Aravaipa Creek from springs and baseflow, with small discharge to wells. Freethey and Anderson (1986) estimated 5 maf of water in storage in the basin.  Depth to water within the basin fill varies from 25 feet bls where the younger alluvium is thin to over 500 feet bls in the uplands in the southern part of the basin (Holmes, 2003).  Two recent water level measurements in the central valley were 64 and 39 feet bls (Figure 3.1-6). Arsenic is the water quality parameter that most frequently exceeds drinking water standards in wells measured in the basin (Table 3.1-5), but groundwater is generally of good chemical quality (Holmes, 2003).

Aravaipa Creek

Aravaipa Creek.  Groundwater Flows toward the head of Aravaipa Canyon where its flow path is geologically restricted, resulting in the perennial portion of Aravaipa Creek

Donnelly Wash Basin

Donnelly Wash Basin is a relatively small basin with few inhabitants. The principal aquifer is a strip of basin fill that covers about 30 percent of the basin.  The rest of the basin is composed of hardrock that surrounds and underlies the basin fill (Overby, 2000). A 16-mile reach of the Gila River flows east to west through the basin, which is also drained by Donnelly Wash and Box O Wash located on the south side of the Gila River. In general, groundwater flow follows surface water drainage patterns, flowing toward the Gila River. Aquifer recharge is from the mountain fronts and streambed infiltration.  Groundwater is discharged from the alluvium into the Gila River and from domestic and stock wells. Storage estimates for the basin range from 140,000 acre-feet to 2 maf (Table 3.4-2)  Depth to water in the basin fill varies from about 150 feet in the north, 256 feet in the center, and about 370 feet in the south. Water levels are more shallow in wells located in the hardrock areas (Overby, 2000).  Elevated fluoride concentrations were measured in two springs in the basin (Table 3.4-7).  Eleven water samples collected by the Department in 1996 and 1997 did not find elevated fluoride levels in groundwater in either the alluvium or the hardrock (Overby, 2000).



San Pedro River in the Lower San Pedro Basin

San Pedro River in the Lower San Pedro Basin.  The streambed alluvium along the San Pedro River and its tributaries is very permeable with high well yields

Lower San Pedro Basin

The Lower San Pedro Basin consists of the northwest-trending San Pedro River Valley bordered by mountains ranging in elevation from 6,000 to over 8,000 feet in elevation. There are two sub-basins; the Mammoth sub-basin and the smaller Camp Grant Wash sub-basin (Figure 3.8-7).  The two major water bearing units are stream alluvium and basin fill. Most mining, industrial and domestic/municipal wells are located in the regional basin fill aquifer while most irrigation wells are located in the stream alluvium.  The stream alluvium along the San Pedro River and tributaries can be quite permeable with high well yields but this aquifer is often less than 50 feet thick south of Redington (USGS, 2006a). Groundwater in the alluvium is unconfined. The hydrologic characteristics of the basin fill aquifer vary widely due to the amount of cementation and occurrence of fine-grained layers.  Both confined and unconfined conditions exist. Artesian conditions exist from about five miles north to ten miles south of Mammoth in wells drilled deeper than 500 feet.

Groundwater flow direction is from the mountains toward the valley floor and to the north.  The estimated groundwater recharge ranges from 24,000 to 29,000 AFA (Table 3.8-6) from mountain front recharge, streambed infiltration and underflow from the Aravaipa Canyon and Upper San Pedro basins. Groundwater is discharged by pumpage, evapotranspiration, evaporation from streams, and springs and seeps.  The estimated volume of groundwater in storage ranges from 11 maf to more than 27 maf (Table 3.8-6).  Water level change data between 1990-1991 and 2003-2004 for 16 wells shows relatively stable levels in most wells (Figure 3.8-7).  (A water level sweep was conducted in winter 2006-2007 and a hydrologic map series report is expected to be completed by fall 2009).Water quality data from selected sites show that fluoride was the parameter that most frequently exceeded drinking water standards, with elevated levels of cadmium found in the vicinity of Hayden and Dudleyville (Table 3.8-7).


Upper San Pedro Basin

The Upper San Pedro Basin consists of the northwest trending San Pedro River Valley and surrounding mountains that range from 5,000 to almost 10,000 feet in elevation. The basin contains two sub-basins: the Sierra Vista and the small Allen Flat sub-basin. Basin fill is the principal aquifer although the stream alluvium is also utilized.  Groundwater in the basin fill aquifer is found in both unconfined and confined conditions. Artesian conditions exist near Palominas, Hereford, and more extensively near Benson and Saint David. These conditions supported modest groundwater discharges for irrigation use primarily in the Benson-Pomerene area. An interesting feature is a limestone aquifer in the Whetstone Mountains that contains a “live” or wet cave, Kartchner Caverns, a state park.  The water level in the cavern is about 700 feet higher than that of the underlying alluvial aquifer (ADWR, 2005a). 


Allen Flat

Allen Flat, Upper San Pedro Basin. The basin consists

of two sub-basins: the Sierra Vista and the small

Allen Flat sub-basin

Groundwater flow direction is from the mountain fronts toward the central valley and to the north. A cone of depression has formed in the Sierra Vista area that has altered flow direction (Figure 3.13-7).  Groundwater recharge is approximately 35,700 AFA from the mountain fronts, underflow from Mexico and streambed infiltration. Two effluent recharge projects in the basin also recharge the aquifer. The most populous basin in the planning area, major discharge is from municipal and agricultural pumpage and from riparian evapotranspiration. (ADWR, 2005a)  The most recent estimate of groundwater in storage is 19.8 to 26.1 maf although estimates of up to 59 maf  exist (Table 3.13-5).

As shown in Figure 3.13-7, water levels declined in most wells measured in 1990-1991 and 2003-2004.  Additional data show annual declines of 0.9 to 2.9 feet in some wells in the Bisbee-Naco area and rises of up to 0.6 feet per year in the Pomerene area north of Benson (ADWR, 2005a).  The Department measured water levels in the basin in 2006 and these data are expected to be released in a water level change map series report in 2009.  Preliminary data show water levels decreasing in most wells in the Bisbee and Naco area; about seven feet in five years from 2001 to 2006.  In the Benson area, water levels west of the San Pedro River have declined most.

Groundwater quality is generally suitable for most uses. Arsenic and fluoride were the water quality parameters that most frequently exceeded drinking water standards in wells sampled in the basin. Localized nitrate contamination near St. David is being remediated as part of the Superfund Program.



For more information on Groundwater in individual basins in the Southeastern Arizona Planning Area see the menu to the right

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