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In highly permeable sedimentary rock formations, U extraction by in-situ leaching techniques (ISR  In-Situ Recovery) is generally considered to have a limited environmental impact at ground level. Significantly, this method of extraction produces neither mill tailings nor waste rocks. Underground, however, the outcome for 238 U daughter elements in aquifers is not well known because of their trace concentrations in the host rocks. Thus, understanding the in-situ mobility of these elements remains a challenge. Two samples collected before and after six months of ISR experiments (Dulaan Uul, Mongolia) were studied with the help of a digital autoradiography technique (DA) of alpha particles, bulk alpha spectrometry, and complementary petrographic observation methods. These techniques demonstrate that before and after leaching, the radioactivity is concentrated in altered and microporous Fe-Ti oxides.Most of the daughter elements of U remain trapped in the rock after the leaching process. DA confirms that the alpha activity of the Fe-Ti oxides remains high after uranium leaching, and the initial secular equilibrium of the 238 U series for 230 Th to 210 Po daughter elements (including 226 Ra) of the fresh rocks is maintained after leaching. While these findings should be confirmed by more systematic studies, they already identify potential mechanisms explaining why the U-daughter concentrations in leaching water are low.

A new approach is proposed in order to spatially localize and determine the equilibrium state of natural decay chains on hand-scale geological samples, thanks to a combination of three techniques: 1) Elementary chemical mapping by microprobe; 2) Alpha autoradiograph by gaseous detectors and 3) bulk alpha particle spectrometry. The quantitative nature of alpha autoradiograph and its comparison with U chemical maps allows to locate radioactive equilibrium state in four samples. This equilibrium state was confirmed by alpha spectrometry analysis.