Chemical denudation, or the uniform lowering of the karst surface, is a dominant karstification mechanism that is also widely recognized as the primary cause of the creation of surface and underground karst forms. That is why the topics of ionic runoff and chemical denudation are two of the classic problems of karstology. The rate of lowering of a karst surface due to bedrock dissolution is commonly referred to as the denudation rate [1
]. This can be expressed in several ways. It is most often expressed as the amount of material removed in mm over a period of 1000 years (also called Bubnoff unit, [4
]), or in m3
/year. The term ionic runoff [5
] has been used to denominate the amount of dissolved rock mass transported by waterways (not depending on the area).
Corbel’s equation was the first to be used to measure the degree of denudation in the karst [6
]. Since Corbel initially reported the issue of regional denudation distribution, many different field methods have been proposed to solve the problem. One of the direct methods is the calculation based on limestone plates or measurement using a micro-erosion meter (MEM). Indirect methods are, e.g., hydrochemical measurements, geomorphological research and recent methods using cosmogenic nuclides (mainly 36
]. Each of these methods has its advantages and disadvantages that limit its use. The denudation rate calculated from tablet measurements was several times (4–5) lower than the denudation rate obtained from the hydrochemical data. Using this method, precipitation was found to be the most important factor in the rate of denudation. Recently, this method was used by Plan [8
], who found that the denudation rates obtained from tablet measurements (10–30 mm/ka) are 2–8 times lower than those obtained from solute flow measurements. A micro-erosion meter (MEM) [9
] is used to directly measure surface reduction. Using MEM, it was found that in addition to climatic differences, denudation rates were also related to the slope and lithological properties of the rock [10
]. The degree of denudation can also be estimated from the height of pedestals (e.g., erratic boulders) protected from corrosion or emerging veins of “insoluble” rocks. However, a reliable estimate of their growth time is needed. Lauritzen [11
] presented a pedestal growth model taking into account condensation corrosion under boulders and the boulder shading effect, concluding that the total denudation rate should be 25–80% higher than the rate calculated directly from pedestal height and growth time. The latest methods are based on cosmogenic 36
Cl produced in calcite from 40
]. The main processes involved are Ca spallation and different muon reactions [7
]. Denudation continuously alters the shielding of the rock; therefore, the denudation rates can be estimated by the model of shielding history.
As ionic runoff is very closely related to the evaluation of the denudation rate, we present an extended overview of previous works. Among the most important works we can mention Corbel [6
], which dealt with factors affecting denudation and considers this a sign of the development of karst in different climatic regimes. Smith and Newson [13
] focused on the dissolution rate based on chemical and mechanical erosion. Priesnitz [14
] found a positive relationship between limestone solubility and yield, which was also confirmed by [5
]. Appelo and Postma [16
] drew attention to the issue of karst denudation in their monograph on the topic of groundwater geochemistry. Gabrovšek [17
] presented a simple mathematical model of the denudation rate in karst. The carbonate denudation rate was reconstructed in dependence on temperature and precipitation [18
]. In the Polish Sudetes, the intense karst denudation in a crystalline basin with small positions of marble was studied [19
]. The seasonal variability of chemical export rates was concluded and there was a positive correlation with surface runoff discharge in the Houzhai karst basin in the southwest of China [20
In a comprehensive work on the chemical denudation of karst [5
], the author presented the ionic runoff values from 14 localities in Europe and Asia and argued that ionic runoff is primarily influenced by mineralization and discharge in various climatic regions of the Earth and in various types of karst e.g., alpine, Mediterranean, karst basins, cold and tropical climate, etc. [21
From the neighboring Czech Republic, more studies are known [25
] which dealt with changes in the intensity of denudation in the Moravian Karst or investigated different dissolution rates for different karst rocks [27
In Slovak conditions, the monitoring of denudation and corrosion studies around the caves were performed [28
]. A pioneer in the research of denudation was Droppa [29
], who focused on the determination of the denudation rate mainly in the karst basins of the Low Tatras. Two studies [32
] have explored the weight loss of limestone tablets at two experimental sites in the Slovak Karst. Both have provided the most in-depth analysis to date of the karst denudation and the first results from the Slovak Karst area. Differences in the denudation rate may be caused by the influence of the content of the dolomitic component on the karstifying of limestones [34
The plateaus of the Slovak Karst are among the best developed karst areas in Europe, yet ionic runoff and karst denudation rates have not been explored in depth (and no results have been published so far). The area is located in the Dfb climate zone (cold, humid continental with the temperature of the coldest month below 0 °C), at the interface of the continental and oceanic climates, in terms of altitude at the border between the lowland and mountain climate [35
Denudation helps us to understand the rate of formation of karst forms and thus the development of karst areas. As it occurs in different areas or climatic zones, it is not possible to determine a global figure that would cover all karst areas of the world. Therefore, we would like to contribute to this overview with the missing new information about ionic runoff (and thus denudation) from the area of the Slovak Karst as an example of a plain in the temperate zone of Europe. These results allow comparison with other localities and thus contribute to the study of regional differences or similarities and, at the same time, to the influence of individual factors on the different degree of denudation.
