And like most (?all) this type of calculation there is an inherent assumption that the solution is in a laboratory beaker and isolated from its surroundings. Otherwise the brines, which generally have relative densities above 1.25, would tend to sink into the porous surface regolith. Of course one postulate negative gravity on Mars, which some models do, and allow the brines to float on lower density regional groundwaters but since probes do tend to land or crash onto Mars, that model can be rejected and we can safely say brines will sink into porous materials just like on earth. Hence back to the beaker.