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Mayo and AssoeJales, LC <br />• anthropogenic "C. The "C compositions of SOM-45-H2 and SOM-13 yield calculated <br />mean residence times of 4,700 and 10,500 years, respectively. The elevated'H contents of <br />these waters mean that they also contain components of modem recharge water or that the <br />constructions of the wells do not completely isolate the screened intervals from shallower <br />groundwater. Regardless of the sources of modern waters, the'H contents mean that the <br />actual ages of the old components of recharge are older than the calculated ages. <br />Except for B-Seam well So.W-1, grottndwaters in F-, E-, and B-Seam wells have essentially <br />no'H and very low "C contents (Table 5). The general absence of'H in these waters means <br />that they recharged prior to about 1954 and that the groundwaters are not part of an active <br />groundwater system that supplies significant baseflow to springs or streams and are therefore <br />• not tributary. Very low 14C contents (i.e. < 4 pmc) are usually indicative of very old <br />grottndwaters (i.e. > 25,000 years). However, "C dating of these waters has proven difficult <br />because there is evidence of additional "dead" carbon, which cannot be accounted for in the <br />"C dating models. This additional "dead" carbon is indicated by S°C compositions that are <br />more positive than about -7 or -8%0. We interpret the unusually positive S"C compositions <br />as the result of CO, associated with methane. The isotopic composition of So. W-1 suggests a <br />groundwater that, like groundwaters in the Barren Member wells, has a mixed recharge <br />history. The elevated'H content indicates a component of modem recharge water and the "C <br />content indicates a component of recharge that occurred more than 8,500 years ago. <br />Unstable isotopic samples from the Rollins Sandstone have been collected from one surface <br />well, two in-mine wells, and five fault-related inflows. The'H contents of five of these <br />Characterization of Groundwater Systems in the Vicinity of the West Elk Mine. Somerset, Colorado <br />29 January 1999 <br />Page 49 <br />