An interdisciplinary perspective of the karst springs’ areas as drinking water potential: Perusal from Norteastern Slovenia

doi:10.21203/rs.3.rs-4956493/v1

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An interdisciplinary perspective of the karst springs’ areas as drinking water potential: Perusal from Norteastern Slovenia

https://doi.org/10.21203/rs.3.rs-4956493/v1

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Two karst water basins, water protection areas (WPA), the Ljubija and Rečica catchment areas, located in the north-east of Slovenia, were studied for one (COVID) year (January 2020-December 2020) to assess their freshwater quality risks facing the existing level of water protection regulations. Both groundwater (GW) and surface water (SW) were monitored twice a month in dry and wet periods to comply with European and national monitoring guidelines. For an interdisciplinary approach, an integrated indicators’ model was created to check the natural and human influences on the studied water sources. We related their water ecological conditions to the precipitation, hydro geographic and landscape characteristics of the WPAs. The Ljubija showed a low nitrate content and low fluctuations in other parameters, which declare a stable area with low human impact (the average nitrate content was 2.6 mg/l). Rečica water sources were more endangered. Their groundwater has been excluded from the drinking water supply due to organic contamination. The pollution was encouraged by the karst hydrogeological characteristics of the area. The average nitrate concentration was 6.0 mg/l. Based on the physic-chemical indicators of the water sources’ quality, we missed un upgraded monitoring regulation, especially for more vulnerable water sources as karst are.

Marine and Freshwater Ecology

karst

drinking water risk

land-use impact

water-ecological indicators

water quality assessment

The nature of karst aquifers, the low retention and self-purification capacity, the rapid infiltration from the surface into the groundwater and the rapid subsurface flow make them very susceptible to long-range pollution (Zwahlen 2004; Worthington 2011; Bonacci et al. 2020), especially in areas with agricultural land use exposed to nutrient loading (Spellman et al. 2023). Groundwater flows and the pollutants they contain can reach speeds of up to several hundred meters per hour (Bakalowicz 2005; Worthington 2009). The risk of groundwater pollution is higher in areas where pollution sources accumulate (Xiong et al. 2022; Ravbar et al. 2023).

When considering the factors that influence the spatial and temporal variation of nitrogen in karst watersheds, the unique karst hydrogeology cannot be ignored as an internal influencing factor, nor can natural factors such as meteorology, hydrology and external factors, e.g. human activities (Pan et al. 2021; Serène et al. 2024). General parameters (indicators) are defined in the provisions for monitoring the chemical status of water (Drinking Water Directive 2020; Water Directive 2000; Groundwater Directive 2006; ARSO 2016) and the methodology for the assessment of the ecological status of rivers based on the general physic-chemical quality indicators (ARSO 2017). Turbidity, pH, electrical conductivity (EC), temperature and hardness have been usually measured to determine the impact of human activities that, together with natural conditions, pose a risk of groundwater pollution.

According to the United Nations Organization's climate change prediction, drinking water supply has been declared one of the most important environmental challenges of the 21st century: a) to halt further degradation of already burdened water sources, b) to reverse the degradation towards improvement and to preserve not yet stressed drinking water sources. European directives oblige member states to protect captured and potential groundwater resources (Janža et al. 2017; Bonacci et al. 2021). The most important and challenging task of water supply is to ensure adequate quality and enough drinking water even in periods of drought when the demand for water increases. Globally, karst water resources cover around a quarter of drinking water demand and in Europe a third (Hartmann et al. 2014; Stevanović 2015; Ravbar et al. 2021). Around half of the world’s population has no access to hygienic and safe drinking water (Farzin 2022).

As karst aquifers are highly vulnerable to pollution, karst water resources must be managed appropriately and consistently. In Slovenia, large karst areas are usually quite remote and less suitable. for intensive settlement and concentration of various activities due to unfavorable topography and climatic conditions. They are mainly forested areas or areas characterized by extensive agriculture (Petrič and Ravbar 2008). The experienced assessment approach of the (integrated) indicators followed the appropriate protocol for the protection of karst water sources, which is not a common practice in Europe to improve legislation. Besides Slovenia, Germany has also introduced special implementing regulations that define the procedure for establishing water protection areas, considering the specificities of karst aquifers (Petrič et al. 2011; Živanović 2015).

Our study was aimed to prove the interdisciplinary model of indicators (IMI) as a research tool for karst waters’ study, where we combine water ecological characteristics, landscape settings, and (after the regulation) drinking water opportunities.

The applied interdisciplinary approach combined geographical and ecological research fields. A methodological set (model) of indicators was developed in the karst water study to assess the land use conditions and karst springs’ quality and evaluated in terms of the established legislation.

