Salt accumulation and flushing in unsaturated fractures in an arid environment

Salt precipitation and dissolution in fractured chalk was investigated thro ugh flow experiments in both the laboratory and the field tin the Negev Des ert, Israel). In the laboratory, a flow-cell experimental setup was used to simulate intermittent infiltration and drying periods along coated and unc oated fracture surfaces. Three infiltration events, lasting 24, 8, and 8 ho urs, were carried out with long drying periods of 82 and 44 days between th em. In the field, two flow experiments were conducted through a single frac ture. Water was percolated from land surface through a discrete fracture in to 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 dur ing the experiments suggest that evaporation triggered capillary forces tha t mobilized water and solutes from the bulk matrix toward the fracture surf ace. As the water evaporated, the solutes precipitated on the fracture surf ace. The precipitated soluble salts were dissolved during the first few hou rs of the subsequent flow event that followed the drying period. This mecha nism, enhanced in arid environments, may result in the transport of salts t hat accumulate in the upper few meters of the unsaturated zone to the groun d water, bypassing the low-permeability matrix. The calculated amount of so lutes 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 v oid during the experiments.