•Advanced application of time series analysis in complex karst systems.•Wide spatial distribution of monitoring points and high temporal data resolution.•Cross-correlation analysis not sufficient to ...study the effects of allogenic recharge.•An innovative approach using partial cross-correlation analysis (PCCF) is presented.•PCCF enables separation of the effects of allogenic and autogenic recharge.
Karst aquifers have very heterogeneous structure and complex hydrodynamics through conduits, fractures and matrix, therefore characteristics of groundwater flow and solute transport differ from those in intergranular and fissured aquifers. Appropriately adapted techniques of hydrogeological research are required for understanding of their functioning. This paper presents an advanced application of time series analysis in a binary karst aquifer known for its complex hydrodynamics and mixing of water from various sources of recharge. A classical approach with high temporal data resolution was extended to a spatial domain with simultaneous monitoring of precipitation, sinking streams, water flow in cave systems, and springs. The main objectives of this study were to define and compare the flow characteristics and storage capacity of selected springs and their catchments, and to determine the influence of different types of recharge (autogenic versus allogenic). The time series of recharge were analysed as input functions in the correlation and spectral analysis. The results undoubtedly show differences in water transfer through the system, storage capacity, and recharge characteristics. However, they also highlight the limitations of using cross-correlation functions to distinguish the influence of autogenic and allogenic recharge. Due to their interference, the interpretation of the results in complex karst systems can be ambiguous, therefore a method of partial cross-correlation analysis was additionally used. Although it has been used before in karst aquifers to spatially characterise groundwater circulation and autogenic recharge, in this study it was used for the first time to investigate the mutual influences of allogenic and autogenic recharge. The results confirmed that this approach provides additional insight into the functioning of binary karst aquifers.
•Studied karst systems are hydraulically linked to the hosting karst aquifers.•The greatest event sensitivity observed during autumn and snowless winter periods.•A typical water year with a snowmelt ...spring flood showed ambiguous CCF response.•Flat topography and shallow vadose zone contribute to complex baseflow interaction.•Matrix-conduit gradient inversion occurs during high flow events.
In Estonia, karst aquifers supply roughly a third of annually abstracted domestic groundwater. Karst systems recharged by allogenic humic-rich streams originating from mire-dominated watersheds are common in Lower Estonia (lowlands). This study aimed to characterize the hydrology and hydraulics of two binary lowland karst systems: Salajõe (SKS) and Tuhala karst system (TKS), and their interaction with the hosting karst aquifer. High-resolution water level, discharge, specific conductance time series data combined with tracer test results were analyzed to assess the two systems. The temporal effect of evapotranspiration, snow deposition and snowmelt floods contributed to the variability in recharge event sensitivity of studied watersheds and aquifers, resulting in time-varying CCF strength between precipitation and discharge. The greatest event sensitivity was observed during the autumn and snowless winter seasons. The celerity and solute transport were the greatest in the preferential flow paths of both karst systems, however, solute transport characteristics varied significantly because of differences in structure and maturity of karst porosity. While the dilute allogenic recharge flushed the karst systems occasionally, the contribution of high-SEC groundwater baseflow was significant. During flood events, the matrix-conduit hydraulic gradient could be inverted, resulting in dilute floodwater being pushed into the hosting aquifer from the karst system.
Water suppliers should provide safe drinking water following preventive measures. This is especially important for karst water sources, as they are among the most vulnerable. Recently, there has been ...a strong focus on the early warning system, which mainly involves monitoring proxy parameters, but does not consider drainage area conditions and other monitoring recommendations. Here, we present an innovative strategy for assessing contamination risk of karst water sources that covers spatio-temporal dimensions and can be integrated into management practices. It is based on event-based monitoring and risk mapping and has been tested in a well-known study area. The holistic early warning system provides accurate spatial hazard and risk assessment and operational monitoring guidelines, including locations, indicator parameters, and temporal resolution and duration. In the study area, the high contamination risk, representing 0.5 % of the area, was spatially delineated. The highest probability of source contamination occurs during recharge events when proxy parameters such as bacteria, ATP, Cl, and Ca/Mg ratio should be monitored in addition to continuous monitoring of turbidity, EC, and T. Monitoring of sinking streams should serve as a preventive measure, since water transfer from ponors to springs has been shown to take about one day, and poor quality water is present for at least another day. Therefore, intensive monitoring should be conducted at intervals of a few hours for at least a week. Although hydrologic systems vary, the proposed strategy is particularly useful in systems where water flows rapidly and where remediation is not feasible.
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•A comprehensive early warning system for water source contamination risk is presented.•It includes spatiotemporal dimension that can be integrated into management practices.•The EWS consists of event-based monitoring and contamination risk mapping.•It provides guidance for contamination risk assessment and operational monitoring.•Monitoring locations, parameters, temporal resolution, and duration are proposed.
