All the wells and springs in the surrounding region for which data are available have temperatures lower than Quitobaquito Spring except No. 1 Well and Papago Well. The mean annual temperature of the air for Ajo, the nearest station to Quitobaquito, 33 miles to the north, may be estimated from the records for 1915 to 1918. The average of these years is 69.1° F. For the same years at Yuma the average is 70.8°, which is 1.5° below the mean of 72.3° for all years observed at Yuma. The departure of the four-year average from the yet undetermined mean of Ajo may not be the same as that at Yuma nor in the same direction, but it is probably of the same order of magnitude. This average (69.1°) may then be taken as correct within 1° or 2° plus or minus. The Al Johns Well, in Gibson, less than a mile from the weather station of Ajo, had a temperature of 76.5° on October 16, 1917, near the middle of the day. The temperature of this well is exceeded in only one of the other shallow wells-Cipriano Well. That well was, however, nearly dry on September 30, 1917, the date of measurement, containing only 1.4 feet of water, and its temperature was probably affected by the air temperature. This water of shallow wells is commonly a few degrees warmer than the mean annual temperature of the air, but the Quitobaquito water is 8° or 9° warmer still. The temperature of the Quitobaquito Spring is also above that normal for shallow ground waters in this area. The abnormal temperature and the lack of any local source for the water in the granite from which the spring flows make it reasonably certain that the water is not derived from rainfall but rises from a fissure penetrating the deeper crust. Near the surface this fissure probably merges with the numerous joints and fractures of the surface rocks. The water then comes to the surface by numerous openings in the broad zone of seepage and is cooled by contact with the surface rocks, so that it emerges with only a small excess of temperature above the normal.

Analysis of a sample of the spring water taken October 1, 1917, shows no special distinctions from other ground waters but considerable differences from the water of Sonoita River. The following table gives the analyses of samples from (1) the main opening at Quitobaquito; (2) Blankenship Well, 12 miles southeast of Quitobaquito (see table, p. 181); and (3) Sonoita River at low stage 14 miles south of Quitobaquito:

Analyses of waters from Quitobaquito Spring and near-by sources

Analyzed by A. A. Chambers and C. H. Kidwell. Parts per million except as otherwise designated]

WATER IN THE CRYSTALLINE COMPLEX

The crystalline rocks of pre-Cambrian and Mesozoic age, which together may be grouped as the crystalline complex, crop out largely in the mountains and surrounding pediments. The pore spaces in such rocks are few and so small that the rocks are practically impervious to water. Near the surface of the ground these rocks are, however, broken by numerous fractures and joints. The origin of these fractures is complex; many are due to weathering (p. 81), others to relief of load, compressive and tensional strains, faulting, cooling, or other causes. Whatever its origin, each fracture is a sheetlike opening capable of receiving and holding a film of water. Where these fractures are numerous and open, wells may be profitably dug.

42

The wells in the crystalline rocks of Connecticut have been closely studied. The rocks of the Papago country are very similar to those of Connecticut in the number, size, and character of their fractures. The total quantity of water is much less in the Papago country than in Connecticut because of the smaller precipitation. Of 237 wells studied in Connecticut only 3 are recorded as obtaining no water; of 137 wells whose yield is known only 17 furnish less than 2 gallons a minute, or enough to supply 200 head of cattle if pumped continuously. The most significant result of drilling in Connecticut is the determination of a limit in depth below which it is not profitable

42 Gregory, H. E., and Ellis, E. E., Underground water resources of Connecticut: U. S. Geol. Survey Water-Supply Paper 232, pp. 54-76, 91-94, 1909.

to go. The fractures, joints, and seams in the rock become fewer and less open as depth increases, and it has been found that below 200 feet there are usually not enough open water-bearing joints to furnish water for a well. The same generalization may be made for the crystalline rocks of the Papago country.

Wells in the crystalline rocks in the Papago country are commonly dug by the ordinary methods of sinking shafts. Such a well has the advantage that its large area cuts a great number of fractures. The well is also a reservoir that stores water, which may seep slowly out of the crevices. It is a common practice to run drifts or galleries from the bottom of the well. This is usually done in dry seasons when necessity compels, and the work is then easy because there is little or no difficulty in keeping water away from the workings. The object of a gallery is to intercept more joints and fractures and thus obtain additional seepage into the well. A gallery also provides additional storage for water, which may be pumped out rapidly and then allowed to accumulate between pumping periods.

The characteristics of wells dug in the crystalline complex are shown in the following table:

Characteristics of certain wells dug in crystalline rock in the Papago country

Less than 10 feet.

Lower part.

The table shows that water occurs in many types of crystalline rock. A large number of wells, however, are dug in felsites and porphyries, some intrusive and others ancient lavas, presumably of Mesozoic age. These rocks are intricately fractured. The joints in many places are so close together that the rock is broken into blocks from 1 to 2 inches in diameter. Close jointing seems to be the determining factor in producing successful wells in these rocks. The prospective well digger should therefore select a place where the rocks are broken by numerous and closely spaced fractures. Such localities are most likely to be found in areas of felsite and schist.

The table shows also that the greater number of these wells are on the pediments that flank the mountains (see pp. 93-101), commonly in or near stream courses. Certain practical reasons have much to do with the location of these wells. Wells dug for stock watering places must be near grazing areas and in a place where they can be reached by easy trails. The rough mountains are therefore avoided. Many mines and prospects are located in the pediment or just at the foot of the mountains, because the ore-bearing rocks are commonly somewhat less resistant to erosion than the granite and gneiss and are thus more likely to occur in the pediment than in the mountains. The location of wells along streams is practical common sense, for a flood on an ephemeral stream contributes to the water in the rocks of a locality as much or more than a rain. Along the streams is found the heaviest vegetation, which is kept alive by this flood water. Certain desert plants also are thought to indicate ground water, as discussed on page 156, and in some parts of the region there are seeps in wet weather that are favorable localities for developing wells. One who wishes to dig a well in the crystalline rocks, giving weight to the evidence of vegetation or of seepage, should place his principal reliance on a careful inspection of the locality for the purpose of finding an area of minutely fractured rocks which is so situated as to receive water from the floods of a wash or ephemerai stream that has a large drainage area.

The rocks of the crystalline complex are very diverse in type, and it is therefore to be expected that well waters derived from cracks and joints in these rocks should also differ chemically. Dobbs Well and Stouts Well, with total dissolved solids of 667 and 579 parts per million, are typical well waters of the crystalline complex. In both waters dissolved silica is high and bicarbonate is the principal acid radicle. The water of Stouts Well is higher in iron and magnesium, doubtless derived from the schist in which the well is dug. The high nitrate content of this well water, which exceeds that of Dobbs Well