Characteristics of water and salt transport in subsurface pipes with geotextiles under salt dischargeconditions

Abstract: Soil salinization is an important environmental problem restricting agricultural development. Fresh water leaching combined with subsurface drainage is one of the most effective, fast and widely used methods for salt removal. However, some existing researches only considered the influence of subsurface pipes on drainage, salt discharge and water-salt distribution, there were few studies about the dynamic change of groundwater and the transport of soil water-salt under subsurface pipes drainage in Xinjiang. Moreover, many existing studies adopted sand gravels as filtering layer of subsurface pipes, there was obviously insufficient research on subsurface pipe technology using geotextile envelope as filter layer. This study designed an indoor drainage experiment of subsurface pipe with geotextile envelope, studied the relative position of subsurface pipe depth and groundwater depth at initial stage of drainage and the condition of water-salt discharge, the water-salt discharge effect of subsurface pipe and the dynamic transport law of water and salt in the soil profile during the whole process from the beginning of leaching to the end of drainage. The results showed that whether the initial state of soil is unsaturated or field-holding, the subsurface pipe would not begin to drain until the groundwater completely submerge subsurface pipe, at this time, the salt discharge was the largest, after that the drainage flow increased first and then decreased, the groundwater level also rose first and declined then, and the salt content of groundwater changed from accumulation to desalination with the increase of leaching water. Meanwhile, comparing the results of two leaching tests, the water-salt discharge effects of subsurface pipes in test 2 were better than those in test 1. When unsaturated soil was leached, the soil desalination rate gradually decreased with the increase of soil depth, the desalination rate of 0-20 cm soil (> 85%) was the largest, and decreased to the non-saline level. the soil desalination rate around subsurface pipe was relatively small (< 60%), and still at moderate saline level. In the horizontal direction, the soil desalinization rate the around subsurface pipe was relatively large, and the desalinization rate of 0-20 cm soil layer had little difference. The further away the 20-40 cm soil layer was from subsurface pipe, the smaller the soil desalinization rate was. Test 2 was carried out on the basis of test 1, the salinity change of 0-20 cm soil was small, and the desalinization rate of 20-40 cm soil layer increased (> 60%). In addition, the desalinization effect of test 1 was greater than that of salt drainage by subsurface pipe, which mainly discharged soil salt above subsurface pipe. For test 2, the salt discharge effect of the subsurface pipe was increased, not only removed salt from soil around subsurface pipe, but also removed salt from the soil and groundwater below subsurface pipe. With the increase of leaching water, the soil changed from desalting type to salt discharging type. Finally, this study showed that the application of subsurface pipe with geotextile envelope is affected by the relative position of groundwater depth and subsurface pipe depth, and affected by initial soil water content. Reasonable increase of leaching water volume can increase the water-salt discharge effect and soil desalination effect, effectively improve soil salinization. In addition, the specific applicable conditions of subsurface pipe with geotextile envelope, the influence of the relative position of subsurface pipe and groundwater on water-salt discharge effect, and the determination of reasonable leaching water amount under different initial soil water contents are all issues worthy of further research. This research results can provide theoretical support and scientific guidance for the popularization and application of subsurface pipe salt discharge technology under different groundwater depths in arid areas of northwest China.