Experimental confirmation of a delta chloride-37 minimum in a hyperfiltration system

Hyperfiltration (or reverse osmosis) is the process by which solution is advected towards a semi-permeable membrane, resulting in the buildup of solute at the membrane face and solute redistribution through diffusion back into the high-pressure reservoir. Steady-state equations were derived that describe the behavior of chloride and its isotopes—as sodium chloride—in such a hyperfiltration system where solute is advected orthogonally towards the membrane. An experimental apparatus was constructed in which sampling tubes were emplaced into the concentration polarization layer (CPL), the region of excess solute in the vicinity of the uncompressed bentonite membrane face. Twelve samples were extracted from the CPL at different distances from the membrane and analyzed for chloride chemistry and isotopes. The experimental results demonstrate the correctness of the primary hypothesis of the derived equations: that a minimum in δ37Cl will occur in an orthogonal-flow hyperfiltration system. In the subsurface, the conservative nature of chloride points towards fractionation of its isotopes being accomplished through hyperfiltration or hydrodynamic dispersion. The differences between the two processes, advection and diffusion moving solute in opposite directions during hyperfiltration but in the same direction during hydrodynamic dispersion, implies that they are not truly comparable on a one-to-one basis. However, a theoretical comparison between the two processes can be constructed and with respect to chloride isotopic fractionation the result is clear. At the low flux rates found in the subsurface in regional-flow systems, fractionation due to hyperfiltration is small even compared to the 2σ precision (±0.20‰) inherent in the isotopic measurement. Hyperfiltration can only be expected to produce significant chloride isotopic fractionation in high-flux, localized aquifer settings; the primary mode of chloride fractionation in regional-flow systems in the subsurface will be due to hydrodynamic dispersion