Dissolved uranium, radium and radon evolution in the Continental Intercalaire aquifer, Algeria and Tunisia

摘要

Natural, dissolved ~(238)U-series radionuclides (U, ~(226)Ra, ~(222)Rn) and activity ratios (A.R.s: ~(234)U/~(238)U; ~(228)Ra/~(226)Ra) in Continental Intercalate (CI) groundwaters and limited samples from the overlying Complexe Terminal (CT) aquifers of Algeria and Tunisia are discussed alongside core measurements for U/Th (and K) in the contexts of radiological water quality, geochemical controls in the aquifer, and water residence times. A redox barrier is characterised downgradient in the Algerian CI for which a trend of increasing ~(234)U/~(238)U A.R.S with decreasing U-contents due to recoil-dominated ~(234)U solution under reducing conditions allows residence time modelling -500 ka for the highest enhanced A.R. = 3.17. Geochemical modelling therefore identifies waters towards the centre of the Grand Erg Oriental basin as palaeowaters in line with reported ~(14)C and ~(36)Cl ages. A similar ~(234)U/~(238)U trend is evidenced in a few of the Tunisian CI waters. The paleoage status of these waters is affirmed by both noble gas recharge temperatures and simple modelling of dissolved, radiogenic ~4He-contents both for sampled Algerian and Tunisian CI and CT waters. For the regions studied these waters therefore should be regarded as "fossil" waters and treated effectively as a non-renewable resource.