Subsequently, the tapered ends of the capillary were sealed to the reservoir screw nuts, and both reservoirs were filled with 50 ml of 0.1 M NaCl solution. At this stage, the cell was allowed to equilibrate with the 0.1 M NaCl solution for one week to ensure uniform equilibration between the reservoirs and the gel pore solution. The choice of a 0.1 M NaCl solution, rather than pure water, was made to maintain the generally observed conditions of rock pore fluid. Since the silica-gel is rather porous and is attached to two large reservoirs, the final concentration in the gel pore fluid after one week of equilibration remains very close to 0.1 M NaCl. After the equilibration step, the diffusion experiment was conducted by spiking the inlet reservoir of the cell with approximately 0.122 M NaBr solution, while keeping the outlet reservoir tracer-free.