The study included a potential and an active water supply area. Therefore, we focused on the comparison of their characteristics and the following research challenges, also to verify the applicable water protection laws: 1) precipitation and the dependence of turbidity and nitrogen concentration indicators in karst areas, 2) existing agricultural production, 3) land use as a potential factor influencing nitrate concentrations in water bodies, and 4) drinking water supply in the context of national (and local) protection regulations. We headed for two research questions: Is there an additional need for more specific restrictions? Are the catchment areas covered by sufficiently large WPA?

For the statistical analysis of natural nitrogen and turbidity dynamics, a field study was conducted at karst springs in north-eastern Slovenia (Fig. 1), where water samples were regularly (y. 2020) taken and collected. We monitored nitrate levels as an indicator of the nutrient content in the water to check the impact of extensive (organic) agricultural activities taking place in the study areas. Additional emphasis was placed on turbidity, which is a key characteristic when talking about drinking water. Since we studied karst regions, the EC indicator was added. Selected indicators were confronted with the environmental conditions, the precipitation data.

Environmental conditions in the study areas

The comparison of indicators to present the study areas’ stay state-of-the-art will a) focus on the precipitation regime with rainfall data, b) considered as vulnerable isolated karst landscape areas with extensive agriculture, and c) concluded with a discussion on the relation to a water supply.

Both study areas, the Ljubija and Rečica catchments, are in the contact karst zone, which is very common for the isolated karst areas of the Slovenian Alpine macro-region (Melik 1954). It can be further defined as closed fluviokarst (Gams 2003), which mostly develops on dolomites. The riverbed of the Rečica has been formed in a small pocket valley, while the Ljubija appears in the middle of a dry valley as a Vauclusian spring. Rečica has been formed by two karst tributaries: Žegnan as the main, and another left tributary of Suha. Nomen est omen also applies to the naming of landscape elements. Suha means ‘dry’ and it really comes from the normally dry riverbed that joins the Rečica torrent after leaving a narrow gorge. Ljubija has also formed a narrow valley, but the area of the wider spring is an open-air classroom for the study of karst phenomena (depressions with blind valleys, a (semi-) dry valley, a karst window, a ravine in the lower part of the valley and a karst underground with a system of water chambers and caves upstream, in the steep slope above a spring and a Vaucluse-type pediment (Špeh et al. 2021a).

According to the Slovene cave registry (2020), both areas are rich in underground karst forms; 16 caves more than 6 meters deep/long were recorded in a 1.5 km radius around the Ljubija spring and 5 more caves around the Rečica spring. Geologically, the catchment areas consist partly of karstified carbonate rock, which is typical of the Dinarides macro-region. Due to the smaller outcrops of igneous rock, it would be assigned to the northern edge of the Karawanks (Kralj 1997). In terms of meteorological conditions, both study areas belong to the Cfb subtype of the Koeppen-Geiger climate classification, with an average annual precipitation of 1400–1500 mm in the Ljubija spring, and 1300–1400 mm in the Rečica area, period 1981–2010. The average annual air temperature in the same period was 6–8˚ C in Ljubija and 8–10˚ C in the Rečica area (ARSO 2022; Ravbar et al. 2021).

More specifically, we examined two areas in the Savinja statistical region, the Ljubija and Rečica catchment areas (Fig. 1), which differ in terms of water supply. The Rečica stream is fed by two springs that were included in the water supply before 1999, when the quality of the spring water was found to be insufficient. As microbiological contamination was detected and after additional tracer tests (Novak 2002) in the wider area, the water supply from the two springs was interrupted. Due to the seasonally increased local water demand, the wider catchment area should be treated as a water protection area with a subsequent protection strategy. On the other hand, the Ljubija spring has proven its constant importance for water supply since the early 1980s and provides drinking water for the Savinja - Šalek subregion (Municipal Company Velenje 2023).

In terms of the self-cleaning capacity of the vegetation (soil cover), the study areas are well positioned. On the steep slopes surrounding the spring areas, forest use dominates with over 50% (Špeh et al. 2021a). And in each area, there was only one active farm with cattle and pastures above. The Ljubija spring area is located on the thick brown soil on pyroclastic rock, and an outdoor sheep farm on the thin and patchy Rendzina soil has been placed above the Rečica springs. Soil characteristics, lithology and the distance between a farm and the water source could also lead to differences in the immunity of the source water to pollution.

The tracer test was used as a toxicologically safe hydrological method in karst in both study areas. The artificial tracers were used to assess the underground water flow. The tracer test in the Ljubija area was carried out as early as 1978 to ensure the safety of the spring, and shortly after the investigation became (and still is) a significant water supply for over 30,000 people. After the tracer injection, the coloured water appeared in the spring within 23 hours, and the groundwater flow was estimated at 73 m/h (Krajnc 1979).

The tests in the Rečica region were applied for the first time in 1991 and resulted in an underground flow of 72 m/h (Novak 2002).