Microclimatic monitoring (air T, air pressure, CO2, ventilation, humidity, methane, and radon) in selected show caves in Slovenia has been a continuous process for more than 10 years, a process that ...aims to supervise the use of the caves for tourism in the sense of sustainable environmental management. After the cataclysmic eruption of the Hunga Tonga–Hunga Ha’apai (HTHH) volcano on 15 January 2022, global propagation of ionospheric disturbances was reported worldwide as barometric pressure changes and seismic noise events. Weather stations in Slovenia reported 2–4 hPa changes in atmospheric pressure 16 h after the eruption at 20:30 CET (19:30 UTC). Changes in atmospheric pressure were also detected at 15 air monitoring sites in 3 different caves (20–120 m below the surface), at 8 water monitoring sites in 4 different caves (1–10 m below the water surface), and on the surface (4 air and 2 water monitoring sites), where we identified a small but significant increase in atmospheric pressure of <1 hPa, with the highest signal at 21:00 CET (20:00 UTC). At some cave monitoring locations, air T fell during this global event induced by a far-field volcanic eruption. Cave CO2, methane, and radon measurements did not show significant changes related to the eruption. This is the first evidence of atmospheric pressure changes due to the HTHH volcano eruption in karst caves and waters.
•We list parameters assumed to control flooding in poljes.•We test them with simple models.•We combine the findings to model a real field case.•Two modelling approaches are used.•The method can be ...applied to other poljes.
Poljes are large flat closed depressions that can be found in most karst areas around the world. They are prone to regular flooding, which occurs after abundant rainfall or snowmelt events. The floods can form several meters deep intermittent lakes that can persist for months. Human settlements around poljes have been traditionally positioned above the level of the most severe floods. However, many neighbouring villages expanded during the last decades to areas that might be endangered by large flood events. In addition, a recent study has shown that several poljes encountered floods of more severe amplitude in mid-Holocene due to a prevailing humid climate. As current knowledge of polje’s hydrology appears to be still insufficient, further research is needed. This work aims to identify the factors controlling polje flooding and evaluates their impact on the flood amplitude and duration. To do so, a list of parameters potentially controlling flooding is established. The relevance of each parameter is tested by using two idealized numerical models. The first one considers exclusively the polje from the springs to the ponors, whereas the second integrates the aquifer adjacent to the polje outflow side. First results show the usefulness of simple numerical models to identify the effect of each parameter on the flood amplitude and duration. The findings are combined to simulate a flood of medium intensity and long duration that occurred on a typical polje. A comparison of the results with real data demonstrates that the aquifer draining the polje needs to be considered to understand properly the flooding dynamics. The approach presented within this work has the potential to be used in other poljes encountering similar hydrological situations, but also lays foundation for flooding modelling in other karst areas.
Karst aquifers are abundant, but vulnerable water resources. Therefore, a deeper understanding of possible mechanisms that determine the properties of karst springs is crucial. In this work, we ...present an example of Unica Spring and Malni Spring, the two main outlets of a large karst system in the Notranjska karst region, Slovenia. Although the two springs share same catchment area, the flow distribution between them shows an interesting behaviour: At low-flow conditions, Malni Spring is the main outlet, while Unica spring receives almost no water. During high water events, discharge of Malni Spring stays limited and Unica Spring becomes the main outlet. We relate these observations to the local geometry of the channels and breakdowns in the remote part of the Planinska Jama (Planina Cave), called Mysterious Lake. There, waters from Rakov Škocjan and Javorniki aquifer merge and further diverge to both springs. At low water conditions, the outflow towards the Unica Spring is restricted by the breakdown, so that most of the inflow is directed towards the Malni Spring. With increasing recharge, the level in Mysterious Lake rises until the water starts to flow over the breakdown along a system of large channels (Rak Branch of Planinska Jama) to the Unica Spring. The breakdown level keeps the hydraulic head and the flow towards Malni Spring limited. To verify this scenario, a hydraulic conduit model was made based on the known and predicted channels, and inflows calculated from the historical data of discharge measurements at related springs and ponors. An inversion procedure was used to obtain a satisfactory fit to the observed discharge data and to constrain the selected model parameters. The model accurately reproduced the observed discharge behaviour under low- andhigh-flow conditions.
