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<br />were obtained from wells as much as 600 feet in depth. Bookman- <br />Edmonston dug a number of shallow test pits in 1975 and the Water and <br />Power Resources Service installed piezometer wells along the river to <br />depths of 130 feet. <br />For conveni ence in studyi ng the ground water of the Colorado <br />River Valley, the Geological Survey divided the water bearing sedi- <br />ments into three zones--a shallow zone extending from the water table <br />down for not more than 30 feet; a principal gravel zone which extends <br />downward to about 120 feet beneath the flood plain; and a deep zone <br />which lies beneath the principal gravel zone. Differentiation between <br />them is largely on the basis of water quality variations. Figure 2 <br />presents block di agrams of the three zones underlyi ng the Di stri ct, <br />with the geographic location and TDS of water quality samples noted on <br />the top of each zone. The depths of the samples vary within each <br />zone. For thi s study, the deep zone has been arbitrarily cut off at <br />the 450-foot depth, the location of the top of the less permeable <br />Bouse formation. <br />The chemical characteristics of the ground water provide an <br />indicator of the origin of the salts. Well water samples having a TDS <br />of over 4,000 mg/L have a combined sodium and chloride ion content of <br />from 65 to 75 percent, in contrast to Colorado River water, which has <br />approximately 28 percent. Such a composition is attributed to the <br />a 1 terat i on of Colorado Ri ver water through concentration by evapo- <br />ration of ponded surface water and plant evapotranspiration, precipi- <br />tation of insoluble calcium and magnesium carbonates, and reduction of <br />sulfate. [4] [14] Ground waters with salinities of 1,000 to 4,000 <br />mg/L generally have Na and Cl contents varying from that of Colorado <br />Ri ver water to 65 percent. Thi s i ndi cates that they are not as far <br />advanced in the a 1 ternat i on process, or that they are a mi xture of <br />saline ground water and river water or irrigation drainage. <br /> <br />002848 <br /> <br />9 <br />