Tracing was carried out again in 2019 and 2020 (Ravbar et al. 2021), as the source was no longer used for drinking due to the detection of contaminated water. Tracer infiltration revealed connections between two water bodies. We included both in the monitoring.

To disclose the natural conditions of the studied catchments, we examined the climatic, hydrogeological, geomorphological, and land-use characteristics in detail. They were linked to the water ecological conditions of the karst water sources in the study areas.

Sampling and analysis

Meteorological data - To present the meteorological conditions of the studied areas, data on precipitation trends were observed. The data on precipitation conditions were collected by the Slovenian Environment Agency (ARSO) and the Statistical Office of the Republic of Slovenia (SORS). Land use, pedological, geological and geomorphological data were analysed from the public database PISO (Spatial Information System for Municipalities).

Water sampling & monitoring - Our investigation centered on the hydrogeological characteristics of two karst regions: Ljubija and Rečica catchments. In the Ljubija catchment, a judicious selection process led us to designate 3 springs and 2 ponors as strategic sampling sites, aimed at facilitating a thorough examination of six pivotal physical and chemical water quality indicators. These indicators encompass temperature (°C), pH, turbidity (NTU), electrical conductivity (µS/cm), water hardness (°d), and nitrates concentration (mg/l).

The rationale behind our choice of monitoring parameters was driven by their intrinsic significance in comprehending the groundwater dynamics reaching karst springs. Previous studies (Petrič, Kogovšek 2010; Ravbar et al. 2011), underscored the importance of temperature and electrical conductivity as valuable indicators of spring water conditions. Moreover, the influence of the underlying bedrock on pH and water hardness was considered, with pH further modulated by water temperature and the biological activity within the water. Water hardness, conversely, was elucidated as contingent on the dissolved mineral content, predominantly calcium (Ca2+) and magnesium (Mg2+) ions, as elucidated (Devetak et al. 2019).

In the Rečica catchment, we meticulously selected 1 spring and 1 ponor as comparative sampling sites to observe variation trends in the selected indicators. To streamline our analytical approach, we categorized sampling sites into two distinct groups: groundwater (GW) and surface water (SW) sites.

Within the Ljubija catchment, the chosen springs were delineated as the Ljubija Groundwater group (LGW1-3), while the ponors were denoted as the Ljubija Surface water group (LSW1-2). Similarly, in the Rečica catchment, the spring Žegnan studenec earned the nomenclature of Rečica Groundwater (RGW1), and the ponor Suha was classified as Rečica Surface water (RSW1).

Table 1

Typology of the sampling sites of the Ljubija and Rečica catchment areas
Area	Sampling site	Site description	Altitude (in m) a. s. l.	Coordinates (North latitude, East longitude)
Ljubija catchment	LGW1	left side of the Ljubija riverbed	720	46º 24' 14º 56'
	LGW2	right side of the Ljubija riverbed	670	46º 24' 14º 56'
	LGW3	Ljubija spring	669	46º 24' 14º 56'
	LSW1	ponor in the Rupa cave	929	46º 23' 14º 54'
	LSW2	ponor below the ecological farm	910	46º 24' 14º 54'
Rečica catchment	RGW1	spring Žegnan studenec	430	46º 20' 14º 54'
	RSW1	ponor Suha	470	46º 20' 14º 54'

A comprehensive summary, inclusive of detailed information regarding each sampling site, is presented in Table 1 for reference.

The lithology and land use characteristics of the catchments exhibit similarities. However, distinctions emerge in terms of soil type and the proximity of inhabited farms to the springs. Specifically, in the Ljubija catchment (LGW1), we calculated a linear (air) distance of 1.46 km. In the case of the Rečica water sources, RGW1 is situated 1.7 km away, while RSW1 is 0.98 km further from the elevated farm.

The monitoring period spanned a year, from January to December 2020, during which measurements were conducted bi-monthly. A total of 23 fieldwork observations were executed for each sampling site, resulting in 161 measurements. Groundwater and surface water samples numbered 92 and 69, respectively.

Field measurements, including indicators such as temperature, pH, turbidity, and electrical conductivity, were conducted using the Vernier LabQuest 2 interface, a data-collection device equipped with appropriate sensors. Water hardness and nitrate concentration (NO3²⁻) were determined spectrophotometrically in the laboratory through rapid cuvette tests (Hach Lange LCK 327 for water hardness and LCK 399 for NO3²⁻) following the manufacturer's guidelines.

In the final exploration of the monitored catchments, we meticulously assessed previously researched data and compared it with the fieldwork indicators' findings to derive comprehensive insights.

