Groundwater Hydrology

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.

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 found in stream alluvium and in basin fill 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.

Click to view Table 3.12-5 Groundwater Data for the San Rafael Basin

Figure 3.0-5 USGS 6-Digit Hydrologic Unit Code Boundaries in the Southeastern Arizona Planning Area

Surface Water Hydrology

The U.S. Geological Survey (USGS) divides and subdivides the United States into successively smaller hydrologic units based on hydrologic features.  These units are classified into four levels. From largest to smallest these are: regions, subregions, accounting units and cataloging units.  A hydrologic unit code (HUC) consisting of two digits for each level in the system is used to identify any hydrologic area (Seaber et al., 1987).  A 6-digit code corresponds to accounting units, which are used by the USGS for designing and managing the National Water Data Network.  There are portions of five watersheds in the planning area at the accounting unit level: Lower Colorado River below Lake Mead; Middle Gila River; Rio Bavispe; San Pedro River; Santa Cruz River; and the Upper Gila River (Figure 3.0-5). 

Santa Cruz Watershed

The Santa Cruz Watershed includes most of the Cienega Creek and San Rafael basins and extends south into Mexico and west to include the Santa Cruz AMA and most of the Tucson and Pinal AMAs. The Santa Cruz River originates in the San Rafael Valley and flows southward to Mexico before turning north and reentering the U.S. east of Nogales, Arizona.  Surface water in the Cienega Creek Basin drains west to the Santa Cruz River from Sonoita Creek and north to tributaries of the Santa Cruz River from Cienega Creek.

The Santa Cruz River is perennial in the planning area.  In the Cienega Creek Basin there are perennial reaches of Cienega Creek, Sonoita Creek and Red Rock Canyon. The only streamgage on the Santa Cruz River is near Lochiel with a maximum annual flow of 12,600 acre-feet measured in 1955. Median annual flow at this gage is 1,410 acre-feet. The only other streamgage in the watershed is a gage on Cienega Creek near Sonoita (see Table 3.3-2).

Major springs are located only in the Cienega Creek Basin.  The largest of the seven major springs is Monkey Spring southwest of Sonoita with a discharge rate of 430 gpm.  A measurement date is lacking for this spring (Table 3.3-5). 

There are several impaired waters in the Santa Cruz Watershed. Parker Canyon Lake in the San Rafael Basin contains elevated levels of mercury.  In the Cienega Creek Basin, a total of 20 miles of impaired stream reaches occur on Alum Gulch, Harshaw Creek, Humboldt Canyon and on an unnamed tributary to Harshaw Creek. These waters contain concentrations of cadmium, copper, zinc or pH that exceed standards (Table 3.3-6).

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