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' Memorandum to Kathy Welt June 10, 1998 ' Page 3 ' cazbonate and sulfate species indicates that mineral precipitation cannot be the basis for the TDS decrease. If the WEM sump contributes to the spring, this water must undergo considerable dilution with low TDS water from other sources. In this case, WEM sump water cannot be the main source of water for the Edwazds Portal spring. Sodium ' The sodium (Na`) concentration of WEM sump water ranges between about 70 meq/L (1600 mg/L) and 100 meq/L (2250 mg/L). Sodium in the Edwazds Portal spring is about ' 31 meq/L (724 mg/L). As with TDS, if the sump is a major contributor to water from the spring, a reasonable explanation must exist for the decrease in sodium at the spring. ' Sodium does not form insoluble compounds and is weakly bound to ion-exchange sites in soils. At the TDS and Na` concentrations observed in the sump and spring, Na` cannot be lost by mineral precipitation or reverse-ion exchange. Thus, Na' is a conservative ' species, with no plausible loss mechanisms between the sump and the spring. The only way for the sodium concentration to decrease is by dilution, and the dilution water must be lower in sodium than the spring water. If meteoric water with no significant concentration of sodium were used, a dilution factor of about 2.7 to 1 would be required. Since water collected from wells and springs in the ' Barren Member aze not sodium-free, amore realistic dilution factor would be considerably greater. The required dilution factor varies rapidly with the sodium concentration of the diluting water, from a factor of about 4 to 1 for water with 200 mg/L ' of sodium to a factor of about 160 to 1 for water with 712 mg1L of sodium. If the WEM sump contributes to the spring, the sump water must be considerably diluted with low ' sodium water from other sources. Carbon-13 ' The carbon-13 content of bicazbonates (HCO;) in the sump and in the spring aze significantly different (Tables 2 and 3). The sump water has a S~~C of about -3 %, ' whereas the spring water has a S"C of about -13 °/~. The S"C composition of sump water has been affected by methane gas (CH,), whereas the S"C content of Edwards Portal spring water is consistent with that expected in soil zone COZ coupled with ' cazbonate mineral dissolution. From observations of the S"C content of fault water in WEM, we know that the S"C content of HCO,- affected by methane gas in this environment is not a large negative number. The difference between the S"C contents of the sump and Edwazds Portal spring waters strongly indicates that the two waters lazgely originate from-different and independent sources. ' Because soil zone rechazge water typically has a S"C content of -10 to -13 °i°o, an almost infinite dilution factor would be required to dilute the sump water to the S"C composition