Data analysis - To assess statistical significance and facilitate a comprehensive comparison of annual indicator values, we employed descriptive statistics. One-way analysis of variance (ANOVA) was specifically chosen to discern significant differences in the selected indicators across distinct parameters, including geographical areas (Ljubija vs. Rečica), site types (GW and SW), and seasons (winter, spring, summer, and autumn). A significance level of p ≤ 0.05 was considered statistically significant. The data analyses were executed using Excel 2020 (Microsoft Corporation, Redmond, WA, USA) and GraphPad Prism 9 (GraphPad Software, San Diego, California USA, www.graphpad.com), ensuring robust and reliable evaluation of the dataset.

Discussing the natural settings of the researched karst catchments, there has been Slovenian Environmental Agency interpolated maps (data) for air temperature available (ARSO 2022). As regards precipitation conditions, the data show quite detailed information and we could incorporate it in results’ comparison and analysis (Fig. S1, Fig. S2, Fig. S3).

Given the limitations associated with the availability of open-access air temperature data from a distant meteorological station (MS), our analysis focused exclusively on the regularly and automatically measured precipitation indicator. The meteorological station is situated at an air distance of 2.6 km in the Ljubija area and 2.3 km in the Rečica area, ensuring proximity to our monitored sampling sites. This approach allows us to glean valuable insights into the precipitation patterns influencing the water quality dynamics in the studied catchments.

Both study areas fall within the temperate continental climate zone of the central part of Slovenia. This climatic classification is characterized by several key features: 1) an average air temperature higher in October than in April, 2) a subcontinental precipitation regime, with peak precipitation occurring in May and June as outlined (Ogrin 1996), and 3) an average annual precipitation ranging between 1000 to 1300 mm.

Precipitation characteristics

The precipitation data obtained from the Bele Vode station (1026 m a.s.l.) in the Ljubija catchment reveals a departure from the typical temperate climate classification, with an average annual precipitation ranging between 1300 to 1400 mm during the period 1981–2010. Similarly, the Radegunda station (794 m a.s.l.) in the Rečica catchment also exhibits a deviation from climatic norms, reporting an average annual precipitation between 1400–1500 mm during the same period. These observed values suggest a transitional climatic position, indicative of a subcontinental climate zone, likely influenced by altitude.

Considering the karstic nature of the studied area, we interpret precipitation data as time series (Fig. 2) influenced by A) monitoring dates, initiated ten days before sampling and organized by season, and B) the discussion of precipitation trends within the rainfall regime. The springs, characterized by a pronounced karstic behaviour, respond dynamically to precipitation events, exhibiting short-term, intense flows that swiftly rise after heavy rainfall and subsequently recede rapidly (Ravbar et al. 2021; Krajnc 1979).

Furthermore, we examined the incline between the spring and farm locations, defining mountain karst conditions. The inclination in the Ljubija catchment measured 156.3‰, while in the Rečica catchment, it recorded 511.1‰ (linear air distance factored in). This topographical information enhances our understanding of the local hydrological dynamics and provides crucial context for interpreting the water quality variations observed during the study.

In the year 2020, the average monthly precipitation in the Ljubija catchment was 133.7 mm, while in the Rečica catchment, it measured 129.2 mm, as reported by the Slovenian Environment Agency (ARSO 2022). Analysing the precipitation data collected 10 days before sampling, we observed consistent rainfall trends in both catchments. Notably, the average annual precipitation during wet conditions was more than twice as high as that during dry conditions, as illustrated in Fig. 2 (embedded). A seasonal perspective revealed a distinct peak in summer precipitation, depicted in Fig. S1, for both areas, commencing notably in May (Fig. 2).

To evaluate the precipitation regime, we conducted a thorough analysis of a longer 10-day data series and plotted linear trend lines, revealing an inclining trend for both meteorological stations (Fig. S1). The promising rainfall data is corroborated by the total precipitation recorded for the summer period (June-August), measuring 381.9 mm in Ljubija and 297.2 mm in the Rečica catchment. Another notable peak in precipitation occurs in the period from September to November, with the Rečica area experiencing higher levels (296.2 mm) compared to Ljubija (252.8 mm). These findings contribute valuable insights into the seasonal dynamics of precipitation, shedding light on the hydrological conditions during the study period.

Water Ecological Monitoring

The comprehensive findings from a year of water ecological monitoring are succinctly presented in Table 2.

The temperature (T) of water samples, measured consistently during each sampling, exhibits a harmonious correlation with the natural attributes of the respective areas. In the Ljubija catchment, the annual water temperature averaged 8.0°C, with LGWs registering 7.9°C and LSWs recording 8.1°C. Contrarily, in the Rečica catchment, higher average water temperatures were identified in RGW at 9.3°C compared to 8.7°C in RSW. Given the limited frequency of temperature monitoring solely on sampling dates, it is worth noting that more regular assessments could potentially reveal more pronounced temperature variations, particularly in response to heavy precipitation. In our study, temperature (T) demonstrated a degree of seasonally insensitivity.

