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The light isotopic signature of the seep water might be explained by precipitation of calcite in a closed reservoir, which would preferentially remove heavy carbon and leave tFi~ residual water enriched in the light fraction. Such a reaction is compatible with the major element chemistry, cooling of the fluids, and the loss of bicazbonate and carbonate. Computer modeling could confirm whether this is a likely reaction or not, given the available data. The light isotopic signature of the seep water might also be explained by methane oxidation, which would enrich the fluid in methane-derived CO,, which would be isotopically light because the methane is isotopically light. The presence of unusually heavy sulfur isotopes, discussed above, and methane oxidation are compatible with this interpretation. This notion deserves further evaluation. Overall, a change from the relatively heavy sump water (S"C = -5.4 %o) and the isotopically lighter seep water (S"C = -12.9 %o) can be accomplished through a variety of mechanisms including (a) dilution with meteoric water containing soil gas CO, (b) precipitation of calcite, or (c) methane oxidation. Uncertainty in the B-seam S"C measurement would provide useful insight. The Barren member S"C value, which is itself very light, indicates that local sources of isotopically light groundwater could likely be available, thus diminishing the need to call on more complicated geochemical measures. Given that the Lone Pine seal water is probably not representative of the sump water as a whole, any shifr in carbon isotope values from the sump to the seep may not require much meteoric water mixing or soil gas CO, to explain the difference in values; rather, the answer may be in the average sump value itself. D. Radioisotope Ages. r'C. Owing to the presence of methane and other carbon compounds, the carbon-14 ("C) age information is rather sketchy. What can be said from the data is that the Sump water and the Edwards Mine portal seep water have a relatively high percent of modern carbon (pmc) whereas the fault waters may have much less. This implies that both have a young atmospheric carbon component. This is compatible with meteoric water mixing with the fault waters as modem meteoric water would provide the high pmc component necessary to shift the values to high pmc in the seep water. The higher pmc in the sump water implies the sump water has more modern water than the seep water. This implies further that there must be an older water diluting the sump water between the sump and the seep, assuming of course that the sump water is the source of the seep, or that the seep water did not evolve directly from the sump water that was collected. Neither interpretation appears to be favored. 'H. The tritium ('H) values for most of the samples, both fault, sump and seep, generally record some component of "post-bomb" water. A few of the fault waters do not. Again, the greater amount of deuterium in the sump requires an explanation similar to that provided for the "C values, namely that there is either a second source of "old" water between the sump and the seep, or the sump water is not representative of water that possibly recharges the seep. Overall, the greater abundance of "bomb" water in the sump and seep relative to the fault implies that the faults contain a higher abundance of non-bomb, i.e. old, water. Nonetheless, the presence of any 9