Near-conservative behavior of 129I in the orange county aquifer system, California

Iodine is a biophilic element, with one stable isotope, 127I, and one long-lived radioisotope, 129I. Radioiodine originates in the surface environment almost entirely from anthropogenic activities such as nuclear fuel reprocessing in Europe and thus provides a unique point source tracer. Very few studies have evaluated the geochemical behavior of I isotopes in the subsurface. In this study, the concentrations of 129I and 127I were measured in wells fed by a series of artificial recharge ponds in the Forebay Area of the Orange County ground water basin (California, USA) to evaluate their potential use as hydrological tracers. To substantiate interpretation of 129I and 127I concentration data, the aquifer system was evaluated using the literature values of aquifer water mass age based on 3H/3He, Xe and δ18O tracer data. The aquifer data demonstrate the nearly conservative behavior of 129I with 129I/127I ratios likely reflecting variations in source functions as well as climatic conditions, and with inferred particle-water partition coefficients (Kd) of 0.1 cm3 g−1 or less.