Trends and Sustainability of Groundwater in Highly Stressed Aquifers (Proc. of Symposium JS.2 at the Joint IAHS & IAH Convention, Hyderabad, India, September 2009). IAHS Publ. 329, 2009.

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Sustainability of groundwater resources in the Ljubljana area, Slovenia BRANKA TRCEK University of Maribor, Faculty of Civil Engineering, Smetanova 17, SI-2000 Maribor, Slovenia branka.trcek@uni-mb.si Abstract A detailed study of groundwater flow and solute transport was performed in the recharge area of the Union brewery groundwater resources in Ljubljana. The main study goal was the assessment and prediction of movement of groundwater and urban contaminants through the unsaturated or/and saturated zone of the Pleistocene sandy-gravel aquifer and the risk assessment of contamination of the groundwater below. For this purpose complex geological, hydrogeological, chemical, geochemical and isotopic investigations were performed. The results were statistically processed and modelled with the help of computer tools to simulate transport, and attenuation of pollutants in the study area. They provided the ability to conduct the scenario simulations and thus to develop the protection strategy of investigated groundwater resources. Key words Ljubljana; drinking water resources; urban gravel aquifer; groundwater flow; contaminant transport; sustainable management INTRODUCTION

Groundwater from a Pleistocene sandy-gravel aquifer is an invaluable water resource for the Union brewery, which is located within a highly urbanised and industrialised area near the centre of Ljubljana (the capital of Slovenia) and supplies quality groundwater from four production wells. A big part of the aquifer recharge area is highly urbanized, which represents a great risk for groundwater quality assurance. In the Union brewery area the discussed aquifer is divided into two parts – the upper and lower gravel aquifer (see the Results section – study of the field geology and hydrogeology). The good quality groundwater is exploited from the lower aquifer that is bounded by an impermeable barrier from the upper aquifer. However, the latter aquifer is contaminated by herbicides, chlorides and other urban contaminants derived from traffic, industry and wastewater systems (Juren et al., 2008). Its contamination level varies from location to location, depending on groundwater flow directions and contamination sources. Fortunately, the upper gravel aquifer has not influenced the present brewery production wells, owing to their successful reconstruction during the last decade. Nevertheless, the managers of the brewery were aware that sustainable groundwater resources management should be assessed and improved. Hence, an extensive study of groundwater flow and solute/contaminant transport was performed in the catchment area of the Union brewery groundwater resources with the intention of: (a) assessing and predicting the directions of groundwater flow and contaminants transport through the unsaturated or/and saturated zone of the Pleistocene gravel aquifer; and of (b) analys-ing the contamination risk of drinking-water resources in the lower Pleistocene gravel aquifer. Complex geological, hydrogeological, chemical, geochemical and isotopic investigations were performed and integrated with results of previous investigations with the aim:

(a) to establish a database, connected with a GIS-system; (b) to prepare the detailed geological and hydrogeological maps of the study area; (c) to construct a hydraulic model of the study area applicable: (i) to ascertain a hydrodynamic functioning of the investigated aquifer and a pollution degree

of groundwater; (ii) to assess the optimal balance between environmental protection and economic use of the

brewery groundwater resources; (d) to develop a protection strategy for the investigated urban drinking-water resources.

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STUDY AREA

The Union brewery is located in the Ljubljana basin, which is a large tectonic depression, surrounded by hills and mountains (up to 1300 m a.s.l.). The Ljubljana basin was formed in the Plio-Quaternary by sequential subsiding. Its northern part is named Ljubljansko polje–Ljubljana Field, while its southern part is named Ljubljansko barje–Ljubljana Moor (Fig. 1). The former is characterized by the Ljubljanica River with a karstic catchment, and the latter by the Sava River with an alpine catchment. The recharge area of the Union brewery groundwater resources is situated close to the boundary line of the Ljubljana Moor and Field, while the Dravlje Valley borders its western part. This valley has a dolomite catchment area. The geological composition of the study area is presented in Fig. 1. The Quaternary fluvial and lacustrine deposits fill the Ljubljana basin. The fluvial sediments form an urban intergranular aquifer. This aquifer is opened in the NE part of the study area (t-wQ1 and š-a sediments in Fig. 1). The impermeable or low permeable clay and sandy-clay layers cover the aquifer in other parts (šg-a, p-mQ2, j-mQ2 sediments in Fig. 1). Locally lenses of these sediments divide the aquifer into two parts – the upper and lower aquifer, as is the case in the Union brewery area. Two isolated hills, the Šišenski hrib hill and the Ljubljana castle hill (Fig. 1), have an important role in groundwater dynamics. They consist of impermeable Carboniferous-Permian sediments (Fig. 1), hence they are characterised by a prevalent runoff. The area between the two hills is called the Ljubljana door (Fig. 1).

