Laserfiche WebLink
Mayo and Assoclafes, LC <br />Elevated Nay 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 Mgt' concentrations. Halite dissolution <br />maybe represented as <br />NaC1= Na' + Ct, <br />(halite) <br />(6) <br />and ion exchange may be represented by reactions involving the sodium zeolite analcime, <br />2NaAlSi2O6•H2O + Ca'~ = Ca(AlSi2O6)2•H2O + 2Na', (7) <br />2NaAISi2O6 H2O + Mg" = Mg(AISi2O6)Z H2O + 2Na', (8) <br />or clay mineral exchange which maybe represented as <br />Ca2+ + 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 dischazging <br />at the Edwazds portal spring. For the WEM fault waters to be the source of Edwards portal <br />spring water wool considerable chemical evo ~ n.-Such chemical evolution would <br />have to both d crease the Na concentration and increase the SO;' c ncentration. Most solute <br />dissolution-precipi a ion reactions may readily proc~eEdrtowar er 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 equations. <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 />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 deternine 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 />