Since the recharge areas of the individual springs of the Jasov Plateau have not yet been delineated, the most appropriate way to obtain more detailed data about this area is to determine the ionic runoff, which is a non-invasive method and is based on relatively simple measurements of spring discharge and electrical conductivity. The delineation of the catchment areas by dye tracing is not possible due to the highest degree of protection of the area and the use of one of the important springs as the main source of drinking water for the surrounding settlements and partly for the second largest city in Slovakia, Košice. That is the main reason why we chose this method.
This paper provides an overview of ionic runoff results based on regular sampling and study of karst water (one per month) from six springs of the Jasov Plateau in the Slovak Karst in the period November 2013–October 2016 (three hydrological years). We also included springs in the secondary calculations that were not analyzed regularly but by expedition, or data on them are known from the literature. The aim of this paper is to provide a comprehensive picture of the ionic runoff from the entire Jasov Plateau (65 km2) and the approximate rate of denudation based on the method of calculating ionic runoff, thus filling the information gap from a typical plateau karst area in the middle of Europe. As similar research on ionic runoff or karst denudation has not yet been conducted in this area, these are pilot results that can still be worked with.
The relationship between the discharge of individual springs and the ionic runoff for the whole period has been examined. These graphs show that there is a clear linear dependence of the ionic runoff on discharge (based on the formula for ionic runoff). However, in some months the values are above the trend line, and we observed the most significant ones at the source Skalistý potok. The spring months of 2014 were characterized by unusually high precipitation (extreme was May 2014, total 162 mm), which caused an increased amount of unsaturated water circulating in the Jasov Plateau massif, and the consequent increased ionic runoff not completely correlated with discharge (Figure 6
). During this period, it is one of the isolated cases where the precipitation affected the discharge almost immediately. Although the relationship between ionic runoff and discharge is directly expressed by the formula, on a more detailed view over the months, there are slight deviations from the trend line that express an increased or decreased effect of TDS on total ionic runoff (during higher or lower discharge).
Based on our measured data from six regularly monitored springs, the average value of ionic runoff for the Jasov Plateau in the Slovak Karst is 639.33 m3
/year. Drienovec main Spring is used for drinking water supply and therefore there is relatively exact information about its TDS and discharge from free available data (www.shmu.sk
, accessed on 1 April 2021). However, to make our results as comprehensive as possible, we decided to calculate the ionic runoff value based on data obtained during measurements in 2013 from other outlets in this area (total mineralization and average yield), giving us a relatively accurate view of the total ionic runoff from the Jasov Plateau. In our opinion, these complete results (measured and available data) are then comparable with existing data from other parts of the world. If we only took into account regularly measured results, we would probably include only half of the real ionic runoff. Thus, the total ionic runoff based on measured and recalculated data from the whole Jasov Plateau represents in the spring period of the year 2664.939 m3
/year (where periodic springs are also active). The measured and calculated values are listed in the following Table 1
presents the relationship between precipitation and average ionic runoff from all measured Jasov Plateau springs. It clearly shows the relationship towards the season (and thus evapotranspiration and the amount of water available for karst processes). In winter, when evapotranspiration is minimal, even the minimum amount of precipitation is reflected in the ionic runoff (e.g., 01–02/2015). During the growing season, a large single amount of precipitation is needed to manifest itself in the ionic runoff at least minimally (e.g., 05–06/2014). This shows how important the evapotranspiration is and thus the vegetation period and the type of vegetation cover for the level of the ionic runoff during the hydrological year.
The measured ionic runoff and spring discharge on the Jasov Plateau were compared to other European sites where previous study results were available [5
] by the same method (localities and their main characteristics are listed in Table 2
). These experimental sites were chosen mainly based on their location, altitude, average annual precipitation and temperature, and the character of the karst area (lowland, middle and high mountains). The ionic runoff is directly expressed through the discharge and mineralization of karst water. However, these variables enter the calculation with varying intensity. Based on Figure 8
a,b, it can be stated that ionic runoff is directly proportional to the discharge of karst springs (R2
= 0.8493) and the overall mineralization also plays an important role here (R2
= 0.4254), but it is secondary. The spring with the highest discharge is Ljubljanica (Notranski kras, Slovenia), with an average discharge of about 71 m3
and the highest ionic runoff (cannot be displayed in the graph, Table 2
). Sites such as Presles Plateau, Wiercica-Julianka, and Vichren (in the right part of Figure 8
a) are typical of high discharge of permanent springs situated here. This causes a significant obvious correlation between discharge and ionic runoff. In contrast, a weak correlation appears between IR and mineralization (Figure 8
In terms of the relationship between discharge and yearly ionic runoff from km2
(A/S), the Jasov Plateau reaches a value of 40.847 L·s−1
, which is just below the trend curve (Figure 9
). Locations above the trend lines (Massif Alek—Caucasus, Plateau Presles—France) are areas with the highest specific runoff from all analyzed springs (Presles—38.8 L·s−1
, Alek—65 L·s−1
, listed in Table 2
The value of the denudation rate for the Jasov Plateau is 40,847 mm/1000 years. Compared to previous research in the area of Slovakia—Low Tatras, the rate of denudation is slightly lower. Demänovská dolina (average annual rainfall 1250 mm) reaches denudation values of 46.8–55.3 mm/1000 years [29
], but it is an area of alpine karst. The area of the Moravian Karst (Czech Republic) reaches values of 6–25 mm/1000 [27
]. It can therefore be said that in the temperate zone with similar climatic conditions, our results are comparable.
