Groundwater quality changes in a groundwater body in a poor chemical status

In December 2000 Directive 2000/60/EC of the European Parliament and of the Council establishing a framework for Community action in the field of water policy (WFD) entered into force. The directive creates a transnational approach within water protection and conditions for significant strengthening of environmental point of view in the process of protection. The main aim of the directive regarding groundwater is to achieve ‘good status’ of groundwater bodies (quantitative and chemical) and to observe principles of sustainable use of water. The target should be met by year 2015 under the first planning cycle, respectively by 2021 under the second planning cycle (2016–2021) or under the third planning cycle by 2027. Currently, Slovak Republic is in the second planning cycle ending in 2021.

Basic characteristics of surface and groundwater bodies was created in 2005 (Kuníková et al. 2005) and followed by characterization in 2013 (Malík et al. 2013). Assessment of groundwater bodies is according to the WFD basic element of the water assessment. A definition of groundwater bodies preceded the first assessment of groundwater status. Groundwater bodies are divided into three separate layers: a) the groundwater bodies in the quaternary water collector; b) the groundwater bodies of the pre-quaternary aquifers; c) geothermal water bodies, which are deep groundwater circuits with temperatures above 15 °C (Kuníková et al. 2005). The result is a total amount of 102 groundwater bodies, including 16 groundwater bodies in quaternary aquifers, 59 groundwater bodies in pre-quaternary aquifers and 27 geothermal formations (Kullman et al. 2006; Malík et al. 2013). The point and diffuse sources of pollution were identified for each groundwater body as well as their chemical status (Bodiš et al. 2008; 2013). Based on the actualized data (Bodiš et al. 2013), bad chemical status was classified in 11 groundwater bodies, including 7 quaternary and 4 pre-quaternary groundwater bodies. Geothermal water bodies were not evaluated.

In this paper Porous aquifer of quarternary alluvial sediments in the eastern area of the Danubian Basin, Danube River Basin District SK1000600P is evaluated. Groundwater body was in the first and second assessment of the chemical status classified as groundwater body in poor chemical status. Bad chemical condition is caused by concentrations of ammonium, chlorides and sulphates in groundwater. Development of groundwater quality was assessed based on the chemical analysis of the monitoring objects of the core network of the Slovak Hydrometeorological Institute (SHMI) in the period 2002–2015. The chemical analyses were evaluated according to Regulation of the Government of the Slovak Republic No 282/2010 Coll., laying down the threshold values and the list of groundwater bodies. Development trends for individual parameters (ammonium ions, chlorides, sulphates) were also made.

Studied groundwater body is located in the south-eastern part of the Danube plain, in the area of Podunajská pahorkatina hills. It is a porous aquifer of quaternary alluvial sediments in the eastern area of the Danubian Basin, Danube River Basin District (SK1000600P), which occupies an area of approximately 515 km². The area is bordered by the Danube River in the south, which in this part creates a natural border with Hungary. North-eastern border of the area is Hronská pahorkatina hills and from the north is the area bordered artificially with the imaginary line between the villages (named from the east) Mužla, Bátorové Kosihy, Modrany, Svätý Peter, Hurbanovo, Nesvady and Imeľ. Eastern border is formed by Nitra river and Váh river, which in city Komárno joins Danube river (Figure 1).

Geological development of the body has been formed by the flow of Danube River and its tributaries Váh, Nitra, Žitava, Hron, Ipeľ and by the complex neotectonic movements. Various genetic types have been allocated within quaternary sediments – fluvial sediments, loess (aeolian, swamp and eolian-diluvial), slope sediments, alluvial cones and organic deposits (Vaškovský et al. 1982). The thickness of these quaternary sediments varies. In the north of the investigated area are sediments only 0–5 meters thick. Nearby Komárno the quaternary sediments reach thickness of 5 to 15 meters, towards the middle part of the investigated area is thickness growing up to 30 m and to the east decreases to 5–15 m. In the vicinity of city Štúrovo is thickness of these sediments increasing to 20 m. The oldest sediments (Pleistocene) are residual gravels, sandy gravels and sands with gravel.

In the groundwater body SK1000600P, alluvial and terrace gravels, sandy gravels and sands of stratigraphic classification Pleistocene – Holocene are mainly presented as aquifers. The most watered are fluvial sediments of river floodplains and older terrace degrees. Hydrogeological nature depends on the particle size distribution, thickness and position related to the surface flow. Other quaternary sediments are from low watered to impermeable. In the hydrogeological collectors of this body prevails intergranular permeability. The average thickness of aquifers is <10 m (Vaškovský et al. 1982). General direction of groundwater flow in alluvial floodplain of quaternary body is more or less parallel with the course of the main stream (Malík et al. 2005).

The basic chemical composition of groundwater is variable. Dominant cations in the chemical composition are Ca²⁺, Mg²⁺ and considerable proportion of the cation concentrations are formed by Na⁺. Among anions dominate HCO₃⁻ ions and SO₄²⁻ ions are also present in significant concentrations. According to Palmer-Gazda classification (Gazda 1971) groundwater of strong basic Ca-Mg-HCO₃ type and faint basic Ca-SO₄ type occur in the groundwater body. The total mineralization ranges predominantly from 0.6 g·l⁻¹ to 1.4 g·l⁻¹ (Ľuptáková et al. 2012).

