Quantifying Evaporation and Salt Accumulation in Fractured Rocks

Evaporation was traditionally considered as the loss of water vapor from land surface and water reservoirs to the atmosphere. Direct loss of water vapor from cracks, fractures and other discontinuities was usually neglected. A few papers published in the early 70's, recent models and field measurements suggest that under typical arid conditions significant amount of water vapor could be transported directly from surface exposed fractures to the atmosphere. Subsequently, salt accumulation along the fracture surfaces is likely to occur as the porewater solution in the upper vadose zone is typically saline. The rare but intensive rain events, often occurring in deserts, could dissolve this accumulated salt and flush it to the underlying aquifers. This process could be of great importance for salinization, especially in low permeability rocks, where without this mechanism salts are likely to accumulate in the upper vadose zone and never reach groundwater. The main objectives of this work is to experimentally quantify the amount of water vapor loss from fractures under controlled conditions and measure the amount and distribution of salts precipitate on the fracture walls. A customized Climate Control Room (CCR) was especially designed and constructed to mimic extreme night-time and day-time temperature conditions, typical for deserts. Within the CCR, two fractured blocks of chalk were installed. The rocks and fractures were instrumented so the temperature at the bottom of the rock is constant. Humidity and temperature within the fracture aperture and within the rock are constantly monitored. A feeding container is attached on each side of the block to supply the rock with pore water solution under constant tension. The inflow of water from the feeding containers into the rocks are constantly monitored and recorded as well as the overall changes oft he water content within the block. Preliminary results indicate a measurable water vapor loss from the fracture surfaces to the atmosphere and subsequent salt precipitation on the fracture walls.