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<br />a. The surface and irrigation drainage <br />water have a constant salt concentration or, <br />as it may be equivalently stated, the salt <br />loading is proportional to the flow. <br /> <br />w <br />w <br />~ <br />..... <br /> <br />b. Concentrations increase with low <br />flows because of less mixing to dilute the <br />relatively poor quality effluent groundwater. <br />Some Grand Valley drains show concentrations <br />as high as 20,000 ppm during low flows. <br /> <br />Mineral weathering in <br />!he Rroundwater system <br /> <br />The salts within the groundwater zone <br />are picked up more freely because of mineral <br />weathering. The weathering rate is influ- <br />enced by the soil moisture level, quantity of <br />percolating water, composition of the parent <br />material underlying the groundwater zone, and <br />the s al i oi ty of the so i 1 water. All these <br /> <br />~ <br />... <br />... <br />1 <br />o <br />C <br />';;; 3.0 <br />c <br />o <br />... <br />~ <br /> <br />CIl <br />o <br />..J <br />o <br />CIl <br />o <br />w <br />::J <br />o 2.0 <br />CIl <br />CIl <br />o <br /> <br />4,0 <br /> <br /> <br />'\ <br />1 \ <br />1 \ <br />1 \ <br />/ L <br />, <br />/.... ..../ <br />....--- <br /> <br />factors should logically be included in <br />quantitative analysis of salt pickup in the <br />soil where the two principal processes are <br />the weathering (dissolution) of residual <br />salts within the soil water - groundwater <br />system and the resolubilization of previously <br />precipitated salts (r'everse weathering) <br />within the soil layers. <br /> <br />Salt loading in streams <br /> <br />The major processes contributing to <br />salt loading between two stream points are: <br /> <br />1. Instream salt pickup as it varies <br />with: a) seasonal flow fluctuations in- <br />cluding associated larger fluctuation in <br />sediment movement, b) manmade fluctuations in <br />flow associated with diversion and return <br />flow patterns, and c) the salt content of the <br />river water. Flow fluctuations that expose <br /> <br />/ <br />I <br />, I <br />V <br /> <br />" <br />1\ <br />/........., I \ <br />',I <br /> <br />Out Fall Drain <br /> <br /> <br /> <br />Anderson Drain <br /> <br /> <br />Olive Lake Drain <br /> <br /> <br />ON 0 J F MA M J J AS 0 N 0 J F M A M J J A SON D J F M A M J J A S <br />1975 1976 1977 <br />WATER YEAR <br /> <br />Figure 2.6. Dissolved solids concentration in the drains of the Palo Verde lrrlgation Dis- <br />trict, California, 1975-1977 water years. <br /> <br />13 <br /> <br />