Study of the Chemical Characteristics of Salt in Soil and Drainage Water Under Subsurface Drainage and Leaching Conditions

In arid regions, intensive phreatic evaporation plays a significant role in promoting soil salinisation. Subsurface drainage is widely used to mitigate this issue. However, most previous studies focused on total salt and neglected the varying transport capacities of individual components. Studying salt in soil and drainage water from the perspective of salt components and their chemical species is essential. In this study, field experiments under subsurface drainage were conducted, and the contents of major salt components were monitored. The PHREEQC 3.0 model was used to calculate the chemical species contents. Leaching efficiency was quantified via the leaching ratio and critical leaching depth. The results showed that Cl and Na presented the highest leaching efficiencies, which are low for Ca, as 90% of Ca existed in the form of a CaSO4<middle dot>2H(2)O precipitate. The leaching efficiencies of SO4 and Mg were sensitive to the irrigation quota, resulting from the high sensitivities of their main species, MgSO40 and SO42-, to the irrigation quota. The constitutions of the salt in the drainage water were closer to those in the groundwater because the drainage water mainly originates from the groundwater. This study provides guidance for determining appropriate design parameters for subsurface pipes and leaching quotas.