Geology and ground-water resources of the Douglas basin, Arizona, with a section on chemical quality of the ground water

The Douglas basin is part of a large northwest-trending intermontane valley, known as
the Sulphur Spring Valley, which lies in southeastern Arizona, and extends into northeastern
Sonora, Mexico. Maturely dissected mountains rise abruptly from long alluvial
slopes and culminate in peaks 3,000 to 4,000 feet above the valley floor, Bedrock in the
mountain areas confines drainage on the east and west, and an arc of low hills to the
north separates the basin from the Willcox basin of the Sulphur Spring Valley. Drainage of
the 1,200 square miles in the Douglas basin is southward into Mexico through Whitewater
Draw.

The mountains include igneous, metamorphic, and sedimentary rocks ranging in age
from pre-Cambrian to Tertiary, including Paleozoic and Mesozoic sedimentary rocks that
total about 10,000 feet in thickness. The older rocks have been metamorphosed, and all the
bedrock has been affected by igneous intrusion, largely in Mesozoic time, and by structural
movements, largely in Cenozoic time and extending into the Quaternary period. By
the early part of Cenozoic time the major structural features were formed, and mountain
ranges had been uplifted above the valley trough along northwest-trending fault zones. Since
that time the physiographic features have resulted through erosion of the mountain blocks
and the deposition, in places, of more than 2,800 feet of unconsolidated rock debris in the
valley.

Ground-water supplies of the Douglas basin are developed largely in the saturated
zone of the valley-fill sediments. The ground water in the valley fill occurs in thin lenses
and strata of sand and gravel, which are interbedded with large thicknesses of silt and
day. Scattered gypsum beds and extensive caliche deposits appear at the surface and
occur within the valley fill at various depths. Although the valley-fill sediments are as
much as 2,800 feet thick, the uppermost 300 feet or so are the most permeable.

Ground water originates as precipitation in the mountain areas. The water collects in
streams that lose much of their flow into the coarse sediments that fringe the mountains.
Part of the water ultimately percolates into the zone of saturation. High evaporation
rates, vegetative use, and the presence of caliche and clay at shallow depth in the interstream
areas of the valley floor prevent important recharge of the ground-water reservoir
from direct rainfall or seepage of water applied for irrigation. The total recharge into the
ground-water reservoir of the Douglas basin was about 20,000 acre-feet in 1951.

Ground water is discharged from the basin by evapotranspiration, by effluent seepage
into Whitewater Draw and underflow out of the basin, and by pumping. In 1951, the total
amount of ground water discharged was about 50,000 acre-feet, of which more than 41,000
acre-feet was pumped from wells. Ground water used in excess of recharge is withdrawn
from storage, causing a decline in the water table. Maximum declines have occurred in
the heavily pumped Elfrida area, where a decline of more than 11 feet occurred in the 5-
year period 1947-51, inclusive.

Most irrigation wells in the Douglas basin are less than 200 feet in depth and usually
produce less than 400 gpm (gallons per minute). The average specific capacity of the
wells is about 12 gpm per foot of drawdown. Although water in some parts of the basin is
artesian, all irrigation wells must be pumped.

Ground water in the basin is generally of excellent to good quality for irrigation use, In
small areas along the southern part of Whitewater Draw and east of Douglas the ground
water is high in dissolved-solids content. Although most of the water is hard, it is generally
satisfactory for domestic use. In many areas the fluoride content is more than 1.5
ppm (parts per million).