Interpretation of the chemical analyses of the ground water of the Khorat Plateau, Thailand

Abstract

The Khorat Plateau of Northeastern Thailand is a shallow saucer-shaped basin tilted slightly to the southeast. It is divided into three structural provinces with the Phu Phan folds at the center and the Sakon Nakhon and Khorat Basins on the two flanks of the folds. The plateau is mainly underlain by Triassic sandstone and shale of the Khorat Series, Permia.n limestone of the Rat Bun Series, and pre-Permian shale, sandstone, phyllite, slate, and quartzite of the Kanchana Bun Series. It is also overlain by shale and siltstone, with interbedded rock salt and gypsum of Jurassic and younger age. Alluvium and terrace deposits occur mainly along the courses of the Mun, Clii, and Mae Khong Rivers and in limestone terrane. Ground water occurs within five aquifers: alluvium, upper shale and siltstone, Phu Phan sandstone, Phra Wihan and Phu Kadung sandstones, and limestone. The ground-water system is recharged by the tropical monsoon rainfall with an annual average of about 1, 330 millimeters. The chemical quality of the ground water varies considerably. The major constituents in water of the alluvial aquifer are calcium, sodium, iron, chloride, and bicarbonate, which give hardness properties to water. The water has no salinity or alkali hazard for agriculture, however. The sources of chemical constituents are mainly from clay minerals in the aquifer and contamination with mineralized water from both the lower aquifer and leached zones0 The shale and siltstone aquifer produces water of most inferior character, among which sodium chloride, calcium sulfate, and calcium bicarbonate types are common. The principal sources of these chemical constituents are rock salts and gypsum layers existing within the aquifer. Percolating water carrying salts which have been laid down on the ground surface through capillary action and evaporation is also considered as another source. Water in the Phu Phan sandstone and Phra Wihan and Phu Kadung sandstones is generally similar in properties and character. The major dissociated ions are iron, calcium, and bicarbonate, with sodium, chloride, and sulfate in places. The principal sources are impurities of cementing materials of sandstone, residual salts in interbedded shale, and contamination by mineralized waters from overlying aquifers and leached zones. The property and character of water are satisfactory for most purposes. The limestone aquifer is free from sodium and chloride, but contains high concentrations of calcium, magnesium, sulfate, and bicarbonate, of which the sources are dolomitic limestone and gypsum. These chemical constituents are usually responsible for carbonate hardness at shallow depths and noncarbonate hardness at greater depths. The water also has salinity hazards for agricultural uses0 For ground-water development, depths of wells within the shale and siltstone aquifer should be limited to not more than 300 feet, because the salty water mostly occurs in the deeper zones. In limestone, the well should not be more than 200 feet deep, due to the occurrence of highly saline water beyond the specified depth0 In other aquifers the location and depth of wells are not critical.