Across all sampling sites in both areas, pH values exhibited negligible variations, remaining consistently slightly basic and stable. In the Ljubija catchment, the average pH was 7.4 (7.5 in LGWs and 7.4 in LSWs), while in the Rečica catchment, it averaged 7.8. The observed pH values denote a remarkable stability, providing an essential baseline for understanding the water quality dynamics in both catchments.

Electrical conductivity (EC), governed by a limit value of 2500 µS/cm at 20°C for drinking water (Decree on drinking water, 2023), serves as a crucial indicator often exhibiting fluctuations linked to various parameters, such as precipitation events, which may contribute to water contamination. In our investigation, the Rečica catchment's groundwater (RGW) and surface water (RSW) displayed the highest average annual EC values, measuring 417.0 µS/cm and 377.3 µS/cm, respectively, with a modest 9.5% average difference between them. Conversely, a significant disparity of almost 50% in average annual EC values was observed in Ljubija between the two site types, with LGW recording 260.0 µS/cm and LSW at 137.2 µS/cm. Intriguingly, LGW exhibited higher EC values compared to LSW, presenting a contrasting trend to the Rečica catchment's dynamics, as summarized in Table 2. This variation underscores the nuanced interplay of local factors influencing EC levels and underscores the importance of detailed site-specific analysis in water quality assessment.

Conductivity serves as a practical surrogate for estimating water hardness, although it provides an assessment of the total dissolved ions in water, unlike hardness which specifically quantifies the concentration of divalent cations, primarily Ca and Mg. In our investigation, we noted a direct correlation between water hardness values and conductivity measurements.

The average annual water hardness across all sampling sites was 5.2 °d in the Ljubija catchment and 10.8 °d in the Rečica catchment. In Ljubija, groundwater (GW) exhibited higher average hardness at 6.8 °d compared to surface water (SW) at 3.6 °d, a trend mirrored in the Rečica catchment with annual GW and SW hardnesses at 10.6 °d and 9.9 °d, respectively. Notably, Ljubija displayed a more pronounced difference in hardness between the two water sources, indicating that SWs are comparatively deficient in Mg²⁺ and Ca²⁺ ions.

Contrastingly, water hardness values for both water types in the Rečica catchment were more comparable, aligning with the characteristics of moderately hard water (°d ≥ 8). It was observed that the surface water source RSW Suha occasionally influenced the characteristics of the groundwater source RGW Žegnan studenec (Novak, 2002). Moreover, the shorter air distance between surface and groundwater sources in Rečica (192 m) compared to Ljubija (LSW2-LGW1 distance: 1.5 km) contributes to a more cohesive water quality profile.

In the Ljubija catchment, approximately 68% of water samples exhibited low hardness (4 ≥ °d ≤ 8), 29.6% were very soft (°d ≤ 4), and only 2.6% fell into the medium-hard category. These findings illuminate the nuanced variability in water hardness, providing valuable insights into the ion composition of the studied catchments.

Turbidity serves as an important indicator of the presence of particles smaller than 1 mm, encompassing both inorganic and organic matter as well as microorganisms. In our study, we specifically focused on the turbidity results for the Ljubija spring (LGW3), a crucial source for drinking water supply. To enhance the context, we compared these results with annual data spanning three years (2019–2021), gathered within the framework of emission monitoring of water quality by the Slovenian Environment Agency (ARSO, 2022) (Table 3).

Comparisons revealed generally comparable data across various parameters, except for turbidity, where our study in 2020 reported a slightly elevated average (9.2 NTU). This disparity can be attributed to our measurement strategy, which focused on the current state during a short sampling period (Table 2). Intriguingly, the Rečica area exhibited even higher turbidity values, averaging 13.4 NTU for groundwater. Despite a similar precipitation regime compared to the Ljubija catchment, the hydrogeological characteristics of the Rečica catchment proved to be more sensitive to precipitation events, elucidating the nuanced response of different geological settings to environmental factors.

