科研之友
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摘要
The radioactivity due to U-238 and U-234 in three aquifer systems occurring within the Parana sedimentary basin, South America, has been investigated. Uranium is much less dissolved from fractured igneous rocks than from the porous sedimentary rocks as indicated by the U-mobility coefficients between 7.6 x 10(-6) and 1.2 x 10(-3) g cm(-3). These values are also compatible with the U preference ratios relative to Na, K, Ca, Mg and SiO2, which showed that U is never preferentially mobilized in the liquid phase during the flow occurring in cracks, fissures, fractures and faults of the igneous basaltic rocks. Experimental dissolution of diabase grains on a time-scale laboratory has demonstrated that the U dissolution appeared to be a two-stage process characterized by linear and second-order kinetics. The U dissolution rate was 8 x 10(-16) mol m(-2) s(-1) that is within the range of 4 x 10(-16)-3 x 10(-14) mol m(-2) s(-1) estimated for other rock types. The U-234/U-238 activity ratio of dissolved U in solutions was higher than unity, a typical result expected during the water-rock interactions when preferential U-234-leach from the rock surfaces takes place. Some U-isotopes data allowed estimating 320 ka for the groundwater residence time in a sector of a transect in So Paulo State. A modeling has been also realized considering all U-isotopes data obtained in Bauru (35 samples), Serra Geral (16 samples) and Guarani (29 samples) aquifers. The results indicated that the Bauru aquifer waters may result from the admixture of waters from Guarani (1.5 %) and Serra Geral (98.5 %) aquifers.
- 出版日期2013-3
