Hyperfiltration‐induced fractionation of lithium isotopes: Ramifications relating to representativeness of aquifer sampling

Abstract

A 10‐m M LiCl solution was hydraulically forced through a 0.3‐cm‐thick smectite membrane at a constant solution flux in order to depict the chemical and isotopic evolution of analogous hyper filtration cells in the subsurface. At steady state (∼19 days) there was, relative to the input solution, a measured 12% buildup of solute within the entire cell and a calculated 248% solute increase at the clay/solution interface. The maximum depletion of 7 Li (about 13‰ relative to the input solution) was calculated to occur about 1 cm away from the membrane out into the high‐pressure reservoir. When scaled to realistic solution fluxes in natural systems, the position of maximum fractionation would occur meters away from a shale/aquifer contact. A steady state hyperfiltration model indicates that a solute's heavy isotopic species will always be depleted on the high‐pressure side of a clay membrane, a prediction corroborated not only by the Li experiment but also by previous hyperfiltration studies involving stable isotopes of C and Cl. The extent of fractionation increases with higher values of the membrane's reflection coefficient, σ. In the experiment a σ of 0.55 for the 90% porosity clay membrane yielded these results. Because shales have substantially lower porosities with attendant higher values of σ, we conclude that the hyperfiltration phenomenon in the subsurface may result in sampling of waters that are not chemically and isotopically representative of the formation penetrated by the well.