Periodic Table--Chlorine

Chlorine has 9 isotopes with mass numbers ranging from 32 to 40. Only three of these isotopes occur naturally: stable ³⁵Cl (75.77%)and ³⁷Cl (24.23%), and radioactive ³⁶Cl. The ratio of ³⁶Cl to stable Cl in the environment is about 700 x 10^-15 : 1 (Bentley et. al., 1986). ³⁶Cl is produced in the atmosphere by spallation of ³⁶Ar by interactions with cosmic ray protons. In the subsurface environment, ³⁶Cl is generated primarily as a result of neutron capture by ³⁵Cl or muon-capture by ⁴⁰Ca (Fabryka- Martin, 1988). ³⁶Cl decays to ³⁶S and to ³⁶Ar, with a combined half-life of 308,000 years. The half-life of this hydrophilic nonreactive isotope makes it suitable for dating in the range of 60,000 to 1 million years. Additionally, large amounts of ³⁶Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of ³⁶Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, ³⁶Cl is also useful for dating waters less than 50 years BP. ³⁶Cl has seen use in other areas of the geological sciences, including dating ice and sediments (Nishiizumi et al., 1983; Phillips et al.,1983).

The attractiveness of chlorine in hydrologic studies is that it is highly soluble, exists in nature as a conservative non-sorbing anion, does not participate in redox reactions, and has some quickly identifiable sources (e.g., seawater). The abundance of ³⁶Cl is usually reported as the atomic ratio of ³⁶Cl to total chloride in the sample. The ratio is always quite small in natural waters, typical values ranging from 10^-15 to 10^-11. The four orders of magnitude range of ³⁶Cl/Cl ratios is due to several factors. Over geologic time an equilibrium will be established between the subsurface in-situ production of ³⁶Cl and its decay (similar to the secular equilibrium of U isotopes). The equilibrium ³⁶Cl/Cl value will depend on the rate of production of ³⁶Cl, which is a function of the U and Th concentrations in the aquifer. Ratios higher than 10^{- 12} in natural waters indicate the presence of thermonuclear ³⁶Cl; maximum global values during sea-level atmospheric nuclear weapon tests were on the order of 10^-11. Thermonuclear ³⁶Cl is likely to become more frequently used as an indicator of young water as the thermonuclear ³H in ground water decays to background levels over the next couple decades.

³⁶Cl has been used to date old ground water in confined aquifers by consideration of the affect of radioactive decay on measured ³⁶Cl/Cl ratios (Bentley et al., 1986; Phillips et al., 1986; Nolte et al., 1991). Potential complications include possible addition of stable Cl isotopes to the water by reactions with rocks, ion filtration (Phillips et al., 1986), or mixing with higher chloride waters. Such additions can substantially change the ³⁶Cl/Cl ratio. An age interpretation also requires knowledge of the initial ³⁶Cl/Cl ratio, which can be difficult due to the wide range in possible precipitation input values (Andrews and Fontes, 1993). In-situ production of ³⁶Cl requires an adequate assessment of the U and Th concentrations in the aquifer. Because precipitation values are in the 20-500 x 10^-15 range, in-situ production values of about 50 x 10^-15 can have a significant affect on observed ³⁶Cl/Cl ratios (Nimz, 1998). The addition of Cl can be evaluated by measurement of the Cl and other ionic concentrations in the water, in-situ production can be estimated after a measurement of aquifer U and Th concentrations, and precipitation input values can be estimated using present-day local values. For the determination of water transit times between two sampling locations instead of the absolute age of the water, the value measured at the upgradient location is used as the input value.

Cl, like Li and B, has two stable isotopes (³⁵Cl and ³⁷Cl) that are highly mobile in the hydrosphere; however, unlike Li and B, these isotopes are not easily fractionated in nature. d³⁷Cl values (ratios of ³⁷Cl to ³⁵Cl) are reported in ‰ relative to the standard SMOC (Standard Mean Ocean Chloride). Small variations in d³⁷Cl values (< 2.1‰) were reported by Kaufman et al. (1984) in several water types. Fractionation can occur during diffusion-controlled processes in ground water (Desaulniers et al., 1986), during high-temperature water-rock interactions (Eastoe et al., 1989), and as a result of temperature variation in the ocean over time (Kaufman et al., 1993). Therefore, there appears to be sufficient isotopic variations in nature to make ³⁷Cl useful as a hydrologic tracer. Depending on local lithology, d³⁷Cl might be a useful tool for hydrograph separation analysis and determination of mixing between regional and shallow ground water mixing (Nimz, 1998).

A multi-isotopic approach is almost always beneficial ("the more isotopes the merrier!"). One recent example is the use the d¹³C and d³⁷Cl of chlorinated solvents such as TCE and BTEX to identify the sources of the contaminants in ground water, and the degradation reactions that may remediate the pollution plumes. One such study found that chlorinated solvents (TCE, PCA, and TCE) supplied by different manufacturers had distinctive C and Cl isotopic signatures (Aravena et al., 1996). This appears to be a very promising avenue of research, although the analysis of d³⁷Cl is still technologically challenging.

Further information can be found in Clark and Fritz (1997), Environmental Isotopes in Hydrology (CRC Press):

• Thermonuclear 36Cl

Source of text: This review was assembled by Carol Kendall, Eric Caldwell and Dan Snyder, primarily from a recent review by Nimz (1998) and Bentley et al. (1986).