Groundwater Hydrology

North/Northeastern Portion

Groundwater basins located in the north and northeastern portion of the planning area are Bonita Creek, Dripping Springs Wash, Duncan Valley, Morenci and Safford. The Safford Basin aquifers are primarily stream alluvium and basin fill, while the other basins also contain aquifers composed of volcanic rock or sedimentary rock (Gila Formation). Groundwater flow is toward the Gila River drainage and the Bonita Creek, Duncan Valley and Morenci basins contribute underflow to the Safford Basin. 

Safford Basin

The Safford Basin is a relatively large, alluvial filled depression bordered by elongated mountain ranges.  Basin fill is the major aquifer in all three sub-basins of the Safford Basin. Sub-basin delineations are shown in Figures 3.10-7 and 3.10-9.  In the San Simon Valley Sub-basin a clay deposit, known as the Blue Clay unit, separates the upper and lower basin-fill aquifers and may be as much as 600 feet thick. Groundwater is found under artesian conditions in the lower aquifer and is generally unconfined in the upper aquifer. Groundwater flow in the sub-basin is toward the north along the San Simon River drainage but also flows toward agricultural pumping centers. The principal aquifer in the Gila Valley Sub-basin, located in the middle part of the Safford Basin, is the upper basin fill, underlain by the Blue Clay unit. Groundwater is also utilized from the lower basin fill, which generally is found under artesian conditions and where well discharges may be quite high. Groundwater flow is from south to north along the Gila River drainage. The main water-bearing unit in the San Carlos Valley Sub-basin, located in the northern part of the Safford Basin, is the upper basin fill, which is found under unconfined conditions.  As with the other sub-basins, groundwater in the lower basin fill is generally found under artesian conditions. Groundwater flow in the sub-basin is toward the Gila River drainage.

Groundwater recharge for the entire basin is estimated at 105,000 AFA. Groundwater discharge is to agricultural and municipal pumping, primarily in the Gila Valley Sub-basin, and to spring discharge.  Estimates of groundwater in storage range from more than 27 maf to 69 maf (Table 3.10-6).

Depth to water is relatively shallow in wells measured near the Gila River, while water levels are generally deeper in wells in the San Simon Valley Sub-basin, the southernmost sub-basin. Water levels declined in most wells in the basin that were measured in 1990-1991 and 2003-2004, with the most significant declines south of San Simon where water levels declined by more than 30 feet during this time period (Figure 3.10-7).  Water levels exceed 600 feet bls at two wells along the western boundary of the San Carlos Valley Sub-basin, the northernmost sub-basin.  In one of these wells, water levels declined over 60 feet between 1990 and 2004 (Figure 3.10-7). Most of the groundwater development in the Safford Basin is in the Gila Valley Sub-basin, the central sub-basin, which contain the basin’s major population and agricultural centers. The median well yield reported on registration forms for almost 1,500 large (>10-inch) diameter wells was 600 gpm.  As shown on Figure 3.10-9, high yield (>2000 gpm) wells are found along the Gila and San Simon river drainages and in the vicinity of Bowie.

Water quality conditions vary in the basin although fluoride and arsenic concentrations consistently exceed drinking water standards. In the San Simon Valley Sub-basin the upper aquifer generally contains elevated total dissolved solids (TDS) and fluoride concentrations.  Groundwater in both the upper and lower basin fill of the Gila Valley Sub-basin may also be high in TDS.  In the San Carlos Valley Sub-basin, elevated levels of TDS have been measured in stream alluvium.

Click to view Table 3.10-6 Groundwater Data for the Safford Basin

Click to view Figure 3.10-7 Safford Basin

Groundwater Conditions

Click to view 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). 

Upper Gila Watershed

The Upper Gila Watershed drains about 7,400 square miles in the planning area above Coolidge Dam and contains the Bonita Creek, Duncan Valley, Morenci, and Safford basins.  Major tributaries include the San Francisco River, Eagle Creek, Bonita Creek, San Simon Creek and the San Carlos River. 

An average of about 160,000 AFA of Gila River water flows into Arizona from New Mexico and over 40% of this flow typically occurs in the winter. Tributary inflows from the San Francisco River are significant, typically over 150,000 AFA.  Inflow to the San Carlos Reservoir from the Gila and San Carlos Rivers averages about 310,000 AFA (ADWR, 2006).  There are three active streamgages on the Gila River. The maximum annual flow recorded was at a gage near Solomon with a flow of 1.56 maf in 1993.  Median flow at this gage is approximately 273,000 AFA (see Table 3.10-2).

The largest spring in the planning area is located in the Safford Basin.  Warm Springs, with a measured discharge of almost 3,400 gpm is located at the headwaters of the San Carlos River.  There are also a number of large springs downstream of Pima near the Gila River (USGS, 2006c). In total, there are 22 major springs in the Safford Basin. Other major springs are found in the Bonita Creek Basin (1 spring), Duncan Valley Basin (2), and Morenci Basin (9).  Most of the spring measurements shown on the springs tables in sections 3.2, 3.7, 3.9 and 3.10 were taken between 1940 and 1982 and may not be indicative of current conditions.

A 15-mile reach of the Gila River in the Duncan Valley Basin is impaired due to elevated selenium concentrations (Table 3.7-7).  In the Safford Basin, a 6-mile reach of the Gila River exceeded the water quality standard for E.coli and turbidity and a 8-mile reach of Cave Creek exceeded the standard for selenium (Table 3.10-7).  In the Morenci Basin, water quality standards were exceeded at Luna Lake and in a 13-mile reach of the San Francisco River near Alpine (Table 3.9-7)

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