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Where: <br />C = average concentration in alluvial groundwater <br />Cb = concentration in backfill waters <br />Ca = historic concentration in groundwater. <br />Vb = volume of water in backfill <br />Va = volume of water in alluvium <br />The results of the calculations for selected species are shown in <br />Table 33, Effect of Backfill on Quality of Alluvial Ground Water. Only <br />those species for which a detectable change was indicated are included <br />in Table 33. Many of the trace element concentrations measured in the <br />leach tests are less than those previously measured in the alluvial <br />groundwater. The calculation indicates that no detectable change in the <br />average concentrations of trace elements will result from the backfilled <br />channel. Only moderate increases in dissolved solids, calcium, sodium <br />and sulfate are indicated. Neither the moderate change in alluvial water <br />quality nor the change in groundwater flow pattern resulting from the <br />backfilled channel will impair the hydrologic function of the affected <br />reach. Alluvial groundwater downstream of the modified reach is not <br />expected to be influenced by the relocation of the channel as proposed. <br />The leaching tests show that the effluent from the backfilled <br />channel will contain a number of chemical species at concentrations <br />substantially above the in -stream values. However, flow through the <br />backfilled channel will be so small in comparison to the stream discharge <br />that no measurable increase in the in -stream concentrations will result. <br />This conclusion remains unchanged even if the stream discharge falls to <br />one-fifth of the value used or if the flow through the backfill were to <br />be 100 times greater than estimated. The probability that effluent from <br />the backfill channel will measurably increase the in -stream concentra- <br />tions is regraded as being virtually zero. Even a greater 'safety <br />factor' can be realized by compacting the channel fill so that the in- <br />place permeability is less than the rather large value of 1.25 x 10-4 <br />cm/s measured and used in this study. <br />RN -6 <br />2.05-89 (Repaginated 10/21/16) <br />