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ISSN : 1225-6692(Print)
ISSN : 2287-4518(Online)
Journal of the Korean earth science society Vol.43 No.4 pp.481-489
DOI : https://doi.org/10.5467/JKESS.2022.43.4.481

Review of Karst Research in the Republic of Korea

Han-Sun Ryu, Sangwook Park, Jin-Yong Lee, Heejung Kim*
Department of Geology, Kangwon National University, Chuncheon 24341, Republic of Korea
*Corresponding author: hydroqueen@kangwon.ac.kr Tel: +82-33-250-8560
June 29, 2022 August 10, 2022 August 14, 2022

Abstract


Various types of karst topographies are found worldwide. Owing to their global distribution, karst areas have been extensively studied by scientists who investigate new discoveries by linking the characteristics of karst topographies with their own research fields. However, there have been only a few studies on karsts in the Republic of Korea, and little research exists on their hydrogeology. Fragmentary studies have been conducted on the hydrochemical characteristics of groundwater in limestone areas, the causes of high arsenic concentrations in groundwater, and the hydraulic conductivity of limestone areas. Research on hydrogeological characterization and flow mechanisms in these areas has only began recently. Identification and the proper management of available groundwater resources in karst (limestone) areas is essential as their unique geological characteristics render it difficult to construct reservoirs or dams at appropriate scales. We have reviewed prior work on karsts in the Republic of Korea to provide information that supports water resource security in the karst areas, to improve the understanding of the equitable use of water resources, and to identify the best management practices for groundwater resource resilience improvement.



초록


    Introduction

    A continuous surface streams or thick soil usually do not develop in karst regions. Limestone and dolomite exposed at the surface is the most representative geology in karst areas. A groundwater system is developed by corrosion, and karst topographies are formed at the ground surface and underground (Kim, 2020). Many limestones can be observed in some areas of Gangwon-do (Samcheok, Yeongwol, Jeongseon, and Sokcho) and in the northern region of Chungcheongbuk-do (Danyang) in Republic of Korea (hereafter, South Korea) (Fig. 1). Unlike other areas dominated by lithologies with porous media, the underground media in the karst areas have different hydrogeological characteristics, such as the flow path and variability of the flow rate, which are difficult to identify. For these hydrogeologic conditions, water resource in karst areas is more sensitive or susceptible to contamination. Its management, thus, is harder due to difficulty associated with evaluation, prediction, and control of the water flow and consequent pollutants transport. The use of groundwater and mine developments in these karst areas can cause disasters, including sinkholes, and may exert substantial environmental impact. While basic research on groundwater resource evaluation and pollution reduction have been conducted in other geological areas in South Korea (basalt areas like Jeju Island and most of the inland areas), that on the karst areas (mainly in Yeongdong and Chungbuk regions) has been limited. Analytical research on the hydrogeological characteristics and flow networks in the karst areas with limited water resource alternatives is extremely important, especially in account of recent climate variability.

    The International Association of Hydrogeologists (IAH), which comprises of approximately 4,500 members globally, had specially organized the IAH Commission on Karst Hydrogeology for researchers, who are actively engaged in research activities and exchanges. The 45th IAH Congress took place in 2018 included recent research developments in karst hydrogeology, flow, and contaminant transport modeling in karst aquifers with special sessions on karst hydrology. Daly et al. (2002) presented in-depth research results on the effects of human activities on karst hydrogeology and proposed possible countermeasures. Mayaud et al. (2014) elucidated the water resources in Austrian karst areas, and investigated changes in the water flow rate and storage level in karst watersheds under future climate conditions. In contrast, research on karst groundwater in South Korea is limited. Fragmentary studies have been conducted on the hydrochemical characteristics of groundwater in limestone areas (Ryu et al., 2017;Ryu et al., 2019;Ryu et al., 2020;Jang et al., 2021), and the characteristics of the hydraulic conductivity distribution in limestone areas (Park et al., 2011). However, hydrogeological characterization and flow connectivity of these areas have been rarely researched (Kim et al., 2020;Ryu et al., 2020;Jang et al., 2021). Therefore, in this study, we reviewed previous work on four topics (hydrogeolgy, geograhy and geomorphology, geo-engineering and ecotourism) about the karst in South Korea to identify best management practices that support water resource security in the karst area, improve understanding of the sustainable use of water resources, and improve the groundwater resources resilience.

