Karst hydrogeological observations in Tra Linh and Quang Uyen districts, Cao Bang province, Vietnam

Abstract

The Tra Linh and Quang Uyen Districts are located in Cao Bang Province, Northeastern Vietnam, where abundant rainfall combined with considerable soluble limestones of Devonian, Carboniferous, and Permian ages support a spectacular tower karst landscape. This paper is a comprehensive study based on geologic, tectonic, and hydrogeological observations in the Tra Linh and Quang Uyen Districts, Cao Bang Province, Vietnam, enhanced with water-quality samples collected from 31 sites. Each location was sampled once, in March 2012 or February 2014, except Hang Ban Lang (HQ-31) and Hang Ban Nua (HQ-32) springs, which were sampled twice to determine variability in water quality. The water samples were analyzed for physical proprieties and chemical and isotopic composition. To better understand the impact of the lithologic factor to the development of the karst landscape in the study area, two rock samples were collected and analyzed during the 2014 trip. Four additional rock samples were collected for comparison from areas located in similar humid subtropical climate as the study area; three from Thailand and one from Alabama, USA. The data and analyses presented herein are performed for the first time in the study area. Results showed that groundwater quality met US drinking-water standards, with only manganese exceeded secondary maximum contaminant levels (MCLs) in seven samples. Specific conductance values for the water samples ranged from 153.2 to 421.6 µS/cm. The pH values ranged from 6.51 to 8.29, total hardness (as CaCO3) was between 85 and 326 mg/L, the total alkalinity as CaCO3 varied from 68 to 207 mg/L, and carbon dioxide varied between 32.4 and 123.4 mg/L. The water chemistry is dominated by calcium. The highest concentrations of calcite were recorded in the rock samples collected in Vietnam. The water samples collected in the Cao Bang Province are depleted in both 18O and deuterium with a d-excess value above ~ 10‰ and have a median value of 13.16‰. Some samples exhibited lesser (three samples with values under 10; a sinking stream, a karst window, and a lake) or higher deuterium excess values due to local variations in humidity, temperature, and wind speed. 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 Cao Bang Province.