Osmotic potential refers to the energy required to pull water away from ions in solution that are attracted to the polar water molecules. In the presence of a semipermeable membrane between two solutions, water molecules will move through the membrane to the side with the higher concentration. This property may be important to fate assessment because in the deep-well environment, shales that serve as confining layers can act as semipermeable membranes if the injected waste significantly changes the solute concentrations.59 In laboratory experiments, Kharaka60 found that retardation sequences across geologic membranes varied with the material, but that monovalent and divalent cations generally followed identical sequences: Li+ < Na+ < NH+ < K+ < Rb+ < Cs+ and Mg2+ < Ca2+ < Sr2+ < Ba2+.
If osmotic effects are possible, several other effects would need to be considered in a geochemical-fate assessment, depending on whether the solute concentration is increased or decreased. If solute concentrations are increased, pressures associated with injection would increase beyond those predicted without osmotic effects. Also, the movement of ions to the injection zone from the aquifer with lower salinity (above the clay confining layer) would increase the salinity above those levels predicted by simple mixing of the reservoir fluid and the injected wastes. This action could affect the results of any geochemical modeling.
If solute concentrations are decreased, the remote possibility exists that wastes would migrate through the confining layer. For this to occur, solute concentrations above the confining layer would have to be higher than those in the injection zone, and movement, in any event, would be very slow. As USDWs have salinities less than 10,000 mg/L, compared with typical salinities in injection zones of 20,000 to 70,000 mg/L, even if this process were to occur it would cause migration only to overlying aquifers that are not USDWs.
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