Development of the chemical composition and quality of groundwater in the groundwater body SK1000600P were evaluated based on data from national monitoring program performed by SHMI. The monitoring program has been carried out since 1982. The changes in the monitoring program were made in 2006, resulted from the requirements of EU legislation, in particular from the Water Framework Directive 2000/60/EC. Monitored objects were divided into 26 important water management areas until 2006. Since 2007, is monitoring of groundwater quality based on monitoring of groundwater bodies.

There are seven monitoring boreholes in the body of groundwater SK1000600P, of which one borehole (253890 Komárno-Komočín) is included in the basic monitoring and six boreholes (38690 Hurbanovo-Malý Vek, 52990 Iža-Bokroš, 251490 Mužla-Kendeleš, 602390 Iža, 602490 Moča, 602690 Kravany) are included in the service monitoring (Figure 1). Frequency of sampling for the groundwater quality monitoring is twice per year. Monitoring objects with their distribution represent the quality of the ground water of the entire investigated body.

During evaluation of the development of chemical composition and quality of water in the body of groundwater SK1000600P 146 chemical analyses of groundwater were used from the period 2002–2015, which were provided from SHMI (analytical processing is shown in Ľuptáková et al. 2012). Groundwater quality was evaluated according to Government Regulation No 282/2010 Coll. (threshold value) and changes in the chemical composition of ground water were assessed based on chemical analysis of ammonium, chlorides and sulphates. Two periods were compared – the first period from 2002 to 2009 and the second period from 2010 to 2015. The individual reference periods were processed into transparent box charts. Development trends for individual parameters of ground water (ammonium ions, chlorides, sulphates) are shown in the map.

Total dissolved solids in the groundwater monitoring boreholes ranged from 676 mg·l⁻¹ to 2,403 mg·l⁻¹ (Figure 2). The lowest mineralization of water and the smallest extend of mineralization were detected in samples from the borehole Hurbanovo-Malý Vek and the highest mineralization as well as the greatest extend in samples from the borehole Iža (Figure 2). Increased concentrations of certain ions were found in the ground water in the body SK1000600P, mainly Na⁺, Ca²⁺, Mg²⁺, NH₄⁺, Fe, Mn, Cl⁻, NO₃⁻ and SO₄²⁻. The pH values were in the range from 6.84 to 7.70. The concentration of dissolved oxygen in the ground water was below 3.37 mg·l⁻¹ and in most cases had this concentration zero value.

According to Gazdás classification of ground water (Gazda 1971), the groundwater can be characterized as a strong basic and faint basic Ca-HCO₃, Ca-Mg-HCO₃ and Ca-Mg-SO₄ type. Waters are also represented by mixed types with predominance of Ca-HCO₃, Ca-Mg-HCO₃ and Mg-Ca-SO₄ components, but also by transitional types. The groundwater changes the chemical composition from the strong basic to faint basic Ca-HCO₃ and Ca-Mg-HCO₃ type in the boreholes Hurbanovo-Malý Vek, Mužla-Kendeleš and Komárno-Komočín and the chemical composition of groundwater in the borehole Iža Bokroš was changing in time from the faint basic Ca-Mg-HCO₃ type to transition Mg-Ca-HCO₃-SO₄ type.

In the boreholes Iža, Moča and Kravany chemical composition of the ground water varies from mixed type with a predominance of Mg-Ca-SO₄ through intermediate type Mg-Ca-SO₄-HCO₃ to the faint basic Mg-Ca-SO₄ type. The dissolution of carbonates, hydrolytic degradation of silicates, gypsum dissolution and possible transfer of water from the Neogene collectors are participating in the formation of the chemical composition of groundwater, especially in the southern part of the investigated area (Iža-Bokroš, Iža, Moča and Kravany). The high level of agricultural activity as well as villages without canalization occurring in this area can also affect the chemical composition of groundwater. Groundwater pollution in the area of village Iža can be indicated also by too great range of groundwater mineralisation in the monitored object Iža. Groundwater from the other monitored objects in the flow direction have also higher mineralisation what can be caused by dilution of the groundwater by surface waters (Figures 2 and 3).

The quality of groundwater in the body SK1000600P was evaluated on the basis of 146 chemical analyses from seven monitoring objects SHMI for the period 2002–2009 and 2010–2015. These two reporting periods were consequently compared with each other.

During the observation of trends of the selected monitored parameters (ammonium ions, chlorides, sulphates) it was found that in the most cases is concentration trend of these parameters growing (Figure 3). Declining trend for concentrations of chlorides and sulphates was recorded in waters from monitored object Komárno-Komočín. For the ammonium ions was development trend constant (Figure 3). It is only monitored object where an improvement was observed in all parameters.

