11

UNDERGROUND WATER.

Valley wells.—For several years water has been pumped from underground sources in the El Paso Valley to supplement the supply from the river at times when little or no surface water is available, and many pumping plants have been installed. Typical valley wells are about 60 feet deep and between 4 and 8 inches in diameter, and water is usually found from 10 to 15 feet below the surface. There are several dozen pumping plants in operation in the valley below El Paso, electricity, crude oil, and gasoline being used for motive power, and the financial returns are reported to be very satisfactory.

It has long been thought that there is a large underflow in the narrows above El Paso, but measurements undertaken by C. S. Slichter in 1904, at the request of the writer, show that the underflow there is insignificant, amounting only to about 50 gallons a minute. Part at least of the water in the valley is derived from the water-bearing beds below the Hueco Bolson, which in the vicinity of Fort Bliss are only about 30 feet beneath the level of the river. But the most important source of underground water in the El Paso Valley is seepage from the river. The stream scours its bed during flood stages, so that there is direct access of water to the underlying sand and gravel, which become saturated. It is a common experience that the water level in the valley wells fluctuates with changes in the stage of the river. During long periods when the river is dry the water level gradually lowers, and after floods it rises. The main well of the old city water company in El Paso furnished an example. This is a dug well 65 feet deep, 18 feet in diameter at the top and 14 feet at the bottom. It is reported that in periods of drought, when the pumps were not working, the water level was at a depth of about 36 feet, and that after high water in the river the level rose within 12 or 15 feet of the surface. It thus appears that the sand and gravel beneath the valley serve as reservoirs which are replenished during flood stages of the river.

Mesa wells,.—Until recently the water supply of the city of El Paso was obtained from valley wells situated within the city limits near the river. The quality of that water is poor, however, and in the fall of 1905 the International Water Company began supplying the city from a series of wells on the mesa just north of Fort Bliss. In 1903 a prospect hole was sunk to a depth of 2285 feet, as recorded on page 5, but besides a small quantity at a depth of about 400 feet, water was not obtained except at the usual zone, between 200 and 300 feet below the surface. A dozen or more wells from 10 to 12 inches in diameter have since been sunk to depths of 500 feet or more, and these are used for the city supply. The water rises in the wells about 40 feet above the water-bearing beds to the normal level of 180 feet below the surface. The wells are connected and the water is raised by compressed air, petroleum being used for fuel. From a near-by reservoir the water is pumped to the city through a 16-inch main, a distance of about 6 miles. The average supply is 1,500,000 gallons a day.

Examination of the records shows that alternating layers of sand, gravel, and clay were encountered in all the wells down to the water-bearing sands, below which clay is chiefly present, with some interbedded sand. The water-bearing zone is not a single stratum, but comprises several beds of sand separated by clay, between 200 and 300 feet below the surface. These sands are reported to be fine, with some intermixture of gravel, and range in thickness from 10 to 28 feet.

A number of other wells in the Hueco Bolson are used chiefly for watering stock, but little information concerning the water horizon is available. The wells range in depth from 200 to 600 feet and are equipped with windmills or gasoline engines. Little or no gravel is reported and the water occurs in sand that locally is so fine as to yield but a scanty supply. This water is generally under pressure enough to cause it to rise a few feet.

To judge from the different depths of the wells and from the varied conditions illustrated by the International Water Company's logs, there appears to be no sharply defined water stratum, but the entire Hueco Bolson is apparently underlain El Paso by lenses of water-bearing sands. More test wells are needed toward the center of the bolson to determine whether a greater artesian head exists than has already been found.

The supply of mesa water is probably replenished by the rainfall only on a small part of the Hueco Bolson and on the contiguous highlands, although from the extent of this intermontane lowland it might be supposed that there is a much larger tributary area extending far northward into New Mexico. But the marked difference in the quality of the underground waters north and south of an indefinite line near the Texas-New Mexico boundary indicates that subterranean circulation is cut off between the two areas. Possibly the south westward continuation of the Jicarilla Mountains constitutes the barrier, which is superficially covered by wash. Presumably little of the rainfall percolates directly downward to the water horizon over a considerable part of the mesa, because of the common presence of clay and caliche at or near the surface. There are, however, on the mesa several broad sand-covered areas that are well adapted to absorb the rainfall, and the porous deposits adjacent to the base of the highlands imbibe the storm waters so readily that the run-off disappears a short distance from the mountains. From these sources that part of the small rainfall that is not evaporated percolates to the water horizon, but the areas of absorption are so ill defined and the loss by evaporation is so little known that it is impossible to estimate the annual increment to the store of underground water.