Fig. 1 Catchment area of the Union brewery groundwater resources with geological characteristics and observation points (modified after Petauer & Juren, 2006a).

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RESEARCH MEHODOLOGY

The research methodology included five tasks that supplemented and upgraded each other. Database establishment

At the beginning of studies the database was established, which collected archive geological, hydrogeological, chemical, geochemical and isotopic data. Data were geo-referenced and adjusted to a geological and hydrogeological GIS system. The database was supplemented with new research data. Study of the field geology and hydrogeology

The structure and geology of the investigated aquifers control hydrodynamic processes and groundwater solute transport. Hence, a detailed geological and hydrogeological mapping of the study area was performed for a precise definition of the recharge area of brewery groundwater resources. The boundary conditions of the river basins were put into focus, as well as the adjacent hill, Šišenski hrib, characterised by a prevalent runoff (Fig. 1). As the groundwater direction was not known in the study area, four new piezometers were drilled. Study of groundwater flow and solute/contaminant transport

Groundwater flow and solute transport were studied based on the so-called quantity and quality monitoring. It was performed in the unsaturated and saturated zone of the observed aquifer. The main goal of the saturated zone monitoring was to investigate sources of contaminants and to identify their flow paths and sinks. However, the main goal of the unsaturated zone monitoring was to study possible contamination in the vicinity of the brewery and to evaluate the role of the unsaturated zone in groundwater protection. The saturated zone of the upper and lower aquifer was monitored in 20 observation wells, located in the Union brewery and its catchment (Figs 1 and 2). The observation wells capture: (a) groundwater of the lower aquifer, (b) groundwater of the upper aquifer, or (c) both of them.

Fig. 2 Observation wells in the Union brewery (modified Google Image Terra Matrics, 2008).

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The aquifer unsaturated zone was monitored at the urban lysimeter (Juren et al., 2003; Trček, 2005), adjacent to the brewery (Fig. 2), in boreholes that are distributed at depths of 0.3–4.0 m and equipped with suction cups for groundwater sampling (Fig. 3). A total of 18 observation points were established on the right lysimeter side, under the industrial railway tracks, while other observation points are located on the left side of the lysimeter, beneath the asphalt surface. The boreholes penetrate four layers: sandy gravel, silt-sandy gravel, clayey silt-sandy silt with gravel grains and gravel with sand and silt. The upper three layers are not autochthones; therefore they are called artificial layers. However, the fourth layer is autochthon and consists of river deposits. The quantity and quality monitoring included: (a) continuous measurements of piezometric head and discharge, of physico-chemical water parameters (pH, temperature, SEC-specific electrical conductivity), and (b) analyses of trace elements, herbicides, 18O and 2H isotopes and artificial tracers in groundwater and surface water. Additionally, precipitation quantity and its isotopic composition were monitored.

Fig. 3 Scheme of the Union urban lysimeter.