The aim of the present study is to characterize and improve the still insufficient knowledge of the recharge processes that have an important influence on the flow and solute transport in karst ...aquifers and thus also on the quantity and quality of karst water sources. A binary karst aquifer in the recharge area of the Malenščica and Unica springs, which covers an area of about 820 km2 in south-western Slovenia, was selected as the study area. Previous research and knowledge of the system enabled efficient planning of the measurements, and the results of the data processing were checked and evaluated using some already known methods and others newly introduced for this study area.A dense monitoring network was established at 20 observation points (six springs, four ponors, seven water-active caves and three surface streams) for simultaneous monitoring of the hydrological characteristics and the physicochemical properties of the water, the so-called natural tracers. Data-loggers were installed to measure water pressure, temperature and conductivity. During selected storm events, samples were taken for chemical and microbiological analyses and discharge measurements were carried out. The meteorological and hydrological data of the national monitoring network of the Slovenian Environment Agency complemented the extensive dataset.Data collected during the observation period of two hydrological years 2016-2018 allowed the analysis and comparison of the spatial and temporal variations of the natural tracers under different hydrological conditions. Frequent discharge measurements allowed the generation of rating curves and proved to be a crucial element for understanding the hydraulic processes that determine the functioning of this system. The calculation of the water budget allowed an assessment of the proportion of autogenic and allogenic recharge of the springs and a quantitative estimate of autogenic recharge under different hydrological conditions. A large share of autogenic recharge was confirmed in the period of low-flow, when most of the water flows out of the Malenščica spring.The hydrological analysis, i.e. the flow duration curve, the hydrograph separation techniques and the recession analysis, revealed that the Malenščica spring has a higher storage capacity, a greater proportion of autogenic recharge, especially at low-flow, and a slower recession than the Unica spring. This was also confirmed by correlation and spectral analyses, which were also used to investigate the relationships between discharges at ponors and springs. However, the results of the cross-correlation analysis showed hardly any difference between the two springs and in this case proved to be unsuitable for studying the influence of allogenic recharge. Instead, an innovative approach of partial cross-correlation analysis was used to control the input parameters of effective precipitation and discharge of one of the sinking streams to determine the contribution of the other sinking stream to the observed spring. The results confirmed differences in allogenic recharge of the Unica and Malenščica springs.Hysteresis analysis has been applied as a complementary method to time series analysis and represents an improved approach to the characterization of the karst hydrological system. In our innovative approach, both the hydraulic and physicochemical responses of the springs were investigated. The former is based on the principle that the relationship between discharges at ponors and the discharges of springs often follows a certain hysteretic behaviour, highlighting the successive activation of different hydrological components during the recharge period and different recession patterns. In the latter case, hysteretic relationships between discharges and physicochemical parameters were analysed at different locations in the karst aquifer to characterise the mixing of groundwater originating from different recharge sources and moving through both conduit and matrix pathways.The hydraulic approach to the construction of hysteresis enabled a detailed analysis of allogenic and autogenic water interaction and its influence on the Malenščica and Unica springs under different hydrological conditions. Narrow shapes of the hysteresis indicate a direct hydraulic connection between the ponor and the spring and thus a well-developed drainage system. Any deviation towards a convex or concave shape indicates a less developed, more matrix-related drainage system or the influence of other recharge sources.Analysis of physicochemical hysteretic function of individual locations confirmed the differences in the recharge characteristics of the two springs. Compared to the Unica spring, the Malenščica spring has specific recharge characteristics that result in lower vulnerability to the effects of the sinking streams. A greater proportion of autogenic recharge in the initial phase of the storm event is important, as it allows for a time delay of the possible negative effects of the sinking stream. However, possible pollution from the area of autogenic recharge can have strong negative effects, as in this initial phase with low discharges the dilution effect is negligible. Based on the results, some suggestions for monitoring and protection planning were given.
The area of the Classical Karst is roughly defined by a triangle with Ljubljana, Trieste and Rijeka as its vertices. This is the area where the first scientific studies of karst phenomena were ...conducted. Two sub-regions that particularly attracted researchers are presented. Kras/Carso plateau with the Škocjan caves and the underground course of the Reka river. The groundwater flow of Reka-Timavo is characterised by high recharge variability of allogenic inflow of Reka River and flow restrictions in the upper part of subterranean flow, which control regional backfloodings observed in cave systems. The recharge area of Ljubljanica Springs is known for a cascading series of poljes in intermediate cave systems. The area has been in focus of hydrological studies for over a century, but many phenomena have been resolved in the last decade based on results of continuous autonomous monitoring in the last decade.
Key words: Classical Karst, Kras, Škocjanske Jame, Reka-Timavo system, Ljubljanica Recharge Area, Polje.
