On the Processes Occurring During Infiltration of Brine into Porous Media

Infiltration of saline solutions and pure water into pre-wetted and air-dried sands was investigated using a light transmission system. Four grades of sand and five solutions were tested. Narrow fingers with a sharp, almost saturated, wetting front were observed in the dry sands for all solutions. The water saturations left behind the "fingertip" of saline solutions was higher than for those of pure water, resulting in a greater lateral expansion of the saline fingers over time. In the dry sand, the rate of lateral expansion scaled with the square root of time, most likely due to classic liquid sorption with the possible addition of water vapor diffusion. Early on, the salty fingers moved faster, but were ultimately overtaken by the pure water fingers. In pre-wetted sand, the wetting fronts were diffuse and never exceeded 26 percent saturation. The elevated surface tension of the brines was the major cause for the observed differences in finger width and velocity, yet appeared to be insignificant in air-dry sand. Additionally, water vapor transport in the vicinity of the infiltrating saline solutions was investigated. Drying around infiltrating brines was observed. The same solutions were applied to layered systems, where two horizontal fine layers were embedded within a coarser matrix, mimicking stratified sedimentary deposits. Water vapor stripping was found to enhance the lateral transport of brine in layered sand, where capillary barrier effects play a major role. Our observations suggest that (1) wetting fronts of infiltrating solutions are significantly different in air-dry and pre-wetted sand; (2) surface tension of the infiltrating solution plays an important role in determining the infiltration rate into pre-wetted sand; and (3) vapor pressure gradients, which develop due to differences in vapor pressure as saline solution infiltrates into pure water or vice versa, drive water vapor transport along them and should therefore be taken into account in places where brine leaks into the vadose zone.