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• decreased the "C content in the troposphere in the northern hemisphere by about 3% <br />(Houtermans and others, 1967). Atmospheric weapons testing greatly increased the atmospheric <br />1'C activity by the mid 1960's (Ferronsky and Polyakov, 1982). <br />The post-industrial revolution atmospheric "C perturbations and laboratory measurement error <br />in measunng [he "C content of ground water make the reliable lower limit for "C dating about <br />450 years. The upper limit of "C dating, using conventional laboratory analytical methods, is <br />about 35,000 years. <br />Estimating the age of dead wood or other organic carbon is relatively simple. The "C activity of <br />pre-industrial revolution organic material is assumed to be 100 pmc. The radiocarbon date is <br />then corrected for systematic variations in atmospheric ~'C that have been established by <br />comparing tree ring dates of the wood of Sequoia and Bristlecone Pines with their <br />corresponding radiocarbon ages (LaMarche and Harlan, 1973; Michael and Ralf, 1970). <br />Estimating the radiocarbon age of ground waters is not as straightforward as estimating the age <br />of dead organic matter. Ground water acquires carbon from numerous sources, many of which <br />had initial "C activities of less than 100 pmc. The "C content of ground water is affected by <br />tour factors: <br />1) the addition of "live" carbon (i.e., "C = 100 pmc) from the biogenic production of <br />CO,t~~ in the soil zone, <br />2) the addition of "dead" carbon from the weathering of minerals in the soil zone and <br />the dissolution of carbonate minerals in the soil zone or aquifer (i.e., "C ~ 0 pmc), <br />• 3) the addition of "dead" carbon from the soil or aquifer during isotopic exchange <br />reactions, and <br />4) the addition of both "live" and "dead" carbon by other processes. <br />The crux of dating ground water is estimating the initial "C activity (Ao) of [he water at [he time <br />of recharge. This may be accomplished by using the solute and isotopic chemistries of the <br />ground water and applying correction procedures. Correction procedures for estimating Ao are in <br />[he form of mathematical equations [ha[ attempt to account for the contribution of "dead" carbon <br />and "C from various sources, and for the effects of the isotopic exchange and fractionation <br />processes. <br />• <br />