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overburden are higher, especially Na. Which cation becomes the dominant one in the Seneca • <br />11-w spoil aquifer will depend on the degree of cation exchange occurring in the spoil and <br />the amount of precipitation and redissolution of calcite and gypsum. The dominant anions <br />are 504 and HC03. 504 concentrations are considerably higher at Seneca II-W in comparison <br />to Seneca II, The spoil water and mixed downgradient spoil/bedrock aquifer water is all <br />expected to be a S04 type. <br />The downgradient spoil/alluvial water is expected to be a CaMg-S04 type as Na only <br />increases slightly and Ca and Mg levels remain the same in the alluvial aquifer below the <br />004 pond at Seneca II. Spoil water discharged through sediment ponds should also be the <br />same water type. A review of the period of record chemical analyses for 1vPDES Sites 002 <br />and 004 at Seneca indicates that most of the major cation and anion concentrations can be <br />defined in narrow ranges of percentages of the TDS values. Table 17-14 presents these <br />ranges and the corresponding concentration values assuming a Seneca II-W TDS value of <br />4,500 mg/1. <br />For the purposes of the last two parts of this section of the PHC, a peak TDS <br />concentration for the spoil at Seneca II-W was needed. Because the undisturbed overburden • <br />aquifer at Seneca ll-W has a higher background TDS and the 004!008 pond TDS levels at <br />Seneca II have not peaked, another approach was necessary for arriving at the projected <br />peak spoil TDS. The approach selected was to use the US CS lysi meter data presented in <br />Water-Resou rc es Investigations Report 86-4350 (Williams and Hammond, 1988). For all <br />lysi meter sites and for all analyses following the initial ones, TDS levels have never <br />exceeded 4,500 mg/1. Since the samples are from unsaturated flow (non-equilibrium) <br />conditions, one would expect the reported concentrations to be higher than spoil water <br />discharge analyses collected from sediment pond outfalls. This is certainly the case at <br />Seneca II where the maximum spoil discharge TDS levels measured at the 004 pond as of <br />August, 1990 is only 3,342 mg/1, and the maximum TDS level discharged through 002 pond is <br />3,174 mg/1. Thus, the maximum TDS level used in the following ground and surface water <br />impact parts of this section of the PHC is 4,500 mg/7. <br />The maximum amount of Seneca II-W spoil discharge was calculated based upon weekly <br />discharge measurements taken at the Seneca II 004 pond outfall since 1984 (Table 17-15). <br />The season selected to be analyzed that would show the greatest impact potential to <br />downstream surface water users was the low flow June through September irrigation season. • <br />Analyses of data collected through 1989 indicate that the highest runoff year was 1984. <br />36 <br />