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' Moryo and Assoclales, LC <br />' Elevated Na` concentrations may result from either the dissolution of small amounts of very <br />soluble. halite or from ion exchange on clay particles or on sodium zeolites. Halite <br />' dissolution will increase the overall solute concentration (i.e. TDS) and will yield equal Na' <br />and Cl- contents when the solute compositions are reported in the meq L-' units. Ion <br />exchange will no[ directly elevate the overall solute content, but will result in increased Na' <br />' concentrations at the expense of reduced Ca" and/or Mg'' concentrations. Halite dissolution <br />may be represented as <br />(halite) <br />' and ion exchange may be represented by reactions involving the sodium zeolite analcime, <br />' 2NaA1SizO6 HZO + CaZ' = Ca(A1Si,O6),•HZO + 2Na', (7) <br />2NaA1SizO6 HZO + MgZ` = Mg(A1Si,O6)_•H,O + 2Na`, (3) <br />' or clay mineral exchange which may be represented as <br />' Ca=' + Na-clay = 2Na' + Ca-clay (9) <br />Mg" + Na-clay = 2Na' + Mg-clay (10) <br />NETPATH Modeling <br />' Differences in the solute compositions between WEM fault groundwaters and Edwards portal <br />spring waters strongly suggest that WEM fault waters are not the source of water discharging <br />at the Edwards portal spring. For the WEM fault waters to be the source of Edwards portal <br />spring water would require considerable chemical evolution. Such chemical evolution would ~~ <br />have to both decrease the Na' concentration and increase the SO;- concentration. Most solute <br />1 dissolution-precipitation reactions may readily proceed toward either products or reactants. <br />However, the overall reduction in Na' would require reverse ion exchange, which is very rare <br />(i.e. reactions such as 7-10 would proceed toward reactants). At the observed Na` <br />' concentrations there are not precipitation reactions which will consume Na'. The substantial <br />increase in SO; would require either appreciable gypsum dissolution (Eq. 5) or pyrite <br />oxidation. Pyrite oxidation is not included in the above list eyuations. <br />In an attempt to determine if chemical evolution from WEM fault water to Edwards portal <br />spring water is plausible, chemical reaction pathways were modeled using the computer code <br />1 NETPATH (Plummer and others, 1994). NETPATH performs mass balance calculations for <br />specified rock water reactions. NETPATH maintains mineral saturation consistent with input <br />and output waters and accounts for isotopic fractionation and exchange. In other words, <br />NETPATH will allow us to determine if the solute composition of one water can evolve to <br />the composition of another water in a specified environment. <br />' edwspr.doc 5 20 January 1998 <br />~ ,; ~ <br />~` <br />