The chemical, geochemical and isotopic analytical analyses were made by the laboratory of JP Vodovod-kanalazicaija d.o.o., laboratory of Acme Analytical Laboratories Ltd, laboratory of Helmholtz Zentrum München – Institute for Groundwater Ecology, and laboratory of Joanneum Research – Institute of Water Resources Management, respectively. The chemical analyses are based on solid-phase extraction and GC-MS analysis, developed at the laboratory of JP Vodovod-kanalazicaija d.o.o. (Auersperger et al., 2008). Trace elements were analysed by the enhanced ICP-ES/ICP-MS methods, while a fully automated mass spectrometry facility was applied for isotopic analyses. For the purpose of defining recharge processes and predicting contaminant transport through and/or between different aquifer zones, tracer tests are of particular value. The so-called new compound served as a tracer in the aquifer-saturated zone. This compound was detected by the SPE and GC-MS method and a scanning technique at the laboratory of JP Vodovod-kanalazicaija d.o.o. (Juren et al., 2008). Groundwater samples were analysed by this procedure at least twice per year for the purpose of controlling the contamination state in the study area. In spite of numerous tests the new compound has remained unknown during the discussed study, but it was found not to be a high volatile compound. Two tracer tests were performed at the lysimeter. In the framework of the preliminary test 75 g of uranin was mixed with 20 L of water and injected into a point in the middle of the right lysimeter side. The principal test was performed in the same location, but it was based on three tracers – 2H, eosin and UV tracer.

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Artificial tracers were analysed at the laboratory of Joanneum Research – Institute of Water Resources Management. A new HPLC-FD method (high performance liquid chromatography combined with high sensitive fluorescence detection) was used for the separation and quantitative determination of the fluorescent dyes and 1,5-naphthalene disulfonate (1,5-NDS) in percolation water samples (Juren et al., 2008). Statistical processing and modelling of data

Geochemical data were processed with statistical methods. The multivariate cluster analysis and R-mode factor analysis were used to reveal the associations of chemical elements in sample media (Šajn, 1999; Juren et al., 2008). The factor analysis was performed on variables standardized to zero mean and unit standard deviation (Šajn, 1999; Juren et al., 2008). As a measure of similarity between variables, the product-moment correlation coefficient was applied. For orthogonal rotation the varimax method was used (Šajn, 1999). Data of the presented study were synthesized in a conceptual model of the study area. It served for a development of the 3-D mathematical model of intergranular aquifers of the Ljubljana barje, Dravlje valley and Ljubljana polje with a VISUAL MODFLOW 4.2 modelling code (Juren et al., 2008). The model was validated and verified on the basis of geological, hydrogeological, hydrological and geochemical data. It was applied for simulations of contaminant transport in the investigated area. The simulations were conducted with the MODFLOW 2000, MODPATH and M3DM modelling tools (Juren et al., 2008). RESULTS

Study of the field geology and hydrogeology

The study of field geology and hydrogeology resulted in new geological and hydrogeological maps of the Union brewery groundwater recharge area and in the map of boreholes of the Union brewery groundwater recharge area on a scale of 1:25 000 (Petauer & Juren, 2006a,b,c). The geological model of intergranular aquifers of the Ljubljana barje, Dravlje valley and Ljubljana polje was illustrated with the map of the pre-Quaternary basement, on a scale of 1:25 000. Data of 178 structural and piezometric boreholes were applied for the model construction. The Ljubljana polje, Dravlje valley and Ljubljana barje are tectonic depressions, filled with Quaternary clayey-silt and sandy-gravel sediments that are locally transformed into lenses of porous conglomerates (Fig. 1). The depth of sandy-gravel sediments increases in the Ljubljana polje from NW to S and SE where it exceeds 80 m. In the central part of the Ljubljana polje the depth of sandy-gravel sediments vary between 30 and 90 m. The aquifer cross-section in the Union brewery area is presented in Fig. 4. The Dravlje depression is filled with sandy-gravel sediments with lenses of conglomerates that are 50–80 m deep. They are covered with clayey layers that are up to 20 m thick. In the central part of the Ljubljana barje a distinctive doorstep is present in the pre-Quaternary basement. The sandy-gravel sediments are 50–60 m deep north of the doorstep. They are covered with silty-clay layers that are more than 20 m thick. The depth of the discussed sediments varies between 100 and 150 m south of the doorstep. The sandy-gravel sediments of the Ljubljana polje represent an important urban intergranular aquifer. It is recharged by precipitation and inflows from the Sava and Ljubljanica rivers. The groundwater flow from the Ljubljana barje direction additionally recharged the discussed aquifer through sediments of the Dravlje valley and the Ljubljana door (Fig. 1). The sandy-gravel sediments of the Ljubljana polje aquifer are very heterogeneous as they are divided with conglomerates and silty-clay lenses. On the basis of archive data the permeability k was estimated to 0.02–0.005 L/T in the central part of the Ljubljana polje where the Union brewery is located. Lenses of clays, silty clays, clayey sands and clayey gravels are present at

