Implementation of Isotope Subroutine to Computer Program PHREEQE and their Application to C-14 Ground Water Dating

Abstract

The age of ground water is defined as the length of time the water has been isolated from the atmosphere. Among the methods for ground water dating, C14 is the most commonly used and the most intensively studied tool. The concentration of C14 in dissolved inorganic carbon can change as a result of chemical processes in nature, hence, an adjustment factor Q is included in the age equation. A = QAo(e^(- λt)) Various models have been proposed to account for this adjustment factor. Among those models, the mass transferbalance approach is the most rigorous method. Wigley, Plummer, and Pearson (1978) formulated a mass balance equation to calculate the evolution of C13 and C14 in natural water systems closed to soil CO2 gas. Deines, Langmuir, and Harmon (1974) used a set of dual chemical-isotopic equilibrium equations to calculate changes of C13 in systems open to soil CO2 gas. This study implements these two models as a subroutine and adds carbon isotope mixing equations to PHREEQE (Parkhurst, Thorstenson, and Plummer, 1980), which is a computer program for general hydrogeochemical calculations. With this program package, it is now possible to simulate the evolution of chemical and carbon isotopic compositions, including C14, of ground water from open to closed systems. These simulations allow much improved inferences of Q factors for radiocarbon groundwater dating.