    Karst Research on Hydrogeology

    Yeongwol, Pyeongchang, and Samcheok areas in a previous study (Kang, 1992), comprise Paleozoic limestone bedrock and exhibit various characteristics of karst topography (Fig. 1). Kang and Yoo (2000) investigated the karst topographies that developed in some areas in Pyeongchang-gun in Gangwon-do. Kang (1994) explained that karst areas exhibit good drainage conditions, and various karst topographies have developed owing to the corrosion of limestones. Moreover, Oh (2007) reported that corrosion is more dominant in karst topographies than other areas; thus, suggesting corrosion as the major shaping factor. If groundwater flow rapidly through a karst conduit, Darcy’s Law, a representative theory in hydrogeology, is not applicable. Unlike other porous material groundwater systems in South Korea, karst areas exhibit complex movement paths in underground media that the evaluation and prediction on the transport of fluid and pollutants is not a simple task (Fig. 2;Oh, 2007). Kim (2004) reported that karst water was “water that continuously fill the earth’s cavities and is only affected by gravity and hydraulic pressure”. Hydrogeological analysis and methods are crucial in the research of karst topographies because water exhibits the highest effect on the formation of karst topographies.

    Kim (2004) classified karst areas according to the geological tectonic environment, depending on the outflow cycle of the karst spring. In addition, Kang (1994) investigated the shapes and depths of dolines and uvalas, and recorded the characteristics of red soils that were also present in these regions. The study findings demonstrated, 1) red clay residues occur in the areas, 2) red soils are polygenetic soils that do not match the modern bioclimatic environment, and thus, 3) the present bioclimatic conditions preserve the paleo-red soils (Fig. 2). Yoo (1993) claimed that the bedrock of the Gosu Cave is composed of Paleozoic limestone, and the cave and its components were formed through weathering, resulting from a combination of paleoclimatic and geochemical conditions. Furthermore, he explained that the formation direction of the cave coincides with the main directions of the surrounding bedding planes, and this cave formation process continues today.

    Song et al. (2011) conducted hydrogeological research using the results of prior geophysical experiments acquired through electrical resistivity tomography. Ryu et al. (2017) explained that karst topographies exhibit complex water systems (fast flow: flow in conduit or slow flow: flow in fault and joint) that are different from those of porous material groundwater systems (Ford and Williams, 2007). Furthermore, they conducted a preliminary study to investigate the flow process of groundwater systems through hydrochemical analysis. They conducted water quality measurements for pH, electrical conductivity, dissolved oxygen, oxidation-reduction potential, turbidity, and water temperature. Later, they studied the groundwater flow process of a karst topography in Samcheok through further in-depth research. Moreover, through field measurements, Ryu et al. (2019) analyzed field water quality data and oxygen-hydrogen isotopes and explained the hydrochemical properties of groundwater and river water in karst areas and the factors that cause such properties. They determined whether the streams in the study were interconnected using isotope analysis. Furthermore, Ryu et al. (2020) examined the effects of limestone mines in karst areas on groundwater flow by analyzing hydrochemical data and through mineralogical analysis. To investigate the hydrogeological complexity of the Samcheok karst area, Kim et al. (2020) examined hydraulic connectivity by introducing a microbiological methodology, whereas Jang et al. (2021) explained the complex hydraulic flow in karst areas using isotope data.

    Karst Research on Geography and Geomorphology

    Research on karst has been conducted more actively than hydrogeological research in the fields of geography and geomorphology. Several studies have investigated multiple unique karst topographies in specific research areas that are defined by scholars. Furthermore, some studies have analyzed the shaping factors of such karst topographies (Yoo, 1991;Kang, 1995;Kang, 1998;Kim and Seo, 2013;Tak and Son, 2014). Park (2011) classified the published karst research results of Korean geographers and divided the last five decades into early youth, late youth, maturity, and old stages. Park (2019) explained that the role of water is more critical in karst topographies than in other terrains, while demonstrating that the developed shape and process vary by climate, and that karst topographies have different characteristics according to the region and altitude. In addition, Park (2019) claimed that training experts in this field is very important because karst topographies are strongly related to human activities (groundwater and land use) and can cause ground subsidence disasters. Kang (1995) investigated the morphological and soil characteristics of mountain karst topographies developed at high altitudes that appear in Sanganmi-ri, Pyeongchang, and Gangwondo with the Jeongseon limestone as the bedrock (Fig. 1). He mainly classified the size and shape of dolines and reported the soil characteristics of the study area.

