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.
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 groundwater. 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.
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. 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 1990-91 and 2003-04, 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 3.5-5).
Click for Table 3.5-5 Groundwater Data for the Douglas Basin
Click for Figure 3.5-6 Douglas Basin Groundwater Conditions