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 stream alluvium. The basin-fill alluvium is a relatively poor aquifer in this subarea with relatively low well yields and interbedded clay layers that create leaky confined and artesian aquifer conditions.
The southwestern part of the basin is the Sonoita Creek subarea where the main aquifer is 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 estimate 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.