Shallow uranium mobilization processes
The terrestrial hydrosphere is here considered in the three established subrealms: surficial, vadose, and saturated zones. Less discussed in the literature is the fact that the saturated zone is underlain by a dry zone to which meteoric water is denied entry because of the geostatic pressure, except along preferred stratigraphic or structural aquifers where flow is restrictively channeled. Water in the surficial realm is well demonstrated to be purely precipitation. That in the vadose and saturated zones is supergene percolating meteoric water contaminated to varying degrees by dissolved minerals, connate water, or hypogene fluid (liquid or gas) emanations. Hydrochemistry suggests that most groundwater is ultimately meteoric (White, 1969). Connate water is, by definition, trapped; it loses its identity, as do hypogene emanations, upon entry into the circulating system, although both can contribute to the salinity.
The dissolving power of pure water, given adequate time, is well demonstrated, and that of brines is greater. Therefore, the solution and redeposition of less stable elements and minerals in the vadose and saturated zones represent a valid process of lateral secretion. Preferred channel systems permit this process to operate in the dry zone. Therefore, the only effective way to distinguish supergene, laterogene, or hypogene processes for low-temperature elements is by the geometry of their controlling hydrologic systems and by the relative effects of variable time, temperature, and salinity. The negligible effect produced by relatively fresh meteoric water is well demonstrated by the absence of significant mineral deposits in active aquifer systems. Mineralization by cold brines is also negligible. However, rising temperature increases mineralization dramatically; that by brines is orders of magnitude greater than by fresh waters. The effects of temperature can be deduced from the distribution of mineralized areas. The source of volatiles and salts cannot be differentiated easily, but distribution in relation to regional geology can suggest hypogene or stratigraphic sources.