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<br />2354 <br /> <br />.' <br /> <br />Reservation. Total thickness of the Bouse formation is generally 450 <br />feet. The upper 300 feet appear to be somewhat hydraulically contin- <br />uous with the Colorado River alluviums, and the lower part is consid- <br />ered an aquitard. [3] <br />Ground Water <br />The Geological Survey made an extensive investigation of ground <br />water under the Reservation between 1961 and 1967. [3] The <br />ground-water aquifer was divided into three sections--a shallow zone <br />extending from the water table downward for not more than 30 feet; a <br />principal gravel zone which extends downward to about 130 feet <br />beneath the flood plain; and a deep zone which lies beneath the <br />principal gravel zone (See Figure 2). Differentiation between them is <br />based on the composition of the sediments and water quality varia- <br />tions. <br />The dissolved solids concentrations in the shallow zone were <br />found to range generally between 600 and 7,600 mg/L. <br />Concentrations are generally less than 2,000 mg/L except for certain <br />areas on the east side of the valley. The high concentrations <br />(greater than 3,000 mg/L) probably result from concentration by <br />evapotranspiration of natural vegetation and evaporation from soil. <br />The low concentrations (less than 1,000 mg/L) seem to be in areas <br />that have been flushed by river water, canal leakage, or possibly <br />past flood waters from washes. The water in the shallow zone tends <br />to become more mineralized as the distance from the river increases. <br />In one area, in the northeastern part of the flood plain, poor drain- <br />age at one time resulted in both a high water table and salt accumu- <br />lations in the irrigated soils so that some farmland had to be aban- <br />doned. However, a drainage network now maintains a water table of <br />satisfactory depth for growing crops in most of that area. <br />The water quality in the principal gravel zone is also variable <br />but generally slightly less saline than in the shallow zone. The <br />dissolved minerals content throughout most of the principal gravel <br />zone underlying the flood plain varies from about 650 mg/L to some- <br />what more than 1,600 mg/L. However, under a 3- to 4-mile wide belt <br />of land on the eastern side between Mohave Road and the foot of the <br />mesa the water varies in concentration from about 2,000 mg/L to as <br />9 <br />