The Origin of High Silica and Low pH in the Groundwater of the Quaternary Terrace Deposits in the Northern Khorat Basin in Northeastern Thailand

Abstract

The groundwater of the Khorat Plateau in Northeastern Thailand is used as a water resource due to its excellent quality. The region is underlain by an aquifer system consisting of two zones separated by a clay lens. The system is composed of unconsolidated sediments that contain different forms (crystalline, cryptocrystalline, and amorphous) of silica-bearing rocks and minerals. The sediments are overlain by highly weathered, laterite soils that allow for rapid recharge to the aquifer system, with subsequent lateral flow. Dissolved silica concentrations (10-68 mg/L SiO2) in some aquifer depths are higher than normally observed in natural environments (35 mg/L SiO2). The system also contains unusually low pH values, as low as 4.6, which may result from the formation of silicic acid resulting from silica dissolution and a lack of a buffering capacity in the system. To evaluate this hypothesis, the saturation indices of silica minerals were determined along a groundwater flow gradient, and modeling was conducted to observe how the water changes as it moves along the flow path. Saturation indices generally increased down gradient for crystalline, cryptocrystalline, and amorphous silica materials, demonstrating increasing dissolved silica concentrations. Different saturation index values for the three silica forms indicate the conditions for dissolution of amorphous silica and precipitation of crystalline silica existing in all locations down the flow path. The conditions for dissolution exist in upgradient wells and precipitation in mid- and downgradient wells for cryptocrystalline silica. This suggests the dissolution of amorphous and cryptocrystalline silica have contributed to the dissolved silica concentrations. Dissolved silica, alkalinity and pH trends indicate that as silica concentrations spike, pH values decrease suggesting the possible formation of silicic acid. Low alkalinity concentrations and pH values in some parts of the aquifer system suggest the lack of a buffer zone in these locations.