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where the value SJiS ~ +15 %o has been arbitrarily selected. <br />The typical S7°S value of magmatic pyrite is about 0 %w (Faure, 1986). A S'°S of -2.2 %o has been <br />reported for pyrite in the Park City, Utah District (Thode and others. 1961). Mayo and Klauk <br />(1991) found a mean SJ°S of +l .3 ``%w in ground water from non-carbonate (crystal line rock) <br />aquifers in north central Utah. Mayo and Kravits (unpublished data) found a S'°S value of pyrite <br />in the Sufco coal mine, Utah of+3.4%. Sulfur isotopic fractionation does not accompany the <br />dissolution of pyrite. <br />Tritium ('H) <br />Tritium ('H ), the radioactive isotope of hydrogen, has been used in ground-water investigations <br />to differentiate between ground waters which recharged prior to or after [he advent of <br />atmospheric thermonuclear weapons testing. Tritium, whose half-life is 12.43 years, forms <br />naturally in the upper stratosphere by [he interaction of10N with cosmic ray neutrons according <br />to the reaction: <br />uN + n ~'H +''C <br />Tritium is rapidly incorporated into water molecules and is removed from the atmosphere by <br />precipitation. <br />• Prior to the advent of atmospheric thermonuclear weapons testing in 1952, tritium activity in <br />prec~pnation ranged from 4 to 25 tritium units (TU). One TU equals one'H atom per lOd <br />hydrogen atoms. In mountainous areas, larger natural concentrations have been observed <br />(Fontes, 1983). During the peak of atmospheric weapons testing, tritium levels in precipitation <br />rose to m ore than 2,200 TU in some northern hemisphere locations (Fontes, 1983). As of 1987, <br />the'H concentrations in rain water varied from 25 to 50 TU. Unpublished data of 1991 and 1992 <br />snow samples collected in the central Wasatch Range, Utah have 'H concentrations of 20 TU or <br />more. <br />Carbon-14 (1°C ) <br />Carbon-14, the radioactive isotope of carbon, has ahalf--life of 5730 f 30 years (Godwin, 1962j. <br />Carbon-14 is produced in the upper atmosphere by a variety of reactions that involve the <br />collision of cosmic radiation (neutrons) with stable isotopes of m[rogen, oxygen, and carbon. <br />The most important of these reactions is between neutrons and1JN according [o the reaction: <br />n+uN~i°C+p <br />where n is a neutron andp is a proton (Libby, 1955). Carbon-14 is incorporated into CO„at and <br />rapidly mixes throughout the atmosphere and hydrosphere where steady state equilibrium <br />between "C production and'°C decay is attained (Faure, 1986). <br />The pre-industrial revolution atmospheric "C content has been assigned the steady state value of <br />100 percent modem carbon (port). The burning of fossil fuels and the advent of atmospheric <br />• thermonuclear weapons testing greatly altered the 1iC activity in post-industrial revolution <br />atmosphere. Burning of fossil fuels, whose 1°C had previously completely decayed away, <br />