Salinity Hazard Assessment of Paddy Soils in Thailand Using Innovative Ground Geophysics.
Jean-Pierre Montoroi Sr., Institut of Research for Development, 32 avenue Henri Varagnat, 93143 Bondy, France, Olivier Grünberger Sr., IRD - Land Development Dept, Phahonyothin Road, Chatuchak, 10900 Bangkok, Thailand, Nordine Bouzid Jr., Zonge Engineering and Research Organization, Inc., 3322 East Fort Lowell Road, Tucson, AZ 85716, Yannick Enet Jr., Paris XII Univ, Avenue du Général de Gaulle, 94000 Créteil, France, Jean-Luc Maeght Sr., IRD-LDD, Phahonyothin Road, Chatuchak, 10900 Bangkok, Thailand, Somsak Sukchan Sr., LDD-Office of Soil Survey, Phahonyothin Road, Chatuchak, 10900 Bangkok, Thailand, Kriengsak Srisuk Sr., GroundWater Research Center, Faculty of Technology, Khon Kaen Univ, 40002 Khon Kaen, Thailand, and Suwanchai Nadee Sr., GWRC, Khon Kaen Univ, 40002 Khon Kaen, Thailand.
Soil salinization in Northeast Thailand is caused by natural phenomenom (climate, rock salt deposit, saline groundwater) (Japakasetr and Workman 1981) as well as by human activities (wood cutting, water storage, groundwater pumping, etc) (Williamson et al., 1989). Salt-affected soils are formed as bare saline patches in lowland paddy soils under groundwater influence and are so called “Groundwater Associated Salinity” by Fitzpatrick (2005). High salinity level in rainfed ricefields depletes the annual crop yields (Yuvaniyama et al., 1996). Much research has been undertaken to survey salinity extent, to explain the causes and to recommend measures to be taken by the farmers. Recent works focused on the explanation of origin and how salts reaches the ground surface, namely based on hydrogeological and geochemical data (Srisuk, 1994; Imaizumi and al., 2001). In a given rice farming area of Isaan region (Khon Kaen province, Phra Yun district, Ban Daeng site), French organization IRD and Land Development Department (LDD) from Thai Ministry of Agriculture and Cooperatives has carried out since three years a local field experiment (LFE) based on water and solute transfer monitoring at short-time steps within two meters soil depth and on the salt-affected soil rehabilitation using improved cultural practices (Saejiew, 2003). Complementary data are provided using ground geophysics at different scales. At a regional scale, a saline soil survey is in process by LDD combining remote sensing and frequential domain electromagnetism (Geonics™, EM34). The paper summarizes our geophysical approach around the FLE to identify the spatial distribution of saline patches at small scales (soil pit, rice plot, watershed lowland), to calibrate the 2D soil cross-sections images using pedological, hydrogeochemical, geological and mineralogical data and to model paddy soil and water processes. In dry season, Time Domain ElectroMagnetic (TDEM) method was performed using a Temfast 32 apparatus on a 9 ha survey area (450 m x 200 m). TDEM data were collected in the center of a 25 m squarred loop using coincident transmitter and receiver loops. The 155 apparent electrical resistivity profiles were inversed and presented according to 9 vertical and 5 horizontal cross-section imageries. Frequential Domain ElectroMagnetic (FDEM) method was performed using a 1 m long EM38 (Geonics™) and a 0.60 m long CS60 (Paris University) on a 1.5 ha survey area and along soil transects. In rainy season, EM38 was sealed in a designed epoxy glass container to be used in flooded conditions. Vertical Electrical Soundings (VES) were carried out within the 1 m upper soil layer. Complementary geochemical and hydrological groundwater data were acquired on site using a 50 piezometer network. Four 20 m boreholes were rotary drilled to describe the deposit distribution and to further analyze the mineralogical composition. FDEM mapping show the salinity distribution in dry and rainy season. As the soil was water-saturated in rainy season, the electrical conductivity values are higher and the salinity distribution is maintained. VES data give the vertical soil salinity distribution in dry season with a salt accumulation in the 10 cm surface layer. TDEM mapping of saline patches allow to localize electrical resistivity anomalies. In the 9 ha survey area, a conductive volume, Southeast-Northwest oriented, is identified with a highly conductive anomaly roughly situated at a 10 m depth. The borehole descriptions are quite similar and show the following deposit distribution: (i) from the surface, a sandy loam layer, light ochre-beige coloured, with local iron oxide accumulation (oxidized zone); (ii) from a 3-4 m depth, a light grey-dark grey clayey layer, locally organic matter enriched (reduced zone); from a 6-7 m depth, a red fractured rock, with secondary minerals (bedrock). The hydrogeological modelling includes two groundwater systems: a deep saline groundwater flowing up through fractures and a shallow saline groundwater overlying a low-permeable clayey layer. Further studies are needed to know the geochemical relationships of these two groundwater systems. References: (1)Fitzpatrick, 2005. 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