Table 2

Aggregated series of the sampling water bodies results: GW - groundwater, SW - surface water
Area	Sampling site	Site type	T_water (°C)	pH	Turbidity (NTU)	EC (µS/cm)	Hardness (˚d)	Nitrates (mg/l)
Ljubija catchment	LGW1	groundwater	8.3 ± 2.4	7.5 ± 0.6	8.8 ± 6.3	272.1 ± 46.7	6.7 ± 1.0	3.2 ± 2.3
	LGW2	groundwater	7.8 ± 2.5	7.5 ± 0.5	8.6 ± 7.7	262.9 ± 48.0	6.9 ± 0.8	3.0 ± 1.2
	LGW3	groundwater	7.6 ± 2.2	7.4 ± 0.5	9.2 ± 5.4	246.0 ± 61.1	6.2 ± 0.5	3.3 ± 1.4
	LGW average	groundwater	7.9 ± 2.3	7.5 ± 0.5	8.9 ± 6.5	260.0 ± 52.8	6.7 ± 0.8	3.2 ± 1.7
	LSW1	surface water	8.4 ± 5.7	7.4 ± 0.4	11.5 ± 8.2	172.1 ± 40.1	3.8 ± 0.8	1.9 ± 0.9
	LSW2	surface water	7.9 ± 5.3	7.4 ± 0.6	9.0 ± 5.4	102.4 ± 31.5	2.3 ± 0.6	1.7 ± 0.9
	LSW average	surface water	8.1 ± 5.5	7.4 ± 0.5	10.2 ± 7.0	137.2 ± 50.1	3.1 ± 1.1	1.8 ± 0.9
AVERAGE			8.0 ± 3.9	7.4 ± 0.5	9.4 ± 6.7	210.0 ± 79.5	5.2 ± 2.0	2.6 ± 1.6
Rečica catchment	RGW1	groundwater	9.3 ± 1.8	7.8 ± 0.4	13.4 ± 7.7	417.0 ± 95.5	11.5 ± 0.4	4.8 ± 1.2
Rečica catchment	RSW1	surface water	8.7 ± 3.9	7.8 ± 0.6	9.9 ± 7.2	377.3 ± 88.2	10.1 ± 0.6	7.3 ± 1.7
AVERAGE			9.0 ± 3.1	7.8 ± 0.5	11.7 ± 7.6	397.2 ± 93.1	10.8 ± 0.9	6.0 ± 1.9
Type of parameter*			/	chemical	indicator	indicator	/	chemical

^{*According to the (Decree on drinking water, 2023), three groups of parameters (microbiological, chemical and indicator parameters) are determined. For indicator parameters limit values are not necessarily correlated with the threat to health hazard and serve as a warning of the potential threat to the water quality}.

Table 3

Annual parameters of Ljubija drinking water supply (ARSO, 2022)
	2019	2020	Our study	2021
T_water (°C)	7.4	7.0	7.6	7.2
pH	8.3	8.2	7.4	8.2
turbidity /NTU)	2.7	2.0	9.2	2.0
EC (µS/cm)	224.1	299.6	246.0	214.2
hardness (°d)	6.2	6.7	6.2	6.1
nitrates (mg/l)	3.4	3.4	3.3	3.3

The evaluation of nitrates content serves as a key parameter for estimating human impact on groundwater ecosystems, with a stipulated threshold for aquifers in Slovenia set below 10 mg NO₃²⁻/l (Nitrates in the Groundwater). In the Rečica area, the observed values slightly surpassed the values of the Ljubija catchment (average 1.8 mg/l for SW and 3.2 mg/l in GW samples), measuring 7.3 mg/l for surface water (SW) and 4.8 mg/l in groundwater (GW), a variation potentially influenced by hydrogeological conditions.

In both study areas, marginal differences of approximately 5% were noted between measurements during dry and wet conditions for both site types (data not shown). It's worth noting that a farm situated at a higher elevation from the measured water sources may contribute to these variations. The alert level of concern is set at values exceeding 50.0 mg/l (Decree on drinking water, 2023). Notably, due to organic contamination, the Rečica groundwater (RGW) was excluded from public drinking water supply in 1986 (Novak, 2002).

In the Rečica area, both sampling sites (RGW and RSW) are positioned almost parallel, while in the Ljubija catchment, groundwater monitoring locations with lower nitrate values are situated below the higher-lying farm, contrasting with the surface water sites in terms of altitude (Table 1).

The indicator of nitrate concentration reflects stable and healthy water ecological conditions in the Ljubija catchment, with an average value of 3.2 mg/l. This aligns with the national monitoring (ARSO, 2022) findings from 2019–2021 (Table 3), signifying a congruence between our study's outcomes and broader regional assessments.

The land use data depicted in Fig. 3 reveals that both catchment areas are predominantly forested, a characteristic that contributes positively to the quality of water sources within these regions. Notably, the proper treatment of agricultural land is evident, aligning with the observed state of the researched water bodies, which demonstrates a satisfactory condition. This favourable scenario is further reinforced by the presence of self-purifying mechanisms facilitated by adequate precipitation levels and the inherent natural regulation of underground water processes.

Seasonal dynamics of the selected indicators

Both study areas exhibited heterogeneous dynamics in the seasonal behaviour of electrical conductivity (EC). Intense rainfall events, particularly during the summer period, resulted in the highest observed EC values in the groundwater (GW) of both regions. Rečica demonstrated a smaller disparity between groundwater and surface water (SW) EC values compared to Ljubija's surface water bodies. The more pronounced EC fluctuations observed at Ljubija sampling sites were consistent across all seasons, as illustrated in Fig. 4.

