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<br />2} <br /> <br />DIFFUSE-SOURCE SALINITY <br /> <br />" <br />1, <br />,'of <br /> <br />W <br />l-"' <br />t".; <br /><:;:;) <br /> <br />Diffuse-source salinity is significantly influenced by the soluble- <br />mineral content (SMC) of soils and rocks exposed at the earth's surface <br />(Peters, 1984), and runoff salinity increases with the SMC of these <br />surface materials. <br /> <br />Origin of Soluble Minerals <br /> <br />"I <br /> <br />The amount and type of minerals present in rocks and soils is a <br />function of storage, leaching and accumulation processes. Some minerals <br />are formed in situ. For instance, marine deposits contain connate water <br />almost identical to seawater. Therefore, prior to appreciable leaching <br />or accumulation, marine shales and sandstones have a high SMC (Garrels <br />and Thompson, 1962). Unlike marine sediments, freshwater sediments, and <br />igneous and metamorphic rocks rarely have a high SMC. Chlorides, <br />bicarbonates sulfates, and nitrates generally comprise less than O.l% of <br />unaltered igneous rocks. <br />Almost all rocks undergo secondary mineralization, and soils, by <br />definition, undergo pedogenic processes that lead to preferential <br />leaching and/or accumulation. In situ accumulation may occur, but the <br />primary accumulation of soluble minerals is rarely by in-place <br />weathering. Cyclicsalts in the form of dry fallout and rainout may <br />contribute large amounts of soluble minerals to the surface. Fallout, <br />which includes wind-blown sand and dust that is derived from saline <br />materials may concentrate soluble minerals. Rainfall and precipitation <br />in general have a very low salinity (Carroll, 1962), and, therefore, <br />they are not the direct cause of the accumulation of soluble minerals. <br />Transpiration of plants and the loss of minerals and nutrients from <br />plant leaves by the leaching action of rainfall, mist and dew has been <br />assumed to contribute large quantities of leachates from living plants <br />and li tter. However, a recent study of Malekuti and Gifford (1978) in <br />the Upper Colorado River 8asin has shown that plants contribute merely <br />0.01-0.02 percent or less of the total annual salt load of the Price <br />River 8asin. Certain species of saltbush, shadscale, greasewood, salt <br />cedar and halogeton provided the highest concentrations of leachate, <br />which can be as high as 721.1 mg/l from 50 g of greasewood litter that <br /> <br />, <br />}. <br /> <br />-~ <br /> <br />1 <br /> <br />,,' <br />-1 <br /> <br />17 <br /> <br />