IZVLEČEK
Območje Klasičnega krasa v grobem objema trikotnik z Ljubljano, Reko in Trstom v ogliščih. Tu se je začelo znanstveno proučevanje krasa. Dve kraški območji sta tu še posebej pritegnili pozornost raziskovalcev. Prvo je planota Kras s Škocjanskimi jamami in podzemnim tokom Reke med Škocjanskimi jamami in izviri Timave. Ta tok močno zaznamuje velika spremenljivost dotoka reke Reke in lokalne zožitve v vodonosniku, ki povzročajo regionalno poplavljanje, kot ga beležimo v jamah. Drugo je območje kraške Ljubljanice z značilnim nizom dinarskih kraških polj in jamskih sistemov, ki polja hidrološko povezujejo. Območje je že več kot stoletje predmet številnih raziskav, zvezno spremljanje parametrov toka v kraških jamah v zadnjih desetih letih, pa je omogočilo nova spoznanja o lastnostih in mehanizmih pretakanja vode v celotnem sistemu.
Ključne besede: Klasični kras, Kras, Škocjanske jame, podzemni tok reke Reke, kraško zaledje izvirov Ljubljanice, polje.
Poljes are flat closed karst depressions prone to regular flooding. The floods can be several meters high, last for months and damage significantly human infrastructures. To predict the maximum level ...reached, the polje water balance needs to be implemented. This technique encounters the difficulty that important part of the inflow and outflow flowing through many poljes is ungauged, as it is challenging to measure accurately the numerous springs and ponors activating temporarily with the rise of water level. This work aims to see whether this problem can be handled and the polje water balance reconstituted. To do so, a typical Dinaric polje is equipped with several water level stations installed over its surface and in the nearby water active caves. Combining a 1*1m digital elevation model of the polje surface with water levels and inflow records of the main two springs allowed assessing the variation of flooded volume and reconstructing the water balance. The highest total inflow values reached during the observed period were of about 140-150 m3/s, with up to a third of it being ungauged. In addition, the effect of a large estavelles group on the polje inflow and outflow could be identified, and helped to characterize the outflow, with values comprised between 65 and 75 m3/s. Finally, intense rainfall over the polje flooded surface showed to be a temporary important source of inflow. The values found by the water balance analysis have been used as input and calibration data in a numerical model reproducing the flood dynamics in the polje and its surrounding aquifer. Results validated both polje water balance and conceptual hydrogeological model. They justify the significance of combining water level measurements with a digital elevation model to monitor the floods. The method can be applied to other poljes flooding in a complex way of superposed input and output signals. Finally, the places to be equipped in priority if the polje has no measurement network or if available funding is limited are discussed.
Various multivariate statistical techniques (MST) can provide valuable insights into water quality variability. Despite numerous studies in which these methods have been used, their potential has not ...been fully exploited. This paper presents an improved approach to better understand the hydrodynamics of karst systems. The integrated application of hierarchical cluster and principal component analysis in combination with factor analysis allowed the construction of an advanced multivariate chemograph. The analytical procedure was applied in a binary karst aquifer known for its complex hydrodynamics and mixing of water with similar hydrochemical composition. In addition, the study area provides access to an integral groundwater flow system (ponor‐cave‐spring) and offers extensive prior hydrogeological knowledge. The approach allowed reduction and discrimination of the main parameters affecting water quality characteristics. Their identification enabled recognition of three predominant recharge components: (a) stored water impact with Cl and electrical conductivity, (b) sinking stream impact with turbidity and bacteria composition and (c) karst aquifer impact with Ca/Mg ratio as principal parameters. The results supported innovative characterization of the dominant processes and isolation of temporal hydrodynamic phases of individual monitoring points within the aquifer system. On this basis, a spatio‐temporal conceptual model was developed and the hydrodynamic behavior of the main springs was revealed. The applied methodology demonstrated to be useful in ascertaining functioning of a complex karst system under flood event conditions.
Plain Language Summary
Karst aquifer systems contain important water resources. The quality of karst springs can deteriorate significantly after rain events, but it is difficult to distinguish how water flows and mixes in the subsurface, especially in large and complex systems. Statistical methods are powerful tools for studying these issues, but most common approaches are inadequate in some cases to reveal the origin of the water and its fate. In this paper, we present an approach in which we combined different statistical methods to explain the dynamics of water flow based on the physicochemical and microbiological properties of water. The application of these methods led to the discrimination of parameters most useful for a reliable interpretation of statistical results, such as turbidity, bacteria, Cl, EC, and Ca/Mg, and to the construction of an advanced diagram that we called a multivariate chemograph. This diagram allowed us to see where the water was coming from at any given time to our monitoring points, which allowed us to construct a detailed explanation of water flow dynamics in space and time. Our contribution is important to better predict the fate of contaminants in karst underground and to develop an early warning system for better water supply management.
Key Points
A new approach to study and explain hydrodynamics of karst aquifers was developed
It offers an innovative solution to distinguish influential monitoring parameters
Multivariate chemographs allowed spatio‐temporal detection of recharge phases
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