The most important factors for ionic runoff based on the results and monitoring of the basic relationships are climatic ones. In the area of the Slovak Karst as a temperate zone, the decisive factor is the discharge of karst springs, which is significantly affected by precipitation (R2
= 0.8721, Figure 10
). The interval of the water residence time in the karst massif is affected by the structural, textural properties and the lithology of the bedrock and the karst conduit system [54
]. This area is built of high quality limestone (as we described in more detail in the geological characteristics of the area). The amount of precipitation in our territory is directly linked to the season, when the largest amount of precipitation in the Slovak Karst falls in the summer months (July–August) and is reflected in fluctuations in the discharge of springs. In addition, the maximum is reached in the spring during snow melting (March–April) and causes episodic floods at all springs in the area.
Based on the chemical composition of liquid precipitation from the territory of Slovakia [58
], we can state that the chemistry of precipitation does not affect the chemistry of groundwater in this area. The geographically closest station with such information is Milhostov (80 km from the Jasov Plateau, Table 3
The second most significant climatic influence is indirect average annual air temperature (Figure 10
), which is closely linked to evapotranspiration and growing season length. Since dense vegetation covers almost the entire plain, this has a significant impact on evapotranspiration. In the cold season, the land cover favorability to evaporation is very low. These facts are also confirmed by [56
]. Rainwater is retained in the vegetation and soil cover during the growing season, and only a small percentage subsidizes the amount of groundwater. As data from evapotranspiration are available closest to the city of Košice (40 km away and situated at an altitude of 200 m a.s.l.), we do not directly analyze this factor here.
From the point of view of climatic factors, the Jasov Plateau is closest in character to the nearby localities in Poland and Sniežnik, Krowiarki Mts. and Wiercica River (Figure 10
and Figure 11
). However, the results are closest to the area of Vračanska planina in Bulgaria, which has a similar altitude of 800 m a.s.l., precipitation of 800 m, but also spring discharge (0.55 m3
Carbon dioxide also has a very important role in rock dissolution [57
]. Understanding its dynamics and distribution in the subsurface atmosphere of carbonate karst massifs and dissolved CO2
provides important insights into dissolution and precipitation processes, and the role of karst systems in the global carbon cycle [59
]. Water is enriched with CO2
mainly in the soil [60
], and CO2
availability depends on the type of vegetation cover and the soil temperature. After [61
] winter and early spring waters have the greatest effect, due to minimal evapotranspiration and increased dissolution of CO2
in the water due to lower temperature.
Fluctuations in karst groundwater can be very different, and as a consequence different types of surface–groundwater interaction can occur [62
]. Some of the factors affecting ionic runoff (and denudation) have different weights in different karst areas. Figure 12
graphically depicts the factors influencing karst spring discharge and mineralization (and thus ionic discharge).
The ionic runoff method is suitable, especially in areas where it is not possible to precisely delineate the recharge area. It is a relatively simple method, and through data on discharge and mineralization, we can determine the amount of denuded material from such a karst basin, in our case from the entire karst area.
The area of Jasov Plateau in the Slovak Karst belongs to the classical and typical plateau karst areas with most spring outlets at the foothills. These have significantly fluctuating flow rates from 0 L/s in summer and autumn up to 192 L/s at the Teplica Spring on February 1, 2014. Episodic events during the snow melting and at the same time heavy rain in the spring of 2013 are also known, where the discharge at the Teplica and Drienovec springs reached more than 380 L/s. The total value of ionic runoff for this area, 40,847 m3/y.km2, is comparable with the Vračanska Plateau in Bulgaria, which lies at a similar altitude and with a similar amount of precipitation.
Based on the differences found during our regular measurements, seasonal differences, and in comparison with other European experimental sites, we consider important factors that are related to the climate zone and the character of the karst area. Discharge and its changes during the year are influenced by static factors such as karst rocks (structure, texture, and lithology) and epikarst character. Variable factors are precipitation, evapotranspiration, season, land cover, and land use. The mineralization is influenced by the chemical composition of karst rock and the variability in water chemistry is affected by the chemical composition of precipitation and ground water with fluctuation in CO2 (study of the precipitation chemical composition was not the aim of this study). These also appear to be some of the most important factors and depend on precipitation, soil seasonal changes, and temperature (air and soil), depending on the season.