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The CDHG has requested an explanation fox the excaedence of the TDS value over the PHC prediction at Well WHAI.7-2. The June-Sept:uohex TDS value measured this year was 1980 • mg/1, while the predicted value ras 1299 mg/1. The 1480 mg/1 value was determined gravime trically (i.e., dry a water sample an~f weigh the residue). According to Hem (1989, pg. 357), .. it is not uncommon for water high in calcium and sulfate concentrations to yield a residue after drying an hour at 180°C that exceeds the computed dissolved solids by several hundred milligrams per liter" antl also, "In many instances, especially if the concentration is greater than 1000 mg/1, the calculated draeolved-aolida value may be preferable to the residue -on-evaporation value". The calculated dissolved aolida value is determined by adding up all the eoncentrationa fox all of th¢ various dissolved constituents. For June-September 1999, the calculated TDS value ie 1180 mg/1, rhich falls below the PHC predicted value. Likewise, in the 1998 ARR, the 1433 mq/1 annual average TDS value (June-September 1440 mg/1) vas the measured (gravimetric) value; the calculated annual average TDS value was 1273 mg/1 (June-September = 1170 mg/1), which both fall below the predicted value. (In the 1998 AHR, annual average TDS values for the alluvium were used. It ass diacovexed by the rri ter thin year that the Seneca II-W PqC uses June-September values. June-September values fox the alluvium aza being used in this year's AHR). - W d O b d F a qe ver ur en. aux cells monitor the Wedge overburden . Wall WOV19 exhibits a recently increasing TDS trend. At Wall WOVlfi, the limited at so faz indicates an increasing TDS trend. (Only six samples have bean collected eo fnx for Well Wt7V16, until recently there ras normally not enough water to sample). Well WOV17 exhibits an increasing TDS trend. Well WOV4-1 (II-W South Expansion area) ras sampled for the first time ewr this Hay. The Seneca II-W PHC pxediote aMUal average TDS values at various Wadqe overburden re119. For Well WOV14, a value of 4385 mg/1 ras predicted, while a value of 3645 mg/1 ras observed this year. For Well WOV17, a value of 4295 mg/1 ras predicted, chile a value of 8935 mg/1 ras observed. This predicted value (42951 ras predicated on a premining value of 6072 vg/1, rhich is an average of several overburden calls. Horever, Wall WOV17 exhibited a preauning TDS average value of 8043 mg/1. The CDHG, in [heir ravier of the 1998 AHR, raqueated on explanation of the TDS axcaedence of the PHC prediction for Well WOV17. The source of the high baseline TDS values is unknorn. This roll had a baseline TDS average of 8043 ag/1, chile the next highest TOS •