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Hydrology of the Southeastern Arizona Planning Area - Groundwater (Southern Portion and Other Basins)

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:

 

Southern Portion

Groundwater from three basins in the southern portion of the planning area flows south into Mexico. These basins are the Douglas and San Bernardino Valley basins in the southeastern part of the planning area and the San Rafael Basin in the southwest corner.

 

Douglas Basin

The Douglas Basin occupies the southern portion of a northwest-southeast trending structural trough that extends from the central part of the Aravaipa Canyon Basin, through the Willcox Basin, to the northeastern part of Sonora, Mexico.  The long alluvial valley in the Douglas Basin, (the southern part of the Sulphur Springs Valley), contains its main aquifer, basin fill, which supplies most of its large-capacity wells.  The basin fill is composed of sand and gravel lenses interbedded with silt and clay lenses. The sand and gravel lenses are the main source of water. Groundwater is primarily unconfined although artesian conditions were reported locally in the upper alluvial deposits in the early 1950s prior to the start of heavy groundwater pumping (Rascona, 1993). Groundwater is also found in the mountain bedrock which provides relatively small amounts of water for stock and domestic use. In and adjacent to the City of Douglas, groundwater is pumped from basin fill with interbedded volcanic rock.  Groundwater flow is generally from north to south although agricultural pumpage has altered flow directions in the vicinity of Elfrida where a cone of depression has developed.

 

Alfalfa in Douglas Basin

Agriculture near Elfrida, Douglas Basin. The basin has been severely over-drafted since the late 1940s and much of the basin is designated as an Irrigation Non-Expansion Area to restrict agricultural expansion.

Groundwater recharge occurs mainly in washes and along mountain fronts (Rascona, 1993) and is estimated at 15,500 to 22,000 AFA (Table 3.5-5). Incidental recharge may also come from infiltration of agricultural irrigation (USGS, 2006b).  Groundwater discharge is primarily from groundwater pumping of almost 53,000 AFA. Groundwater in storage estimates range from 26 to 32 maf.  The basin has been severely over-drafted since the late 1940s and much of the basin was designated as an Irrigation Non-Expansion Area in 1980 to restrict agricultural expansion.  As mentioned previously, concerns about the future availability of water in the basin is a subject of an investigation to compile hydrologic data and information (USGS, 2006b). Between 1990-1991 and 2003-2004, water levels declined in most wells measured in the basin, particularly in the Elfrida area and north of Douglas (Figure 3.5-6). Groundwater quality is generally suitable for most uses although elevated fluoride concentrations have been measured in a number of wells (Table 3.5-6).

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San Bernardino Valley

The San Bernardino Valley Basin is covered by volcanic flows and cinder cones with some relatively thin alluvial deposits. Groundwater is obtained from sand and gravel interbedded with basalt flows or from shallow alluvium. Springs and artesian wells support wetlands designated as the San Bernardino National Wildlife Refuge adjacent to the international border. Groundwater flow is from the mountains toward the valley center and south to Mexico. Estimated groundwater recharge is 9,000 AFA and groundwater storage estimates range from 1.6 to 2.0 maf (Table 3.11-3).  Most wells in the basin are located immediately north of the international border where water levels are generally less than 100 feet below land surface. The depth to water increases to the north and toward the mountains along the basin margins on the west, north and east. Little groundwater data are available for the basin.

San Rafael Valley

San Rafael Valley, San Rafael Basin

San Rafael Basin

The San Rafael Basin consists of a broad north-trending valley surrounded by block-fault mountains and drained by the Santa Cruz River whose headwaters are in the northern portion of the valley.  Groundwater is obtained from stream alluvium and basin fill. Groundwater is found in stream alluvium along the Santa Cruz River and its major tributaries. Basin fill occupies most of the valley and is composed of clay, silt, sand and gravel. The basin fill has been estimated to be as much as 1,900 feet deep based on well logs.  Bultman (1999) estimated that the San Rafael basin may contain an aquifer up to approximately 1,000 feet thick over a substantial area consisting of upper basin fill.  Groundwater flow is from the mountains toward the Santa Cruz River and then south. Groundwater recharge is from mountain front recharge and infiltration of runoff in stream channels. Groundwater recharge is estimated at 5,000 AFA (Table 3.12-5). Estimated groundwater in storage ranges from 4 to 5 maf.  Water levels are relatively shallow (25 feet bls or less) in the streambed alluvium and generally at levels over 100 feet bls in the basin fill. Well yields are generally higher in the streambed alluvium. There is little water quality data available for the basin but drinking water exceedences of arsenic, antimony, lead and radionuclides have been detected in wells in the western part of the basin, an area of historic mining activity.

Other Basins

Two basins, Cienega Creek and Willcox, have hydrogeologic conditions that are unique in the planning area.  The Cienega Creek Basin has three groundwater sections based on the presence of distinctive aquifers and groundwater flows to the north and to the southwest. Groundwater in the Willcox Basin is generally isolated from surrounding basins, with groundwater flow primarily to the center of the basin, the Willcox Playa.