    Among the studies on geography and topography, we found many papers and reports on caves formed in karst topographies. Seo et al. (1986) described the existence of karst topographies in Mungyeong, Gyeongsangbuk-do and described the minerals found in several caves in this region. Moreover, in “A study of karst topography and limestone cave”, Kang (1997) listed the areas in North and South Korea where karst topography developed and explained its cyclical process. Furthermore, according to Kang (1997), the water that permeated into the bottom of dolines created caves by dissolving rocks, and explained other karst topographies formed by corrosion. Oh (2007) claimed that the morphology of calcite found inside the caves suggested changes in groundwater levels, at least during the Quaternary period and that the karst topography in South Korea has changed to an overburden alpine karst. Oh (2008a;2008b;2008c) insisted that the geological structure plays a key role in determining the stratigraphy and arrangement of karst topography. Oh (2008a;2008b;2008c) also claimed that because karsts are created by the influence of flowing water, the development of karst topography becomes more active if there exist geological structures, such as folds, faults, cracks, and joints, which are involved in water flow (Fig. 2). Besides, they explained that the sea level change in South Korea and the uplift of the Taebaek Mountains during the Quaternary period were the causes of the formation of various karst topographies on the surface. The folds influencing groundwater flow patterns that drive dissolution and cave formation, producing various multi-level and steep-slope caves. As most of the karst areas in South Korea are distributed in mountainous areas, they are also called alpine karsts. Lim et al. (2009) described karst topographies in South Korea and other countries and classified overseas karst world heritage sites according to topographic formation. They highlighted that continuous efforts are needed to ensure that natural landscapes with karst topography remain intact into the future.

    Karst Research on Geo-engineering

    As karst topographies are formed by the dissolution of bedrock by running water, the ground is highly unstable and vulnerable to sinkholes and subsidence, which can damage life and property. To avoid such damages, various geotechnical studies on stability have been conducted. Active geotechnical karst research began since the 2000s, which is later than that in other geotechnical fields.

    Song et al. (2003) performed geophysical explorations in an area with limestone bedrock and examined the applicability of various exploration methods. They discovered the structure of dolines in the study area using a proprietary software application (raw data setting: Autocad, FORTRAN, Visual Basic), and conducted geophysical explorations using electrical resistivity surveys, borehole imaging, and air-to-air tomography. They also analyzed that to determine the efficiency of geotechnical exploration methods. They highlighted the necessity of combined geophysical exploration technology rather than unit exploration technology using one ground property. Thus, they performed geophysical explorations using various methods after selecting a place possessing a history of subsidence in the study area. Through geophysical exploration (electrical resistivity surveys, borehole imaging, and air-to-air tomography), they more accurately identified distribution of the underground cavities by combining the exploration results. In addition, they evaluated more useful exploration method among the available methods (Table 1). Farooq et al. (2009) examined the utility of electrical resistivity exploration, and although the exploration advanced well, confirmed, in some anomaly zones, the filling of cavities with clay because of drilling (Kim et al., 2006). The exploration indicated that electrical resistivity exploration is useful in subsidence evaluation and recorded the degree of subsidence vulnerability of the ground near the study area.

    Song et al. (2011) conducted electrical resistivity tomography exploration to investigate the cause of sinkhole type subsidence that concured with groundwater level reduction, and to identify the distribution characteristics of cavities. In addition, they examined the hydrogeological characteristics and classified the limestones in the study area by the degree of crushing (cracked: 103-161, weathered: 218-277, fresh: 597-662 Ω-m), claiming that limestone cavities or fracture zones were observed approximately 10-20 m underground. Ahn and Sung (2017) explained that sinkholes are a type of karst topography caused by the dissolution of limestones and can cause damage to life and property. To construct a spatial database of sinkhole locations, data were collected from 279 sinkholes located in Samcheok and used to construct a spatial database. Subsequently, eight factors (slope angle, distance to caves, bedrock lithology, distance to faults, soil depth, soil drainage, distance to mines, and distance to traffic routes) affecting the sinkhole were selected. Based on the spatial database, the sinkhole sensitivity of the study area was analyzed using the nearest neighbor distance model and frequency ratio model. They aimed to identify and locate areas that were potentially vulnerable to sinkholes by using a ArcGIS 10.3 program. Furthermore, they selected areas with a particularly high sinkhole risk based on the evaluation results.

