Hydrological changes in the polder system   Chaiyanam Dissataporn and Pramote Yamclee   ABSTRACT        This paper considers a case study for assessment of salinity in the Northeast Thailand from January1995 -December1996 by polder system which composed of earth embankment and drainage channel (ring levee system) for controlling of groundwater and surface water as well as the salinity hazard inside the system.The salt-affected areas were studied using data combined from electromagnetic physical surveys and piezometric data from piezometer and staff gauge installed in the drainage channel.      The result of the electromagnetic conductivity meter survey indicated that the source of salt is in the polder area. The apparent electrical conductivity at the depth of 30 meter from soil surface was more than 160 mS/m. Groundwater flows from recharge area in the south of the study area with the electrical conductivity of 0.25-3.5 dS/m. The electrical conductivity of groundwater increases along the flow path. The electrical conductivity of groundwater in the polder system was 15-35 dS/m. The source of salt maybe come from salty groundwater of confined aquifer beneath the vicinity of the polder system.      The fluctuation of groundwater in the recharge area was in the range of 3-5 meters from the soil surface depending on the distribution of rainfall while the depth of groundwater in the polder system was nearly constantly in the range of 0.5-1 meter from soil surface. The respond of water in the drainage channel surrounding the polder system in term of salinity and water level was the same as groundwater in the polder system.      From the above results, it may be conclude that the process of soil salinization in the polder system is due to the upward movement of the salty groundwater to the depth that capillary fringe can reach the soil surface. The polder system consists of embankment and drainage channel can be implemented to control the salinity by manipulating groundwater level in the polder system through pumping of water in the drainage channel and leaching of soluble salt by rain fall.        Keywords : Polder, Saline soil