Karst geology and isotope hydrology of the upstream section of Nam Hinboun River, Khammouan Province (Central Laos)

Abstract

The study area is located in the Khammouan Province of Central Laos, along the Nan Hinboun River, in the Hinboun District. The Khammouan Formation exposed in Central Laos hosts a variety of karstic landscape including limestone massifs that rise from the alluvial plains. The availability of groundwater in the Nam Hinboun region varies widely, largely due to the geologic complexity of the area. Extensive and productive karst aquifers occur in the Khammouan Formation of the upper Nam Hinboun river basin. The Khammouan Formation consisting of Carboniferous-age carbonate rocks exhibits little or no intercrystalline porosity. Groundwater flow occurs along solutionally enlarged fractures, cavities, joints, and bedding planes. Thirteen water samples were collected and analyzed for anions, cations, and oxygen and hydrogen isotopes from caves (dripping water), rainwater, streams, and sumps. The pH values range between 6.80 and 7.56 that is typical for cave waters. Specific conductance values range between 192.60 and 444.70 μS/cm, the total alkalinity as CaCO3 varies from 107 to 244 mg/l, whereas the values for the total hardness (as CaCO3) are between 58 and 266 mg/l. The water samples collected in the upstream section of Nam Hinboun River are depleted in both 18O and deuterium with a d-excess value below 10 (7.2 ‰). This suggests a long travel distance of monsoon air masses (high degree of rainout) and represents a typical example of predominantly SW circulation. The data set reported here only represents a local and time-limited snapshot of the water isotopic compositions. Further studies are required to confirm our preliminary results from Central Laos and a comprehensive regional study of the karst geology and isotope hydrology is highly desirable given the rapid economic development and population explosion of the SE Asian region.