    Karst Research on Ecotourism

    Kim and Lee (2008) evaluated the tourism potential value of karst caves (Fig. 3a-c) in South Korea by assessing the associated factors like entrance fees using a questionnaire survey and the contingent valuation method. Bae and Jo (2017) explained that although paleokarsts show evidence of sea-level changes through the geological periods of the past, classifying them is not easy as it is difficult to recognize a paleokarst. This is because karsts below the surface are often formed along lithostratigraphic boundaries, which can be misunderstood as traces of modern karsts. In addition, the shape of a paleokarst is similar to the traces of pressure dissolution, and therefore, it is not easy to distinguish them. Moreover, it is difficult to specify when the dissolution action occurred. Thus, there is uncertainty in determining whether the discovered paleokarst topography was formed by the current or subsequent dissolution action. Despite associated difficulties with these research, paleokarsts are important, and hold research value in various geological fields for the following reasons: 1) they provide information on sea level changes and conclusive evidence of how long rocks were exposed to the surface. 2) paleokarsts, formed on a large scale underground, provide a space for useful minerals to accumulate or can function as a reservoir for petroleum. 3) contemporaneous paleokarsts that formed over vast areas are a key bed for comparing strata. Thus, there is a need for research on the paleokarsts formed previously in addition to the karsts that are forming.

    The bedrock of most karst topographies is composed of limestone. If the properties of the bedrocks change, the properties of the soils are also altered, thus affecting the vegetation and fauna properties that inhabit the area (Fig. 3d-f). Kim et al. (2011) investigated various insects that inhabit karst topographies. Bae et al. (2014) classified the major forest vegetation types in the study area and analyzed their properties. They found that forest vegetation was classified into five types and that the syngeographical characteristics were a combination of phytogeographic characteristics and characteristics of the habitat. Furthermore, they claimed that the vegetation in karst topography would help increase the diversity of habitat types on South Korea.

    Conclusions

    Limestone areas with developed karst topography are vulnerable to stable water supply, whicht is subject to change under the effects of climate change. Unlike most areas with porous material groundwater systems in South Korea, karst areas clearly show the simultaneous development of fast preferential flow and extremely slow diffuse flow of underground media. Consequently, it is extremely difficult to evaluate and predict the flow of fluids and the movement of pollutants. Due to the mining of limestone as well as excessive development and use of groundwater in these karst areas can cause disasters, such as sinkholes that could be greater in the karst areas than in the areas of other geological formations. This literature review on karst areas in South Korea can help disseminate results on the complex geological properties of karst areas based on the results of hydrogeological, geographical, topographical, environmental, geological, and geotechnical research. These previous studies can be utilized for understanding the preliminary research on karsts. Furthermore, the importance of research on hydrogeological characterization and the analysis of flow networks in karst areas with little water resource alternatives only increases as climate change deepens. In addition, continuous follow-up research should be conducted that can contribute to the acquisition of water resources and the improvement of managerial elasticity in mountainous and rural karst areas. The findings of this review could also be used as important indicators for field training related to complex geological properties and tourism resources in karst areas.

    Acknowledgments

    This research was funded by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (grant numbers 2019R1I1A2A01057002 and 2019R1A6A1A03033167). This study was supported by 2021 Research Grant from Kangwon National University. We appreciate Mr. Jun-Hong Min’s efforts for preparing the picture in Fig. 3.

    Figure

    JKESS-43-4-481_F1.gif

    Distribution of karst areas, limestone caves, and dolines (sinkhole) in South Korea (NGII, 2020).

    JKESS-43-4-481_F2.gif

    Schematic diagram of fluid flow in karst area (Hartmann et al., 2014).

    JKESS-43-4-481_F3.gif

    Tourist caves and fauna inhabiting caves in South Korea: (a) Cheongok cave, (b) Baengnyong cave, (c) Seongryu cave, (d) Plecotus auritus (bedrock: shale, limestone), (e) Antrokoreana gracilipes Verhoeff (bedrock: limestone, limestone silicate rock, sericite schist, quartzite), (f) Tomocerus (Plutomurus) gul Yosii (bedrock: limestone, dolomite).

    Table

    Scope, advantages, and disadvantages of geophysical exploration methods

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