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.
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