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depths between 8 and 18 m. Their permeability is very low so they separate the aquifer into the upper and lower part (Fig. 4). The upper aquifer is recharged by precipitation and surface and underground inflows from the Šišenski hrib hill (Fig. 1). The groundwater level of the intergran-ular aquifer of the Ljubljana polje is 6–22 m below the surface. It is at depths between 18 and 22 m in the Union brewery area (Fig. 4).

Fig. 4 Schematic aquifer cross-section in the Union brewery area (N–S direction).

The sandy-gravel sediments with lenses of conglomerates compose an intergranular aquifer of the Dravlje valley with the permeability coefficient k > 10-4 L/T. The groundwater level is 17–25 m below the surface. The aquifer is protected by a cover of low permeable or impermeable clayey layers that are 14–20 m thick. Owing to the varied geological composition three aquifers are in existence in the Ljubljana barje area: the Holocene, upper Pleistocene and lower Pleistocene aquifers. They are separated with thick layers of silty and sandy clay that have very low permeability. Hence, the hydrostatic pressure is different in each aquifer and the process of groundwater exchange occurs only in aquifer contact zones. Study of groundwater flow and solute/contaminant transport

The comparison of parameters, contaminant presence and concentrations at and between different observation points in the study area (Figs 1 and 2) gave important information on the recharge conditions, solute/contaminant behaviour, mixing processes and groundwater residence times and with that on the hydrodynamic connections among individual aquifers. The water table varied between 276 and 280 m a.m.s.l. in most observation wells, except in Pu-37 and Pu-40, which are influenced by the Ljubljana barje, and Pu-8 and Pu-43, which should be recharged from hanging aquifers (Fig. 5). The SEC values ranged in observation wells between 200 and 2500 μS·cm-1 at 25°C. High values (600–1100 μS·cm-1 at 25°C) were measured in piezometers that capture groundwater of the

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Fig. 5 Oscillation of water table in observation wells of the study area. upper gravel aquifer. These values indirectly reflect local aquifer contamination with chemical compounds. Chemical analyses of groundwater samples from the brewery and its vicinity indicated the local contamination with herbicides, chlorides and some urban contaminants derived from traffic, industry and wastewater systems (Juren et al., 2008). During the monitoring period some decomposition products of metholaclor and oxygenates were detected. The lowest SEC values (150–350 μS·cm-1 at 25°C) refer to piezometers that are influenced by surface and subsurface runoff from the Šišenski hrib hill (Fig. 1). The SEC data of remaining wells (350–500 μS·cm-1 at 25°C) reflect a little contamination load. Most of them are recharged from the lower gravel aquifer. The SEC values of the lysimeter sampling points are presented in Fig. 6. They varied between 200 and 600 μS·cm-1 at 25°C. The highest values were measure in sampling points on level III (Pu-vr14, PU-vr23), which is located near the sandy-clayey layer. The measurements indicated that this layer acts as a semi-permeable stratum and significantly influences groundwater dynamics. The water balance of the lysimeter sampling points demonstrated that the bulk of water was discharged to sampling points on level III that is located near the contact between two structurally different layers: silt-sandy gravel and underlying clayey silt-sandy silt with gravel grains (Trček & Juren, 2007). The hydraulic conductivity of the upper layer is higher than the lower one, which could result in a development of a lateral flow component near this contact. It is presumed that this is the main reason for high SEC values of groundwater near level III. In the brewery area the highest SEC averages refer to piezometers of the upper aquifer, Pu-9 and Pu-11 (Figs 3 and 4). These piezometers also have the highest parameter ranges, which reflect an intensive groundwater dynamics. The same findings result from isotopic investigations. The variation of oxygen isotopic composition (δ18O) in sampled waters is illustrated in Fig. 7. It is the most intensive in piezometer Pu-9, which reflects intensive groundwater dynamics and mixing of waters from different aquifer layers. Figure 7 indicates a breakthrough process that is particularly evident in January 2006 (after a period of very high water conditions) – groundwater with the similar δ18O to that in piezometer Pu-10 (located in the upper aquifer) was pushed into piezometer Pu-9. Influences of other water masses are evident also in Pu-10 and Pu-4 (production well).

