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Morro and Assoelafes, LC <br />• Two unstable isotopes, tritium ('H) and carbon-14 ("C), have been used to evaluate mean <br />residence times. Tritium is a qualitative tool indicating if groundwater has a component of <br />water that recharged since about 1954. Groundwater that recharged prior to about 1954 will <br />contain essentially no tritium. Carbon-14 provides inforniation regarding the number of <br />years that have elapsed since the groundwater became isolated from soil zone gases and near- <br />surface waters. Like tritium, "C can indicate if groundwater has a component of water that <br />recharged since the 1950s. Groundwaters with '°C contents greater than about 50 pmc <br />contain anthropogenic (human-induced) carbon associated with atmospheric nuclear weapons <br />testing. Occasionally for groundwater issuing from a spring or occumng in a well is a <br />mixture of old (i.e. containing no'H) and younger groundwaters. <br />• Samples for unstable isotopic analysis have been collected from 29 locations including: 13 <br />springs (nine landslide, three Barren Member and one Lower Coal Member), three Barren <br />Member wells, seven F-, E-, and B-Seam wells, seven in-mine fault-related Rollins <br />Sandstone wells and discharges, one Rollins Sandstone well, and one Dakota Sandstone well <br />(Table 5). All of the spring waters contain appreciable'H and all but one of the springs <br />(WCC-24) contain anthropogenic14C indicating a modem origin. <br />Springs issuing from landslides and from Ban•en Member bedrock contain both <br />an[hropogenic'H and '°C and thus contain modern recharge water (Table 5). <br />Two wells in the Barren Member SOM-45-H2 and SOM-13 have unstable isotopic <br />. compositions indicating mixed recharge origins, whereas SOM-23-H4 contains'H and <br />Characterization of Groundwater Systems in the Vicinity of the West Elk Mine, Somerset, Colorado <br />29 January 1999 <br />Pale 45 <br />