Laserfiche WebLink
The fault waters are characterized as sodium bicarbonate waters whereas the seep waters are <br />sodium potassium calcium bicarbonate sulfate waters. The same meteoric water that contained <br />dissolved dolomite could also contain dissolved potassium and sodium, both of which are <br />probably readily available in local Cretaceous rocks. The relative increase in K (from about 3 <br />mg/L in the fault water to 8 in the seep water) would indicate that the meteoric water contained <br />the higher K content. The minor, if any, decrease in Na likewise can be attributed to the relative <br />concentrations of Na in the fault waters and the meteoric water, the meteoric water having the <br />lower concentration. <br />The introduction of meteoric water could also explain the drop in pH. Meteoric water has a <br />typical pH in the range of 5.5 - 6.5 (see for instance. A.W. Hounslow, 1995, Water Quality Data, <br />Analysis and Interpretation, CRC/Lewis Publishers, New York, p. 169). The Edwards Portal <br />seep has a pH of 6.79 compared with pH values in excess of 8 for the fault waters. Easily, <br />dilution of the fault waters by rain or snow could explain this shift, and as noted above, meteoric <br />water mixing is consistent with several findings. <br />C. Isotope geochemistry. <br />S"S. At first glance, an increase in sulfate in the water might indicate gypsum (or <br />anhydrite) dissolution. However two points argue against gypsttm as a source. First, there is not <br />enough Cato balance the sulfate, so that eliminates a gypsum source unless a parallel mechanism <br />is developed to remove Ca from solution. However, that approach may be unnecessarily <br />complex. The second point lies in the sulfur isotope composition of the dissolved sulfate. The <br />S"S values (read: del 34-5), which range up to +44.4 %o, aze up to 15 %o enriched over even the <br />heaviest seawater sulfate (Note: %o =per mil). (Seawater is the source from which most <br />gypsum/anhydrite derives, especially in the Paleozoic and pre-Tertiary rocks of this area which <br />are dominantly marine in origin). Furthermore, the S'aS values are about 25 %o enriched over <br />late Cretaceous seawater, which arguably is the most likely ultimate source of gypsum locally <br />(see Holser and Kaplan, 1966, Chemical Geology, vol. 1, No. 2, p. 93-135). Thus, gypsum or <br />anhydrite in the sedimentary column are not the likely source of the sulfate in the Lone Pine seal <br />and Edwards Portal seep waters. <br />As mentioned above, the enriched (isotopically heavy) S"S values of the sulfate argue for a <br />source area where sulfate, possibly in the presence of methane oxidation, has undergone <br />enrichment due to the preferential reduction of sulfate containing the lighter sulfur isotope, and <br />precipitation or removal of that reduced sulfate as pyrite or other sulfides. A yellowish color, <br />which has been observed in the seep water, and which could indicate the presence of iron, <br />supports this notion because such pyrite precipitation would not be expected to be l00 percent <br />efficient, so some iron should still be in solution. <br />SD and 5180. Values for SD and 5180 of all the samples lie remarkably close to the <br />meteoric water line. With the possible exception of values which are more than about 3 per mil <br />from the meteoric water line, most of these values could represent local meteoric water. Even <br />the isotopically lightest samples. which are from the fault waters, are only about ~ %o lighter, in <br />SD, than Colorado meteoric waters such as those reported in Lawrence and Taylor (1971; <br />7 <br />