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These data point out the risk of contamination spread through the upper gravel aquifer, as well as the possibility of contaminants breakthrough into the lower gravel aquifer. Namely, chemical analyses indicated that piezometer Pu-9 is overloaded with chemical contaminants, mentioned at the beginning of this subchapter. Moreover, the highest concentrations of the so-called new compound were measured in piezometer Pu-9, which is illustrated in Fig. 8. Its concentrations varied in a wide range between the sampling locations, as well as between sampling intervals. A wastewater leakage near Pu-9 could be a possible source of the new compound. This was also inferred from measurements of groundwater temperature. Therefore, the brewery wastewater system should be overhauled and wastewater should be sampled during further investigations.

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Pu-1 Pu-2 Pu-3 Pu-4 Pu-5 Pu-6 Pu-7 Pu-9 Pu-10 Pu-11 Fig. 8 Oscillation of the so-called new compound in sampled groundwater.

Fig. 9 Distribution of Factor 5, reflecting anthropogenic impacts of urbanisation on groundwater, sampled in the recharge area of the Union brewery groundwater resources.

Data of trace elements in sampled water were applied in combination with archive data of trace elements contents in soil and water (Šajn, 1999) for geochemical studies, which enabled the defini-tion of the geogenic background of sampled waters and the evaluation of anthropogenic impacts. The geochemical database refers to 72 trace elements. They were statistically processed, as described in the previous section. In the final multivariate factor solution 46 statistically significant elements were retained. They were divided into five groups that are presented with five statistical factors (Šajn, 2008). The statistical factor 1 represents a natural distribution of trace elements that

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are typical for sediments that compose flysch (Co, Dy, Er, Ho, Lu, Ni, Y, Yb), statistical factor 2 reflects anthropogenic impacts of industry (B, Cd, Cu, Li, Mo, P, Pb, Sb, Se, Sn, U, V, W, Zr), statistical factor 3 associates trace elements of clay origin (Al, Ce, Gd, La, Nd, Pr, Sm), statistical factor 4 presents a variation of the iron group elements (As, Fe, Mn, S, Sc, Si), while statistical factor 5 reflects the anthropogenic urban impacts (Ba, Br, Ca, Cl, Cr, Cs, K, Na, Rb, Re, Sr). The most important results of geochemical investigations are summarised in Fig. 9, which illustrates the anthropogenic impacts of urbanisation – particularly wastewaters, on sampled groundwater (Factor 5). Factor 5 demonstrates that piezometers of the upper gravel aquifer (Pu-6, Pu-7, Pu-9) are significantly impacted by urban contamination. These impacts are very low in shallow production wells of the lower gravel aquifer (Pu-1 and Pu-2) and practically negligible in deep production wells (Pu-3, Pu-4). Modelling of data

The mathematical model of the Ljubljana polje and barje was constructed (Juren et al., 2008). It consists of the network of 320 × 276 square elements that have a 50-m side length. Each element is defined with the groundwater horizon thickness, the permeability coefficient (k), the specific outflow (Sy), the porosity (n), the effective porosity (ne), the infiltration of precipitation, and the initial water table. The thickness of groundwater horizon was determined on the basis of measurements of water table (Fig. 5) and the thickness of sandy gravel sediments given with the thickness to the Pre-Quaternary basement. The modelling area was vertically divided into four layers that were determined on the basis of thickness of the permeable and impermeable strata in the research area. In areas of the Sava and Ljubljanica River beds the elements were described with a bed bottom, a bed water level and a permeability of the river bed, which represents a flow resistance in the zone between surface and groundwater. In addition, the mean annual quantities of discharged groundwater were taken into account for production wells of the Ljubljana waterworks and Union brewery. In the second phase the mathematical model of the Ljubljana polje and barje was validated on mean water table and verified with data of the quality monitoring.

