Thermonuclear bomb testing generated not only a whopping peak in tritium, but also in the radioactive isotope of Cl — 36Cl. The high neutron flux of hydrogen bomb tests in marine settings during the early years of testing elevated concentrations by over two orders of magnitude above its natural atmospheric abundance. By consequence, 36Cl can be used — like tritium — to identify modern recharge. However, unlike 3H, its long half-life (t½ ~ 300,000 years) precludes the use of 36Cl decay to date modern waters.
Natural atmospheric production of 36Cl occurs due to activation of atmospheric argon by solar radiation and arrives at the earth's surface as a dry fallout or in precipitation. Natural production and fallout varies with latitude from close to 20 atoms m–2 s–1 at mid latitudes to less than 5 near the equator and poles. It is also produced epigenetically through activation of Cl, K and Ca on the land surface by solar radiation (see Chapter 8). Thermonuclear 36Cl was produced by activation of atmospheric Ar and marine Cl– by the high neutron fluxes accompanying low altitude tests in the 1950s. A record of this fallout is preserved in glacier ice (Fig. 7-14), reconstructed by Bentley et al. (1986). High levels of 36Cl in groundwater indicate, like tritium, that recharge has occurred since this time. Fig. 7-14 shows that since the period of maximum 36Cl contamination, concentrations have been steadily declining. This washing out of atmospheric Cl has brought concentrations, at least by 1980, back closer to natural levels. However, storage and recycling of Cl in the biosphere seems to maintain a background 36Cl activity chronically elevated above natural levels (Gwen Milton, Atomic Energy of Canada Ltd., pers. comm.).
Concentrations of 36C in groundwater requires conversion from the fallout rate (atoms m–2 s–1) to concentration in water (atoms per litre). For example, with a natural 36Cl fallout rate of 15 atoms m–2 s–1, annual precipitation of 750 mm, and 75 mm loss to evapotranspiration, the concentration in runoff and/or groundwater can be calculated. The annual fallout rate would be: