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<br />BASIC STUDIES <br /> <br /><=> <br />w <br />~ substantially decreased. The computed loads indicate that as water moved <br />~A through Lake Mead it lost calcium bicarbonate by precipitation and gained <br />calcium sulfate by solution, with the rate of solution somewhat exceeding <br />the rate of precipitation. <br /> <br />All ionic loads appeared to have decreased very slightly as the water <br />flowed from Lake Mead through Lakes Mohave and Havasu to Parker Dam. Some <br />of the decrease probably was a result of bank stor~e when Lake Mohave <br />filled, and some may have been the result of calcium-carbonate precipita- <br />tion. Water diverted to the Colorado River aqueduct carries salts out of <br />the Colorado River system; so that upstream and downstream loads can be <br />compared, allowance for the diversion has been made. The computations <br />indicate a 7 to 9 percent increase of sodium and chloride in the river <br />between Parker Dam and Imperial Dam and a smaller increase of bicarbo- <br />nate. Loads of all other constituents declined slightly. <br /> <br />4. Chemical Quality of Ground Water in the Lower Colorado River Basin <br /> <br />Downstream from Hoover Dam, the Colorado River flows through a suc- <br />cession of alluvial valleys underlain by saturated sediments containing <br />millions of acre-feet of ground water of highly variable quality. Although <br />some areas are underlain mainly by high salinity ground water (southeast <br />Barker Valley, southern part of Palo Verde Valley) and some areas are <br />underlain by water of rather uniform quality comparatively low in dis- <br />solved solids (northwest Parker Valley, southern Yuma Mesa), the concen- <br />tration and chemical composition of dissolved solids in the ground water <br />usually vary laterally and with depth in most of this part of the Colorado <br />River Basin. Consequently, useful generalizations about the quality of <br />ground water can be made only for small sections of the valleys and mesas. <br /> <br />Table C lists chemical analyses selected to represent typical water <br />quality for each indicated area and depth; they reflect, in general, the <br />range in concentration and chemical composition of the ground water. <br /> <br />Rather commonly, the chemical composition of the ground water pumped <br />from wells within a mile or two of the river (analysis 1) resembles that <br />of the present river water, suggesting recent infiltration from .the river. <br />In other areas the composition and concentration. of the ground water re- <br />sembles water characteristic of flows prior to construction of Hoover Dam. <br />Analyses 11 and 19 are only slightly different from the low dissolved- <br />solids water of the calcium-bicarbonate type which was characteristic of <br />river flood flows. Analyses 2 and 17 resemble the calcium-sulfate water <br />characteristic of the minimum flows. The existence of these water types <br />suggests that the underground reservoirs were charged in past decades by <br />infiltration from particular stages of the naturally flowing river. <br /> <br />A study of several thousand chemical analyses of ground water indi- <br />cates that the lateral variation of chemical quality appears to be irreg- <br />ularly related to the proximity of the sampled wells to the river or to <br /> <br />32 <br /> <br />