Quality of the water.—The accompanying analyses show the general character of the underground waters of the quadrangle:

There is a marked contrast between the mesa and the valley waters, averages of the analyses showing 323 parts per million of dissolved solids in the former and 1240 in the latter. There is also a difference in the relative abundance of the several dissolved salts. In both waters sodium and calcium are the most abundant bases, but the valley water contains preponderating chlorides with abundant sulphates and minor carbonates, whereas the mesa, water contains predominant carbonates, subordinate sulphates, and only small amounts of chlorides. These differences in composition are accounted for by the character of the materials with which the waters come into contact. The mesa water passes through sands and gravels containing comparatively little soluble matter. There appears to be a local source of sodium chloride above El Paso to cause the abundance of that substance in the valley waters. This is further indicated by an analysis of water from a valley well belonging to Z. White, 10 miles above El Paso, showing 17,200 parts per million of dissolved solids, chiefly sodium chloride. It should be noted that the El Paso sewage is allowed to escape directly into the bed of the river below the city, thereby subjecting the lower valley wells to contamination.

BIBLIOGRAPHY.

The following is a summary of the principal publications relating to the geology of the El Paso district.

Wislizenus appears to have been the first geologist to report on the area. In 1846 he visited the Franklin Mountains (?) and noted the occurrence of Silurian rocks. (Wislizenus, A., Tour of northern Mexico; Senate Misc. Doc. No. 26, 30th Cong., 1st sess., 1848, pp. 40-42.)

More complete collections of rocks and fossils were made by the surveying parties engaged in exploring routes for a Pacific railway and in establishing the boundary between the United States and Mexico. C. C. Parry, of the Mexican Boundary Survey, made observations in the Rio Grande valley in 1853 and 1854, and James Hall and T. A. Conrad reported on the collections. This work made known the occurrence of rocks of the Cretaceous and Carboniferous systems, and again called attention to the presence of the Silurian. (Report of the United States and Mexican Boundary Survey, Win. H. Emory, vol. 1, pt. 2, 1857, Senate Ex. Doc. No. 108, 34th Cong., 1st sess.),

Captain John Pope, in charge of the military survey for a Pacific railway adjacent to the 32d parallel, in 1853 traveled eastward from El Paso via the Hueco Mountains to Pecos River. No geologist was attached to this expedition, but specimens of rocks were collected which were reported upon by W. P. Blake. (Explorations and surveys for a railroad route from Mississippi River to Pacific Ocean, vol. 2, 1855, Senate Ex. Doc. No. 78, 33d Cong., 2d sess.),

Captain Pope made further explorations in the same region between 1855 and 1857. He was accompanied by G. G. Shumard, who made geologic notes along the route of travel. Publication of the latter's report was long delayed, and it was finally printed by the State of Texas. (A partial report on the geology of western Texas, Austin, 1886. Short notices of Shumard's more important results appeared in Trans. St. Louis Acad. Sci., vols. 1 and 2, 1860 and 1868.)

W. P. Jenney in 1874 measured a section of the rocks in the Franklin Mountains and first called attention to the occurrence of the Cambrian in that region. (Am. Jour. Sci., 3d ser., vol. 7, 1874, p. 25.)

T. W. Stanton and T. W. Vaughan in 1896 studied the section of Cretaceous rocks in the Cerro de Muleros west of El Paso. (Am. Jour. Sci., 4th ser., vol. 1, 1896, pp. 21-26.)

R. T. Hill has referred to the El Paso district in a number of papers. (Notes on the Texas-New Mexican region: Bull. Geol. Soc. America, vol. 3, 1891, pp. 95-96. The Cretaceous formations of Mexico and their relations to North American geographic development: Am. Jour. Sci., 3d ser., vol. 45, 1893, p. 307. Physical geography of the Texas region: Topographic Atlas U. S., folio 3, U. S. Geol. Survey, 1900. Geographic and geologic features of Mexico: Trans. Am. Inst. Min. Eng., vol. 32, 1902, p. 163. Geologic and geographic aspects of Mexico: Mining World, vols. 25, 26, 27, 1906, 1907.)

W. H. Weed in 1900 examined the tin deposits north of El Paso. (Bull. U. S. Geol. Survey No. 178, 1901.)

G. H. Girty in 1901 made a collection of fossils from the Hueco Mountains in the course of his expedition to the Guadaloupe Mountains. (Am. Jour. Sci., 4th ser., vol. 14, 1902, p. 363. Dr. Girty also visited the El Paso district in connection with the work of the present writer, who is much indebted to him.

G. B. Richardson in 1903 made a reconnaissance survey in trans-Pecos Texas north of the Texas and Pacific Railway. (Bull. No. 9, Univ. Texas Min. Survey, 1904.)

C. S. Slichter in 1904 made observations on the ground waters of Rio Grande valley, near El Paso. (Water-Supply Paper U. S. Geol. Survey No. 141.)

W. T. Lee in 1905 studied the Rio Grande valley in New Mexico. (Water-Supply Paper U. S. Geol. Survey No. 188, 1907.)

March, 1908.

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aGirty, Gr. H., The Gruadalupian fauna: Prof. Paper U. S. Geol. Survey-No. 58, 1909.

a Richardson, GL B., Report of a reconnaissance in trans-Pecos Texas: Bull. Texas Univ. Min. Survey No. 9, 1904, p. 29.

aWeed. W. H., The El Paso tin deposits: Bull. U. S. Geol. Survey No. 178, 1901.

aCompiled from measurements made by the United States Geological Survey.