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W I-' W C.H ~ The time required to obtain accurate measurements of SMC depends on the sediment-water ratios used in the analysis. Fig. 2-2 shows that release rates of soluble minerals from samples of Mancos Shale and associated' alluvium (Laronne, 1981) are variable and that many hours may be required before all soluble compounds are released from a sample. Rates of dissolution were initially very rapid, but they slowed with time. Nevertheless, some samples continued to produce dissolved solids after 48 hours. The larger the water to sediment ratio the more soluble material is released (Nezafati et al, 1981). Whitmore (1976) found a similar relation, and in addition, he showed that solutes are released more rapidly from fine-grained sediments (Fig. 2-3) (Table 2-2). A long contact time between water and sediment is needed to approach an equilibrium SC. This indicates that in the field "runoff is unequilibrated with respect to the major soluble minerals -- during the first one or as much as a few hours of contact. Hence salinity yields derived from in situ conductance readings of runoff may be inadvertently but significantly underestimated (Shen et al, 1981). Also dissolution from sediment in transport should be a major source of solutes when sediment concentrations are high" (Laronne, 1981, p. 551). The previous description of the effect of sediment-water ratios on EC may be summarized as follows. Increased sediment concentration is the sourqe of more soluble minerals, which, in turn, increases the salinity (SC) and, the EC of the aqueous solution. It is apparent that not all potentially soluble matter dissolves under conditions of partial equilibrium. The amount of this undissolved matter decreases as the sediment concentration decreases. Laronne (l977) used l.l, 1.9, 1.99, and some 1.4 and l.999 sediment- water ratios for samples of surficial shale and alluvium to obtain data on the effect of sediment concentration on EC. 'The specific ion concentrations consistently increased with an increase of sediment-water ratio (Table 2-3). This is, of course, expected for unsaturated solutions to which soluble minerals are added. When the EC of a shaken sediment-water mixture does not increase appreciably with time, it is concluded that equilibrium is approached. A series of experiments were undertaken by Laronne (1977) to determine the validity of referring to such solutions as equilibrated (or " 1 j ~ J , , , ~ -1 i , ~ , ) , I , :l i -;i i , ~ " 'j j ~ . , I J I :~ j 1 '1 A " ~ ',0 i " ; ) 1 j :~ ". 23