Salinity migration of saturated media under density-driven conditions

In order to investigate the salt migration rule in one-dimensional saturated media under density-driven conditions, a density driven test of NaCl solution released from the top of soil column and a concentration dispersion test of NaCl solution released from the bottom of soil column were carried out respectively. Among them, the density driven test included two parts: the dynamic test of high concentration salt migration under density driven conditions and the salt migration characteristic test of different concentrations. The salt dispersion test included two parts: the dynamic process test of high concentration gradient dispersion and the dispersion characteristic test under different concentration gradients. The test device with a column made of perspex in a height of 115 cm and an inner diameter of 11 cm was established. The water inlet and outlet were respectively arranged at the bottom and top of the column, and the water isolation bottom plate was arranged at a position of 5 cm above the bottom. Holes were drilled every 10 cm from bottom to top on the side wall of the column and soil solution extractors were placed to extract soil solutions. The filling medium of the glass column was cleaned and air-dried quartz sand (0.5-1.0 mm). After filling, deionized water was used to saturate the soil column from bottom to top. The results showed that the effect of density-driven on salt migration was much greater than that of concentration dispersion during the test period. Driven by density, the salt released from the top quickly migrated downward, and the salt concentration in the upper layer was greater than that at the bottom within 5-120 min. After 330 min, the concentration increased with the increase of depth, and the salt gradually accumulated towards the bottom. The bottom concentration reached the maximum after 720 min. In addition, during the whole migration process, the peak of salt concentration moved downward with the increase of time, the peak value decreased from 54.3 g/L at the beginning of the experiment (after 5 min) to 24.2 g/L at the end of 21 600 min. However, the change of salt concentration by dispersion was slow, and only the concentration at the within 20 cm of the bottom of the soil column changed significantly within 30 days. The law of salt migration was different for different concentrations of salt solutions. The higher the concentration released from the top, the more obvious the density driving effect was, and the salt content in the depth range of 70-100 cm at the bottom of soil column was higher. After 15 days, the proportion of salt content at the bottom of 150, 100, 60, 40, 12 and 6 g/L decreased successively, and were 47.1%, 40.2%, 40.0%, 38.0%, 36.7% and 33.3%, respectively. The higher the concentration of salt released from the bottom, the greater the upward diffusion flux, but the effect on the redistribution of concentration was not obvious. After 15 days of testing, the salt released at concentrations of 100, 60, and 40 g/L was mainly distributed at depths of 80-100 cm, and the salt concentration at other depths was low, while the salt concentration at concentrations of 6 g/L gradually decreased with depth decreasing, which was similar to the distribution rule driven by positive density. The salt content at the within 20 cm of soil column of NaCl solution with concentrations of 100, 60, 40 and 6 g/L decreased successively, were 82.7%, 72.7%, 62.4% and 37.5%, respectively. Based on four sets of laboratory one-dimensional saturated soil column tests, this study quantified the density driven salt migration rule and the dispersion dynamic rule under different concentration gradients of salt solutions, which can provide theoretical basis for salt balance research in arid irrigation area.