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~ Mayo and Associates Consultants in Hydrogeology 710 East 100 North Lindon, Utah 84042 (801) 785-2385 • (801) 785-?387 (Fax) Application of Selected Isotopes to Hydrogeologic Problems Alan L. Mayo 10 April 1996 Oxygen-18 (5180) and Hydrogen-2 (SZH ) Worldwide, the S'H and 5180 of precipitation (rain and snow) generally follow the empirical relationship: S'H = 8(5180) + d (%o) Where s is the slope and rl is the deuterium (hydrogen-2) excess (Merlivant and Jouzel, 1983). Craig (1961) and Dansgaard (1964) have shown that, on the global scale, s approximates 8 and rt approximates l0 for coastal meteoric water. The Meteoric Water Line (MWL) is therefore • defined as: S'H = 8(5180) + 10 (%) The 5180 and S'H composition of ground waters can be used to help evaluate the origin and flow and mixing patterns of ground waters. Ground water recharged during cooler climates or at higher elevations will have more negative isotopic compositions than ground water which recharged during warmer climates or at lower elevations. Ground waters which have been heated above about 100°C during deep circulation will exhibit a positive 5180 shift relative to the S=H composition. Ground water of non-meteoric origin (i.e. connate and magmatic) will not plot along the M W I,. Carbon-13 (S°C) Most ground water acquires 50 percent of its carbon from soil zone water and 50 percent of its carbon from [he dissolution of carbonate minerals in the soil zone or aquifer skeleton. Because the S"C of marine carbonate minerals is about 0 °~.o (Muller and Mayo, 1986) and soil zone CO, gas has a S°C of -18 to -27 °/W, most ground waters have a S°C of = -9 to -13 °.~,. Sulfur-34 (S"'S) The anticipated range of S"S values in Mesozoic early Tertiary gypsum and anhydrite is +10 to +20 % (Holser and Kaplan, 1966). At non-thermal aquifer temperatures, isotopic fractionation accompanying gypsum dissolution may be represented as: • CaSO,~„ "S enrichment Ca`' + SO;"