Flow reversals, groundwater systems

Aldstadt et al. described the determination of U in groundwater using a FI system with a TRU-Resin column coupled on-line to an ICP-MS. The system used three 2-position valves and a flow-reversal scheme to load the sample in one direction and elute in the reverse direction. The system provided an enrichment factor on the order of 30-fold compared to direct groundwater analysis. Separate elution of Pu and Th in a group prior to eluting U was also discussed. 

System assumptions that should be valid for such applications include fluid flow in the porous media is isotropic and adsorption is fast, reversible, and linear (cf. Freeze and Cherry 1979). Given these constraints, the comparative transport of a conserved (nonadsorbed) tracer, such as Br , and an adsorbed or retarded species, such as Am, can be described as shown in Fig. 10.29. A comparison of migration distances of the two species after time t, is made at concentrations where C(measured)/Co(initial) = 0.5 for the conserved and adsorbed species. The migration distance X of the conserved species after time r is a measure of the average groundwater velocity (U), or X = vt. Similarly, the migration distance of the adsorbed species (X,) i related to its velocity of movement (v ) by Xf = vj. The retardation factor (/tj for the adsorbed species is then given by... 

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