ON THE CONNECTION BETWEEN SURFACE EXPOSED FRACTURES, ATMOSPHERIC CONDITIONS AND GROUNDWATER SALINIZATION IN ARID ENVIRONMENT

This research investigated a possible mechanism for groundwater salinization via surface-exposed fractures, and the effect that these fractures have on estimates of arid land evaporative losses. Groundwater overlain by thick vadose zones of low permeability are generally thought to be protected from typical surface salinization problems. However, a fracture venting process could effectively shorten the protective thickness of the vadose zone. If salts accumulated within the fractures are dissolved during infiltration events, these dissolved salts might reach the groundwater. Essentially, this mechanism could draw the contaminant salts from the matrix to the fracture surface by capillarity, and from there rapidly to the water table, effectively circumventing the natural barrier the matrix is presumed to provide. Intermediate-scale experimental measurements of evaporation from fractures were performed using a specially designed apparatus located in a climate control room. The experiments showed significant salt accumulation along fracture surfaces due to evaporation, and a reduction in long-term evaporation rate caused by salt accumulation. Evaporation rates were found to be strongly dependent on atmospheric temperature, with highest evaporation rates during cold nights when convective conditions within the fracture develop. Field measurements carried out inside a surface exposed fracture in the Negev desert of Israel demonstrated that "convective conditions" occurs naturally and during large portions of the year. Numerical methods were used to establish the limits of the convective process and explore a greater number of parameters. The mechanism for convective venting of moist air from fractures driven by diurnal changes in atmospheric temperature was investigated by experimental and numerical methods. The mechanism was shown to have a significant potential to concentrate and transport salt and contaminants, and to enhance evaporative losses from the landscape.