Not if the brines are less dense than the porous regolith.

MARTIAN REGOLITH SIMULANT JSC MARS-1.
"Density and Porosity The particle density of JSC
Mars-1 is 1.91 ± 0.02 g/cm3. The bulk density is 0.87± 0.02g/cm3. This value can be increased to 1.07 ±0.02 g/cm3
by vibration. These data, compared to the particle density, infer an uncompacted porosity of 54% and a compacted value of 44%.
Drift material near the Viking 1 landing site has a bulk density of 1.2 ± 0.2 g/cm3while blocky material has a value of 1.6 ± 0.4 g/cm3[11]. The bulk density of <2 mm fines after delivery to the Viking x-ray fluorescence spectrometers was 1.10 ± 0.15 g/cm3,
implying a bulk regolith porosity of 60 ± 15 % [12].
Mars Pathfinder experiments indicate an average bulk
soil density of 1.52 g/cm3[13]."
http://www.lpi.usra.edu/meetings/LPSC98/pdf/1690.pdf

Bob Clark