The concentration of ammonium ions in groundwater exceeds the threshold value only in waters from the monitored objects Iža-Bokroš (both periods) and Iža (period 2010–2015) (Figure 4). Ammonium ions in groundwater from the Iža-Bokroš monitoring object have a very large range of values. It can be caused by leaching from soil into groundwater. This state may depend on precipitation or on the use of fertilizers in the agriculture. It is certainly not a natural phenomenon.

The chloride concentration in groundwater in the most of monitored objects exceeds the threshold value (Figure 5). The declining trend in chloride concentration in groundwater was recorded only in the waters from the monitored object Hurbanovo-Malý Vek and Komárno-Komočín (Figure 3). The highest concentrations in groundwater were recorded in the waters of the monitored object Iža (Table 1). These highest concentrations of chlorides were recorded in both observed periods. It is likely to be groundwater pollution because the range of values is very pronounced (Figure 5) and chlorides cannot be found naturally in the geological environment.

The sulphate concentration in groundwater exceeds the threshold value in all monitored objects, except for the object Hurbanovo-Malý Vek. In this object, the sulphate concentration in groundwater exceeds the threshold value only in the maximum value (Figure 6, Table 1). The lower concentration of sulphates has also underground water from the Komárno-Komočín borehole, where sulphates have even a declining trend (Figures 3 and 6). From this it could be assumed that the source of sulphates is located at the top of the flow, but under the city Komárno. The largest range in concentration of sulphates was found in the waters of the Iža, Iža-Bokroš and Mužla-Kendeleš. Observation objects along the river flow had lower concentration without major changes. There is not necessarily the source of pollution in this area. Sulphates are coming here from the top of the stream and groundwater is diluted with the surface water.

Monitored groundwater body SK1000600P was ranked among 7 quaternary groundwater bodies which are classified as bodies in poor chemical status based on exceeding of threshold concentrations of contaminants NH₄⁺, Cl⁻ and SO₄²⁻ (Kuníková et al. 2005; Bodiš et al. 2008; 2013). For chemical status assessment of groundwater SK1000600P were processed data from 7 monitoring network objects SHMI (Figure 1). Groundwater quality was observed through changes in the concentration of contaminants in groundwater within two observation periods (2002–2009 and 2010–2015). Throughout the period 2002–2015, 438 samples were collected (for NH₄⁺, Cl⁻ and SO₄²⁻ analyses) from 7 monitoring objects. Development trends have been made for each parameter. These trends pointed out that chemical composition of the groundwater does not improve. The groundwater body will be probably included among bodies in bad chemical status also in the next evaluation cycle. Declining trends were recorded only in the monitored object Komárno-Komočín.

The most contaminated groundwater was found in boreholes Iža-Bokroš and Iža and the least contaminated was groundwater from the borehole Hurbanovo-Malý Vek. The source of contamination is probably located in the vicinity of the village Iža and subsequently groundwater flows through the environment in the direction of surface water flow. Concentrations of the monitored elements in the groundwater in the lower part of the flow are lower. This can be caused by the dilution of the groundwater by surface waters. In the future, it will be necessary to explore the area of Iža and to locate a possible source of groundwater contamination. It would be also diserable to focus on the quality of surface water in the upper and lower flow in the monitored area.

When we compare two time periods we can see that the concentration endpoints in groundwater boreholes Iža-Bokroš, Iža and Mužla-Kendeleš are in the second observation period significantly higher than in the first one. Development trends of the monitored parameters in the groundwater are growing as well.

According to Water Framework Directive 102 groundwater bodies were allocated in Slovakia, of which are 16 groundwater bodies in quaternary sediments, 59 bodies of ground water in the pre-quaternary rocks and 27 geothermal bodies. Based on the assessment of the chemical status, 7 bodies were classified as bodies in poor chemical status among 16 quaternary groundwater bodies.

The paper deals with the evaluation of body – porous aquifer of quaternary alluvial sediments in the eastern area of the Danubian Basin, Danube River Basin District (SK1000600P). 146 chemical analyses of groundwater (2002–2015) were conducted during chemical status assessment in seven boreholes of the monitoring network SHMI. Monitoring objects are deployed to represent the quality of the entire groundwater body. These objects are placed far from the sources of pollution, so the quality of groundwater is not influenced by this factor. However, during groundwater quality evaluation, it was discovered that in the groundwater from the boreholes, thresholds values set for ammonium, chloride and sulphates were exceeded. The largest concentration of mentioned ions was in the groundwater in boreholes Iža-Bokroš and Iža and the lowest concentrations in the ground water in the borehole Hurbanovo-Malý Vek. When comparing the time periods, it was observed that the concentrations in ground water in boreholes Iža-Bokroš, Iža and Mužla-Kendeleš were significantly higher in the second observation period in comparison with the first. Growing development trends point to deterioration of the status (Figure 3).

The source of groundwater pollution could be found in the vicinity of village Iža. It would be appropriate to carry out an additional survey in this area as well as to situate appropriate measures to avoid contamination of groundwater. Ground waters in the lower part of the flow can also be affected by surface water, which can dilute them partially. Therefore, it is advisable to study the chemical composition of surface water in the area in the future survey.