Electrical conductivity (EC) values are intricately linked to precipitation, particularly heavy rainfall. Initially, precipitation mixes with the inflow of mineralized water stored underground, referred to as old water. Subsequently, a noticeable decrease in EC occurs due to the infusion of newly infiltrated water, as elucidated by prior studies (Ravbar et al. 2021). The outcomes of tracer tests, coupled with additional precipitation events (ARSO, 2022; Novak, 2002; Krajnc, 1979) revealed that the maximum flow velocity of underground water in the Ljubija area surpassed that of Rečica (73 m/h compared to 49 m/h). Interestingly, the inclination data, considering the topography between the nearby elevated farm and the spring, indicated a steeper landscape in the Rečica area, characteristic of mountain karst conditions.

In tandem with EC, turbidity also responds to heightened rainfall, facilitating the arrival of new water to the spring, while the conventional flow of so-called old water remains relatively unaffected (Fig. S2, Fig. S3). As our sampling occurred on selected dates, we documented seasonal turbidity fluctuations, noting higher turbidity values during the dry season (Oct-Apr) for both groundwater (GW) and surface water (SW) samples in the studied areas of Ljubija and Rečica. The Rečica groundwater (RGW) attained its maximum turbidity value at 13.4 NTU (Table 2/ Fig. S3).

The variation of nitrate content according to location (Ljubija and Rečica catchments) and type of sampling point (surface water and groundwater) is shown in Fig. 5. The annual variability of nitrate content is significantly lower in the Ljubija area compared to Rečica. It was lower within each sample site and between both sample site types. However, both observed differences are statistically significant. An even more statistically significant difference was observed between sampling sites in the Rečica area compared to Ljubija, with nitrate levels almost doubling for GW and SW site types. Moreover, the trend of the differences between the two types of sites was reversed; in comparison to Ljubija, nitrate levels in SW samples were higher than in GW samples. Consequently, the differences, and the statistical significance between the same site types, are even more pronounced between the investigated areas.

Comparison of two karst catchments (WPAs)

In delineating the water sources into ground (GW) and surface (SW) categories across the Rečica and Ljubija research areas, our discussion revolved three key aspects: 1) the significance of wet and drought periods, 2) variations across the year (seasonal dynamics), and 3) intercorrelations among the surveyed indicators.

In periods characterized by minimal rainfall and sustained evapotranspiration, the consequential decline in water levels manifests in elevated turbidity and increased nitrate concentrations in Ljubija catchment area. Notably, the most pronounced instances occurred at LGW3 in April 2020, where turbidity exceeded 800 NTU, and nitrate content peaked at 11.5 mg/l. This divergence from average rainfall events indicates the potential influence of leachate reaching the water source, a phenomenon masked during typical rainfall due to dilution. Speaking of nitrate indicator, our findings have shown its accumulation downstream. While the risk of impact from settlement and agriculture in the immediate hinterland persists, the extent of waterlogging introduces an additional layer of complexity.

To compare, turbidity indicator for Rečica water sources, on average, was higher than of Ljubija water bodies. This observed trend, alongside higher nitrate levels in Rečica surface water (7.3 mg/l), raises concerns about potential leachate influx, particularly considering the proximity of agricultural activities. Despite the application of water protection regulations and adherence to organic farming guidelines in both catchment areas, Rečica's water sources demonstrated a marginally less quality at times (Table 2), emphasizing the challenging landscape of WPAs.

In our assessment of nitrate content, the Ljubija area exhibited very low concentrations (< 4 mg/l), indicating a natural origin. Conversely, slightly elevated nitrate levels in Rečica surface water align with the location's topography, situated bellow agricultural land. The overall quality of water sources in both areas has been well maintained, with Rečica demonstrating a degree of variability. Our investigation underscores the intrinsic vulnerability of WPAs, where the convergence of karst features, water supply dynamics, and agricultural demands necessitates nuanced management. Thus far, the collaborative efforts in implementing drinking water protection strategies have been commendable, particularly in the researched communities of Rečica ob Savinji, Šoštanj, and Mozirje. The ongoing commitment to safeguarding karst water resources, despite occasional fluctuations, attests to the resilient co-management of these critical landscapes.

The remaining selected indicators consistently demonstrated a highly satisfactory state:

Temperature Data: Throughout the year, temperature data exhibited minimal fluctuations, attesting to the stability of the hydrological conditions.

pH Values: The pH values consistently aligned with the natural background, affirming their appropriateness within the ecological context.

Recommendations and proposals

As regards the research goals, the experienced assessment of two WPAs followed the (integrated) indicators method to support the legislation improvement, which is not a frequent practice in Europe. As we predeterminated, a serious need has been proved for comprehensive understanding of the complex interactions within the water bodies in the karst catchments.

