Salt accumulation and flushing in unsaturated fractures in an arid environment

Salt precipitation and dissolution in fractured chalk was investigated through flow experiments in both the laboratory and the field (in the Negev Desert, Israel). In the laboratory, a flow-cell experimental setup was used to simulate intermittent infiltration and drying periods along coated and uncoated fracture surfaces. Three infiltration events, lasting 24, 8, and 8 hours, were carried out with long drying periods of 82 and 44 days between them. In the field, two flow experiments were conducted through a single fracture. Water was percolated from land surface through a discrete fracture into a compartmental sampler. The duration of the two field experiments was 5 and 119 hours, respectively, with a drying period of seven months between them. The percolating outflows in both the laboratory and field experiments were collected and analyzed for electrical conductivity. The electrical conductivity of the outflows and its temporal variations during the experiments suggest that evaporation triggered capillary forces that mobilized water and solutes from the bulk matrix toward the fracture surface. As the water evaporated, the solutes precipitated on the fracture surface. The precipitated soluble salts were dissolved during the first few hours of the subsequent flow event that followed the drying period. This mechanism, enhanced in arid environments, may result in the transport of salts that accumulate in the upper few meters of the unsaturated zone to the ground water, bypassing the low-permeability matrix. The calculated amount of solutes transported by back-diffusion from the chalk matrix (190 g) could not account for the large (1200 g) amount of salts released from the fracture void during the experiments.