Karst regions are not only valuable in terms of landscape, tourism, and culture but also offer a variety of natural resources such as freshwater and biodiversity. In particular, groundwater from karst aquifers has proven to be the major freshwater source for drinking-water supply and agricultural irrigation in many countries and regions throughout the world. In tropical and subtropical areas, like in Southeast Asia, the special climate conditions with dry and humid-rainy seasons have created numerous typical tropical karst landforms, but also hamper a continuous freshwater supply. In highland districts of Ha Giang province which is located in the Northern part of Vietnam, although the total surface water discharge is high, the large variations in rainfall, as well as discharge between dry and wet seasons, result in seasonal water scarcity. Since 2010, when Dong Van Karst Plateau was designed as a UNESCO Global Geopark, regional economic development, the number of visitors has strongly increased which has positively contributed to changing the lives of the local people, but also enhances the pressure on the already tensed water supply situation. ... mehr
In order to overcome this issue, plenty of solutions have been applied to address water scarcity in the dry season, but this is not effective or sustainable due to high costs, easy damage, and easily polluted water sources. Therefore, adapted water supply strategies such as the KaWaTech project model has developed and implemented to meet the increasing water demand. However, these areas belong to the small catchment which is located in the northeastern part of the geopark and consist of favorable geological conditions for groundwater availability. Meanwhile, the karst plateau south of Dong Van town is still facing serious water scarcity. In this context, studying and understanding the processes that take place in the Dong Van karst systems is a key basis for developing adequate solutions for the protection, management and sustainable use.
Extensive field studies combined with analysis of the huge cave survey database was carried out and resulted in a karstification model which is presented in Chapter 2 of this thesis. Accordingly, the cave classification based on the cave conduit's geometric parameters indicated that the karst evolution in Dong Van Karst Plateau is in a youthful stage, with cave systems developed mainly of the isometric form (developed almost vertically) in the vadose zone, and not yet deepened to the local base level. The development of horizontal cave passages is related to two levels of planation surfaces at 1000-1250 and 1250-1450 masl. Meanwhile, at an altitude of 240 masl, the Na Luong cave which belongs to the Na De spring system could be the local base level for karst erosion. Additionally, cave passage orientation shows that the cave system formed and developed under the influence of tectonic activities in the Cenozoic.
The second study of this thesis (Chapter 3) focuses on the approach of using concentrations of major ions, trace and rare earth elements to clarify chemical signatures, and the ability for connections among sampling areas in the study area. To this end, a combination of different methods was applied, including mapping, hydrochemical analyses, using trace elements as the natural tracers, and multivariate statistical. It results in a basic hydrogeologic conceptual model for the Dong Van karst aquifer system. Accordingly, the Na De spring system and Na Luong cave are identified as belonging to the same discharge zone of Bac Son Carboniferous karst aquifer, while the southwestern springs and Tia Sang cave were identified as typical of shallow groundwater movement in the weathered surface zone in non-karst formations. In addition, it is possible to show a connection between the mining area and Na Luong cave by the abnormally high arsenic concentrations combined with location and geologic data. Meanwhile, Hang Rong cave has the features of a carbonate reservoir that developed in Triassic siliceous limestones of the Hong Ngai Formation, but is mainly associated with the Bac Son karst aquifer. Notwithstanding the significant differences in the water-chemistry signatures, the commonalities in REE distribution patterns between Tia Sang cave and the Na De spring system, combined with geologic data, lead to speculation that it could be one of the recharge sources of the Na De discharge zone.
The results presented in this thesis have essentially clarified the development of karst and caves, as well as the hydrogeochemical processes and connectivity ability of karst aquifers on the Dong Van Karst Plateau. Although there are still some incompletions, the results obtained are rather positive, which provides the basis for future detailed hydrogeologic studies in this area.
1