On Combined Evaporation and Salt Precipitation In Heterogeneous Porous Media

Evaporation is a major process in the global water cycle. The literature on micro-scale evaporation mechanisms and the influence of the resulting salt precipitation within pores on the evaporation processes is limited. Moreover, the role of heterogeneity and roughness of the evaporating surface on the combined evaporation-salt precipitation processes is poorly understood. This research focuses on salt deposition occurring inside matrix pores during the evaporation process and its effect on evaporation rates for heterogeneous matrices and surfaces. Two methods are being used: (1) X-ray CT scanning to quantify salt deposition within pores; and (2) long-term monitoring of evaporation and salt deposition in soil columns with different scales of heterogeneities.

Investigation of the salt deposition mechanism shows that salt crust properties depend on salt species. Salts with high solubility appear to form crusts at the top of the matrix that is very thick and dense, with a resulting decrease in the evaporation rates due to reduction in the matrix permeability. Surface roughness also affects the way in which the salt precipitates. Preliminary results suggest that salts tend to precipitate in micro cracks on the matrix surface. Column experiments indicate that the evaporation rates from heterogeneous matrices with vertical textural interface between fine coarse textures are 1.5 times higher than the evaporation rates from homogeneous fine textured matrices, with salt crust forming in both cases. This was observed for heterogeneous columns which were vertically divided into half fine and half coarse sand, as well as for columns which were divided into quarters.

Our observations show that matrix heterogeneity moderates the effect of salt crust on evaporation relative to homogeneous media, as the large pores in the heterogeneous media remain open for the flow of water vapor.