Geology and the availability of water in the lower Bonita Creek area, Graham County, Arizona

Abstract

The lower Bonita Creek area lies in the southeastern part of the valley between the Gila and Turtle Mountains, about 15 miles northeast of Safford, Graham County, Ariz. Most of the area, about 70 square miles, is drained by Bonita Creek, a tributary of the Gila River. The city of Safford obtains its water supply from an infiltration gallery in the alluvial fill of Bonita Creek about 4 miles above the confluence of Bonita Creek and the Gila River. Discharge from the infiltration gallery during 1939-56 ranged generally from 900 to 2,500 acre-feet per year. There has been a small steady decrease in production since 1953. Virtually no other ground water is pumped in the area except from a standby well which is actually a part of the infiltration-gallery system. Igneous and sedimentary rocks of possible Cretaceous age, volcanic rocks and alluvium of probable Tertiary age, alluvium of Pliocene and Pleistocene age, and younger alluvium of Quaternary age are not exposed in the area. Only the rocks of the upper part of the Tertiary volcanic, sequence and the late Quaternary alluvium that forms the channel fill along Bonita Creek yield water in moderate quantities. The channel fill is the more important producer at the present time. Bonita Creek is chiefly perennial within the area. Surface flows at times are less than 1 cfs (cubic feet per second) and have reached a short-term peak of 17,000 cfs. The greatest sustained surface flows, averaging about 6 cfs during 1955 and 1956, are encountered in The Box, a constricted portion of the Bonita Creek canyon. The amount of surface flow into the Gila River at the mouth of Bonita Creek, the diversions at the infiltration gallery, the estimated underflow into the Gila River from Bonita Creek, and losses due to evapotranspiration also total about 6 cfs. The alluvial deposits of the channel fill of Bonita Creek are composed of intertonguing lenses of heterogeneous gravel, sand, and silt whose permeabilities determine the transmission and storage of water in the deposit. The long narrow shallow shape of the deposit limits the amount of water it is capable of transmitting and storing. The limited storage capacity suggests that much of the water required for the sustained level of diversions at the infiltration gallery must come from continuous recharge. At the infiltration gallery the amount of water within the channel fill may be augmented by infiltration from the adjacent volcanic rocks. Near the mouth of 2 GEOLOGY AND AVAILABILITY OF WATER, BONITA CREEK, ARIZ. Bonita Creek the underflow of the channel fill is estimated to be about 0.8 cfs, which is a measure of the probable sustained productivity of the channel fill in this vicinity. Chemical analyses of surface, underflow, seep, and gallery water indicated that the chemical content and proportions of constituents are similar in water from all sources. The mineral constituents range in concentration from 173 to 462 ppm (parts per million) and are predominantly calcium and magnesium bicarbonate. The silica content generally ranges between 40 and 60 ppm.