The application of the interdisciplinary indicator’s method performed effective in assessing the current state and advocating for potential enhancements in water supply legislation. Particularly noteworthy was its utility in the Rečica catchment, where human pressures on the environment intermittently impacted water ecosystem quality. However, acknowledging the complexity of the system, further integration of disciplines, such as geology, would enhance the depth of our understanding and contribute to more nuanced recommendations for improvement. The comprehensive integration of various scientific perspectives has shown as a robust strategy to fortify our insights into the intricate dynamics of water ecosystems and strengthen legislative frameworks accordingly.

In our paper, the exploration of the water-ecological dynamics within the karstic water areas was highlighted. We discussed the issue of additional treatment that karst catchments deserve. We grounded our investigation on a meticulous comparison of existing water sources in two catchments situated in Subalpine Slovenia. Our objective was to conduct a nuanced analysis of the karst water potential, addressing associated challenges. To involve natural settings in our assessment, we juxtaposed data on precipitation levels, and connected it with physic-chemical parameters and the characteristics of land use, thereby gauging the existing protection measures for water supply. To conclude:

Two researched karst catchments were determined and monitored: Ljubija by five distinct water bodies, delineated as two ponors, designated as LSW1 and LSW2, and three lower-lying springs, denoted as LGW1, LGW2, and LGW3, and in parallel, Rečica catchment area underwent sampling, targeting one ponor (RSW1) and one spring (RGW1).

The analytical outcomes unearthed intricate connections between environmental variables and various water ecological indicators, notably including precipitation, temperature, pH value, water hardness, electrical conductivity (EC), turbidity, and nitrates.

Three indicators - pH, turbidity, and electrical conductivity - adhered to the stipulations outlined in the drinking water regulations (Decree on drinking water, 2023). As anticipated, electrical conductivity demonstrated a proportional increase in tandem with the total hardness of the water. The monitoring data on water hardness revealed that the ponors (LSW1 and LSW2) exhibited very soft water (< 4°d), while the springs recorded a medium hardness range (6–8°d) due to natural chemical processes, including corrosion. Furthermore, in the Rečica catchment, the average water hardness of groundwater (GW) exceeded even more that of surface water (SW), measuring 11.5°d and 10.1°d, respectively.

In the Ljubija catchment, a downstream increase in nitrate concentrations was observed, aligning with the expected pattern. Notably, the variables of hardness and nitrate concentration, while independent of each other, both exhibited an increase downstream, reflecting the influence of groundwater flow dynamics.

Comparative analysis reveals that, on average, nitrates values for Rečica water sources surpassed those of Ljubija samples; Ljubija catchment’s average was 1.8 mg/l for SW and 3.2 mg/l in GW samples, and Rečica area average measurements were 7.3 mg/l for surface water (SW) and 4.8 mg/l in groundwater (GW).

Supplementary Information

The online version contains supplementary material available at https://ssrn.com/abstract=4843155.

Data Availability Statement

We have already submitted the manuscript to a preprint server (SSRN) to enable sharing of all relevant research data.

Authors Contribution

All authors contributed to the study conception and design.

Anja Bubik: Conceptualization, Methodology, Validation, Data curation, Formal analysis, Visualization, Investigation, Writing - Original Draft and Review & Editing.

Natalija Špeh: Conceptualization, Methodology, Validation, Data curation, Formal analysis, Visualization, Investigation, Writing - Original Draft and Review & Editing.

Funding

The research fildwork was a part of LEADER program contract project "Sustainable management and endogene landscape sources (living lab of water sources)", acronym ŽEGNAN STUDENEC (name of the funding agency: Savinja Institute for rural development and tourism/ https://www.savinja.si/prva-stran-2014-2020/ ; and grant number: 13 297.12 €).

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Competing Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Ethical Approval

This is an observational study. The Faculty of Environmental Protection Research Ethics Committee has confirmed that no ethical approval is required.

Consent to Participate

There were no individual participants included in the study.

Consent to Publish

Since there were no human research participants participants included in the study the authors affirm that all the Figures were provided/made by authors themselves.

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The authors declare no competing interests.

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Graphical Abstract
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Fig. S1 Seasonal variation of precipitation (in mm) in Ljubija and Rečica catchments, summary 10 days before sampling (ARSO href="https://meteo.arso.gov.si/met/sl/archive/">2022)
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Fig. S2 Turbidity dynamics of the observed Ljubija water bodies
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Fig. S3 Turbidity dynamics of the monitored spring and ponor in the Rečica area

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An interdisciplinary perspective of the karst springs’ areas as drinking water potential: Perusal from Norteastern Slovenia

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Abstract

Figures

Introduction

Materials and methods

Environmental conditions in the study areas

Sampling and analysis

Results and discussion

Precipitation characteristics

Water Ecological Monitoring

Seasonal dynamics of the selected indicators

Comparison of two karst catchments (WPAs)

Recommendations and proposals

Conclusions

Declarations

References

Additional Declarations

Supplementary Files

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