Hydrological implications of deep production of chlorine-36

Abstract

Among the possible origins of 36Cl in ground waters: (1) cosmic radiation in the atmosphere, (2) cosmic in the subsurface, (3) deep underground, (4) marine nuclear tests, the deep component is generally taken as negligible. This could be the case in rocks with low or medium uranium and thorium contents and low content in elements with high cross sections for neutron production. However, under favourable geological conditions (crystalline or metamorphic rocks, sedimentary rocks of detrital and endoreic origin), the deep neutron flux may be significant and thus give rise to an underground production of 36Cl by reaction on 35Cl atoms from ground water and possibly from fluid inclusions. This production is easily recognizable when the 36Cl content per liter of old (tritium free) water is greater than the “modern” 36Cl content of precipitation corrected for evapotranspiration. A clear example is the deep ground water system of the Stripa granite, in Sweden, where values as high as 7 × 10 8 atoms of 36Cl per liter are observed. In this particular example, the initial content of the recharge may be neglected and a residence time of the chloride ions may be estimated if the neutron flux is known. Such favourable situations may be encountered when saline intrusions reach the ground water system in uranium and thorium rich rocks. An ambiguity may arise if the deep 36Cl production corresponds to values of apparent decay of meteoric 36Cl. This is illustrated by 36Cl content of deep ground waters from the Sahara (aquifer of the Continental Intercalaire). Values of 6 and 20 × 10 8 atoms per liter may be interpreted in terms either of very high concentration of rain water by evatranspiration during recharge processes, or of deep 36Cl production. A discussion based on additional paleohydrological information (stable isotopes, noble gas contents) suggests an intraformational generation of 36Cl. However, in such a case, the differences in activity ratio 36Cl/Cl tot and in absolute 36Cl content (atoms per liter) might be interpreted in terms of heterogeneity in the recharge, circulation and mixing processes. No age estimation can be proposed. General limitations of ground water dating with 36Cl are discussed.