Geological Survey Water-Supply Paper 1475-K Ground-Water Reconnaissance at Pinnacles National Monument, California

The adequacy of the alluvium as a water-bearing aquifer was explored by a test well drilled in July 1959. The test well was drilled by a commercial drilling company using the rotary method. The well was drilled to a depth of 41 feet, cased with extra-heavy-duty 8-inch casing to a depth of 35 feet below ground surface. The casing from 14 to 35 feet below ground surface was perforated by 8-inch-long acetylene-torch cuts, spaced in a pattern of 4 cuts per linear foot of casing. When pumped, the well yielded 11 gpm at a pumping level of 14 feet below the top of the casing and a drawdown of 4 feet below the standing level in the casing.

Difficulties were encountered in the development of the well as shown by the record of development and test pumping (p. 381). The highly permeable character of the alluvium coupled with a low hydraulic pressure of the ground water undoubtedly were conducive to permitting the drilling mud to invade a large volume around the well bore. It is probable, even though the water did clear, that the mud has sealed a part of the adjacent gravel and sand, and thus has reduced the yield of the well.

To minimize the possibility of introducing mud into the permeable alluvium, future wells drilled in Chalone Creek could be constructed by the cable-tool method, using 12-inch-diameter screen or standard casing that is machine perforated throughout the interval 10 to 25 feet. Depth of these wells may be about 30 to 35 feet below ground surface. They should yield more than 10 gpm.

To permit measurement of the water level it would be advisable to place an access pipe through the foundation into the casing.

The most readily available source of water supply within the boundaries of the monument is ground water contained in the alluvium of Chalone Creek.

The test drilling indicates that the alluvium is at least 30 feet thick near the Chalone Creek campground and that it is composed of very coarse sediments—predominantly gravel, cobbles, boulders, and some very coarse sand. Two wells penetrate the alluvium: 35R1 (sec. 35, T. 16 S., R. 7 E.), the test well drilled in July 1959; and 25J1 (in the same section), a dug well constructed by the Civilian Conservation Corps in the early thirties. Depth to water in these two wells was about 10 feet below land surface on July 13, 1959.

The dug well consists of a concrete- and wood-cribbed shaft about 5 feet square and 17 feet deep. Gravel and sand and 2 or 3 large boulders are visible at the bottom of the shaft. This well could provide an additional supply of water if it were deepened from 17 feet to 20 or 35 feet. Deepening the dug well could be most effectively accomplished by the cable-tool method using 12-inch casing, thereby avoiding any possibility of introducing drilling fluid into the formation and also simplifying the drilling of large gravel and cobbles. The volume between the casing and the cribbing could be backfilled with gravel to the top of the water (10 feet) and then filled to the surface with clay or concrete to provide a sanitary seal.

Another potential source of ground water may be the Temblor formation exposed northeast of Chalone Creek and across the fault. Test drilling would be required, however, to provide information concerning the yield and quality of water in this formation.

Although many springs serve as proof that ground water occurs in fractures, fissures, and other openings in the volcanic formations of the monument, it is doubtful whether adequate supplies of water could be obtained from wells drilled in the volcanic rocks.