Fig. 10 Main directions of groundwater flow in the catchment of the Union brewery groundwater resources.

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The model was applied for simulations of groundwater flow and contaminant transport in the study. It indicated that groundwater flows into the Union brewery area from the Ljubljana door direction (Figs 1 and 10). Hence, if the contamination occurs in the area of the Ljubljana door and the contamination is not sanitized in a time of more than one year, then the quality of the brewery groundwater resources could be threatened. The results illustrate that observation wells that are more distant from the Šišenski hrib hill are more vulnerable to contamination, which is in accordance with results of the quality monitoring. CONCLUSIONS

The presented results indicate anthropogenic impacts on groundwater and directions of possible contaminant inflow into the Union brewery area. They point out the risk of pollution breakthrough into the lower gravel aquifer and served for development of the local hydraulic model that enables scenario simulations on movement of contaminants towards the saturated zone of the lower intergranular aquifer. The results were used for improvement of exploitation methods of brewery groundwater resources as well as for the methodology improvement of the quality and quantity control of brewery water resources. In a case that groundwater is contaminated in whichever location in the Union catchment area the model could be used to find out the threading degree of groundwater resources that are supplied with brewery production wells and hence to develop a protection strategy in water treatments. The synthesis of project results enabled: (a) a precise definition of the Union brewery water body; (b) optimization of groundwater protection zones; and (c) the protection strategy improve-ment. If mechanisms of flow and solute transport are adequately understood, then efficacious monitoring and protection strategies for sustainable management of investigated groundwater resources could also be recommended. Acknowledgements The author would like to acknowledge Union brewery, Slovenian research agency, the European commission under the Fifth Framework Program and the Competence network “Water resources and their management GmbH (KnetWater)” for the financial support of the research. REFERENCES Auersperger, P., Kus, J., Lah, K. & Marsel, J. (2005) High precision procedure for determination of selected herbicides and

their degradation products in drinking water by solid-phase extraction and gas chromatography-mass spectrometry. J. Chromatogr A. 1088, 234–241.

Juren, A., Pregl, M. & Veselič, M. (2003) Project of an urban lysimeter at the Union brewery, Ljubljana, Slovenia. RMZ-mater. geoenviron. 50(3), 153–156.

Juren, A., Trček, B., Harum, T., Leis, A. & Petauer, D. (2008) Final Report, Net Node 6, Work Package 6.2.1: Best Management Practices – Pivovarna Union d.d. (BMP-PU). K-net Water Open File Report 01-34.

Petauer, D. & Juren, A. (2006a) Geological Map of the Union Brewery Groundwater Catchment Area in scale 1: 25 000. GeoSI CD-Rom, Ljubljana, Slovenia.

Petauer, D. & Juren, A. (2006b) Hydrogeological Map of the Union Brewery Groundwater Catchment Area in scale 1: 25 000. GeoSI CD-Rom, Ljubljana, Slovenia.

Petauer, D. & Juren, A. (2006c) Map of Boreholes of the Union Brewery Groundwater Catchment Area in scale 1: 25 000. GeoSI CD-Rom, Ljubljana, Slovenia.

Šajn, R. (1999) Geochemical properties of urban sediments on the territory of Slovenia. Geological Survey of Slovenia Open File Report 01-136.

Trček, B. (2005) Investigations of flow system and solute transport at an urban lysimeter at Union Brewery, Ljubljana, Slovenia. RMZ-mater. Geoenviron. 52(4), 685–696.

Trček, B. (2006) Isotopic investigations in the Union brewery water body. Geologija 49(1), 103–112. Trček, B. & Juren, A. (2007) Flow and solute transport monitoring at an urban lysimeter of Union brewery, Ljubljana, Slovenia.

In: Urban Groundwater – Meeting the Challenge: (ed. by K. W. F. Howard), 19–28. IAH Selected Papers on Hydrogeology 8, Taylor & Francis, London, UK.