Method for Improving Leaching Efficiency of Coastal Subsurface Drainage Systems

Abstract

Soil salinization is a major and long-standing environmental problem worldwide. Leaching through subsurface drainage systems under ponding conditions has been widely adopted to ameliorate saline soils in coastal areas with shallow groundwater tables. Because the seepage rate decreases by orders of magnitude from the near-drain area to midway between subsurface drains, to achieve a uniform leaching outcome requires a large amount of freshwater. This paper introduces a new method for improving the leaching efficiency by setting up a low-permeability soil layer over the subsurface drain. Numerical simulations of the water flow and solute transport in soils quantify the improved leaching efficiency provided by the new method. The results show that for the subsurface drainage system under the condition of complete and continuous ponding, an introduced low-permeability soil layer of a certain length can reduce the surface water infiltration rate near the subsurface drain but increase it in the midway area. This leads to a relatively uniform salt leaching condition in the upper soil layer. Based on the removal of salt over the top .6-m soil leaching depth, the new method is found to save water by 82 and 52% for homogeneous and layered (a silt loam layer overlying a sandy loam layer) soils, respectively. This study sheds light on physical processes in coastal subsurface drainage systems and provides guidance for future optimal designs of soil amelioration schemes.