Chlorine-36

Abstract

The chloride anion is an extraordinarily stable ionic species. Chloride is the thermodynamically favoured form of the element under virtually all terrestrial aqueous conditions. Furthermore, the negative charge of the chloride anion discourages adsorption onto silicate surfaces, which are also typically negatively charged. Due to this behaviour, chloride, once introduced to natural water, is usually advected at the same rate as the water and is not normally removed from the water by geochemical processes. These properties have led to extensive use of Cl as a hydrological tracer (see Herczeg and Edmunds, Chapter 2). The utility of the Cl-tracer has been greatly extended by the recognition that the element has a long-lived radioactive isotope: 36C1. With a half-life of 301 000 ± 4 000 years (Endt and Van der Leun, 1973; Bentley et al., 1986a), 36C1 can be used to date groundwater with subsurface residence times up to one million years, and also has a myriad of uses for tracing subsurface water at shorter time scales. Most of these applications have been realised within the past 20 years.