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Mayo and Assotla/es, LC <br />• years. We believe that the F-, E-, and B-Seam and Rollins Sandstone groundwaters are of <br />similar antiquity. <br />4.4 S2H and 5180 <br />The SZH and S180 composition of a water molecule falling as precipitation is determined by <br />[he temperature at which nucleation of the water droplet occurs. The stable isotopic <br />composition of water is usually analyzed relative to the Meteoric Water Line (MWL). The <br />MWL is empirically derived from the worldwide plotting locations of coastal zone <br />precipitation and is defined by the equation S'H = 8 5180 + 10%0 (See Appendix B for further <br />discussion of the MWL). Precipitation forming daring cooler conditions will plot more <br />negative than will precipitation fomming under warmer conditions. <br />n <br />U <br />In addition to the nucleation temperature of the water molecule, several other factors may <br />affect the isotopic composition of recharge water. These factors include rainout and <br />orographic effects and the sublimation of snow prior to the springtime snowmelt. <br />Except for unusual conditions such as geothermal heating above about 100°C, the S'-H and <br />5180 composition of a groundwater is set at the time of recharge and is no[ affected by <br />subsurface conditions such as residence time and mineral dissolution and precipitation <br />reactions. In other words, the recharge and flow history of a groundwater can be evaluated <br />independently of the solute content of the water. <br />• <br />Characterization of Groundwater Systems in the Vicinity of the West Elk Mine, Somerset, Colorado <br />29 January 1999 <br />Page 51 <br />