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' Memorandum to Kathy Welt June 10, 1998 Page 4 ' of water from the Edwazds Portal spring. The abundant carbonate minerals in the soil zone and bedrock in the region make it very unlikely that any source of dilution water would have a S"C composition appreciably more negative than about -13 °/~. The carbon-13 data indicates that most of the water issuing from the Edwards Portal spring originates from sources not associated with the WEM sump and that any contribution from the WEM sump is too small to quantify. Sulfur-34 ' The S9dS content of sulfates (SO Z-) in both sump and Edwazds Portal spring waters is about 13 %. This composition is consistent with the dissolution of marine gypsum or anhydrite. Combined with this observation, the elevated SO Z-concentrations in both the ' sump and spring waters suggest that dissolution of CaSO° minerals is the source of the elevated SO,Z- levels and that pyrite oxidation is not a major factor. Gypsum dissolution and the subsequent ion exchange of CaZ` and MgZ' for Na' on clays readily account for ' the elevated SO°'-- and Na' concentrations and the relatively low concentrations of Ca" and Mg" in WEM sump water. Water discharging from the Edwards Portal spring has higher CaZ` and ~~IgZ' concentrations and a lower Na` concentration than the sump water, indicating that ion exchange of CaZ' and MgZ` for Na` on clays is less important in the dilution water. The CaZ' and MgZ' concentrations of both the sump and the spring are ' only a fraction of the HCO3 and SO°Z-concentrations of the waters (Tables 1 and 3). Carbon-14 and Tritium ' Both the sump and Edwards Portal spring waters have low, but consistently measurable, tritium ('H) levels of about 1.5 and 1.9 TU, respectively (Tables 2 and 3). The ' differences in the'H contents of the two waters are not great enough to distinguish them; however, the presence of'H does indicate a component of post-1954 water. ' On the other hand, the small differences in the carbon-]4 (1°C) contents of the sump and spring waters aze significant. The sump water has a '°C content of about 7 percent modem carbon (pmc) and the Edwards Portal spring water has a 14C content of about 4.5 pmc. Because 1°C has ahalf--life of about 5,730 years, and the content of typical rechazge water is about 50 pmc, at these low pmc values, a small pmc difference suggests a large age difference between the waters (about 1,000 to 2,000 yeazs). However, the age issue is confounded by the S"C contents of both waters. The S"C content of sump water has been affected by CH°; thus a 1°C content does not only reflect the age of the coal. On the other hand, the S"C composition of the Edwazds Portal spring water indicates a normal and predictable carbon history and water that has a large component of water that recharged 20,000 or more years ago. ' If the sump makes a significant contribution to the water flow from the spring, the "C difference requires that sump water be significantly diluted with very old groundwater