Cienega Creek Basin

The Cienega Creek basin consists of a narrow northeast trending alluvial valley, drained by Cienega and Sonoita creeks, and surrounded by fault-block mountains.  There is a surface water divide southwest of Sonoita, with Cienega Creek flowing northeast and Sonoita Creek flowing to the south and west.  Hydrogeologic conditions in the basin are complex. The basin has been divided into three subareas based on the presence of a distinctive aquifer or set of aquifers: upper Cienega Creek, lower Cienega Creek and Sonoita Creek.  “The Narrows” (T18S, R18E, S6), where bedrock outcrops on both sides of the Cienega Creek channel, divides the lower and upper Cienega Creek subareas (Bota, 1997). The upper Cienega Creek subarea includes most of the basin’s central valley. The main aquifer is basin fill, which is deepest in the southern part of the subarea between Sonoita and Elgin.  To the north, the lower Cienega Creek subarea extends to the northern basin boundary. It contains three aquifers: stream alluvium, basin fill and the Pantano formation. The main aquifer in this subarea is the stream alluvium.

Cienega Creek Riparian

Cienega Creek Riparian Vegetation.  Hydrogeologic conditions in the basin are complex.  THe basin has been divided into three subareas based on the presence of a distinctive aquifer or set of aquifers: upper Cienega Creek, lower Cienega Creek and Sonoita Creek.

The basin-fill alluvium is a relatively poor aquifer in this subarea with relatively low well yields and interbedded clay layers that create a leaky, confined and artesian aquifer conditions.  The southwestern part of the basin is the Sonoita Creek subarea where the main aquifer is the stream alluvium that forms the floodplain of Sonoita Creek and its tributaries and may be up to 90-feet thick.  Wells drilled in the basin fill are generally low yielding. Groundwater flow follows the surface water flow direction with flow toward the northeast, north of Sonoita, and to the south, south of Sonoita.

Groundwater recharge comes from mountain front recharge and streambed infiltration along Cienega and Sonoita creeks and their tributaries.  Groundwater recharge estimates vary from 8,500 to 25,500 AFA, although this does not include the Sonoita Creek subarea (Table 3.3-5). Estimates of groundwater in storage range from 5.1 to 11 maf.  Water level trends are generally stable with some declines noted near Patagonia and east of Sonoita (Figure 3.3-6).  Groundwater quality is generally good although cadmium and copper concentrations exceeding drinking water standards have been measured in several wells in the vicinity of Patagonia.

Willcox Basin

Excerpt from ADWR Water Level Change Series Map No. 1 on the Willcox Basin (Jacobson and others, 2008)

Willcox Basin

The Willcox Basin occupies the northern part of the Sulphur Springs Valley and is hydrologically separate from the southern part of the valley, the Douglas Basin. Groundwater in the Willcox Basin is found in alluvial deposits consisting of stream and lake-bed deposits.  The stream deposits are the most productive water-bearing unit. The clay-rich lake bed deposits outcrop in the Willcox Playa. There they create localized artesian conditions.  Where the coarse-grained stream deposits are underlain by the lake-bed deposits, perched groundwater conditions may occur.  A playa is a nearly level area at the bottom of a closed desert basin, sometimes temporarily covered by water. 

The Willcox Basin has internal surface water drainage and groundwater flow is thought to have mirrored surface drainage under predevelopment conditions; moving from the outer margins toward the Willcox Playa (Oram, 1993).  However, groundwater flow conditions have been altered significantly due to groundwater pumping for agriculture. Several relatively large cones of depression have developed in the basin including one southeast of the Willcox Playa and another north of the City of Willcox (Figure 3.14-6). Groundwater recharge has been estimated at 15,000 to 47,000 AFA primarily from mountain front recharge and also from agricultural irrigation and stream channel runoff (USGS, 2006b).  Groundwater discharge is primarily from groundwater pumping of more than 176,000 AFA. Estimates of groundwater in storage range from 42 to 59 maf (Table 3.14-6).

Declines in groundwater levels (in excess of 200 feet measured in nine wells between 1954 and 1975), may have caused land subsidence in the basin (USGS, 2006b).  Figure 3.14-6 shows groundwater level changes between 1990-1991 and 2003-2004. A number of declines of greater than 30 feet were measured in wells in the central part of the basin during this period.  Concerns about groundwater level declines and future availability of water for all uses has led to an investigation of the geology and hydrology of the Willcox and Douglas basins (USGS, 2006b).  As part of this effort, the Department released a Water Level Change Map Series Report (No. 1) in 2008 summarizing depth to water measurements taken at 578 wells in the Willcox Basin in November/December 1999 and November/December 2005.  Most of the wells (549 of 578 or 95%) showed a water level decline. Forty had declines of more than 40 feet and most of these were located in the area southeast of the Willcox Playa in a predominantly agricultural area (Jacobson and others, 2008).  A summary of the water level changes and a water level change contour map from the map series report are shown in the graphic below.  As shown, most water levels declined between 0.5 and 20.4 feet. A median well yield of 750 gpm was reported from over 1,000 large diameter wells in the basin (Table 3.14-6).

Elevated TDS concentrations exist in some areas and fluoride and arsenic concentrations above drinking water standards have been reported in a number of wells (Table 3.14-7).

 

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

 

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