Konstantina KATSANOU1, Euaggelos NIKOLAOU2, George SIAVALAS3, Eleni ZAGANA1, Nikolaos LAMBRAKIS1
1Laboratory of Hydrogeology, Section of Applied Geology and Geophysics, Department of Geology, University of Patras, Greece, e-mail: [email protected]
2Institute for Geology and Mineral Exploration, Preveza, Greece
3Section of Earth materials Department of Geology, University of Patras, Greece
Abstract: In the frame of the present study the hydrogeological and hydrochemical conditions that prevail in Louros River drainage basin are analyzed and interpreted. The karstified basin occupies an area of 926 km2 and is located in the Epirus region at the northwestern part of Greece. It covers the drinking water needs of four towns constituting one of the most important karstic hydrogeological systems of this region. Geologically the basin is hosted in the formations of the Ionian geotectonic zone, which is the western part of the External Hellenides, which in turn constitute the extension of the Dinarides towards the South. The older formations of the study area consist of evaporitic layers of Triassic age, which outcrop at the western part of the Louros drainage basin. The Triassic evaporites are overlain by a thick sequence of carbonate and clastic rocks reflecting a continuous sedimentation from Late Triassic to Upper Eocene. Oligocene flysch outcrops at the margins of Louros basin, whereas at the lower part these formations are overlain by Neogene and Pleistocene sediments as well as Holocene fluvial deposits. The basin is delimited by large north–south trending faults, which affect the land topography and play a principal role in the function of the karst springs. The karst system is developed in the upper part of the carbonate sequence (Middle Jurassic-Upper Eocene), mostly consisting of thick-bedded and hydraulically interconnected formations, due to tectonic activity. The chemical composition of the groundwater showed the prevalence of Ca-HCO3 and Ca-HCO3-SO4 water types. The presence of SO4 is attributed to the evaporites. Schoeller and Piper diagrams also show that the chemical composition of groundwater is affected by the Triassic gypsum, which is expected to expand underground, throughout the largest part of the area. In the southern part, the reduction of sulphates, contributes to the rise of hydrogen sulphide-rich waters. R-mode factor analysis reveals that the most important process that controls groundwater chemistry is the dissolution of minerals along with redox environment. Reducing conditions are predominant in the periphery of the basin, mainly to the western and southern parts. Nitrates, which derive from agricultural activities, affect groundwater quality.
Keywords: karst, Ionian zone, hydrochemistry, statistic analysis, hydrogeology
1 Introduction
The study area is located in the Epirus region at the northwestern part of Greece, including the drainage basin of Louros River that occupies an area of 926 km2. It supplies the drinking water needs of four towns constituting one of the most important karstic hydrogeological systems of this region.
Louros River has a length of about 75 km and an average flow of about 10.6 m3/s. It discharges into Amvrakikos gulf to the south and its water irrigates about 120 km2 of cultivated area and supplies a small hydroelectric dam with 10 MW installed capacity. Louros karstified aquifer is discharged by 17 main springs, with yields ranging between 2.5 m3/sec and 6.7 m3/sec.
At the estuaries of Louros there are lagoons such as Tsoukalio, Rodia, Logarou, and Tsopeli, which are wetlands designated under the Ramsar Convention and the European
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Communities Legislation. About 2.5% of Louros catchment surface is cultivated and a number of large and small agricultural industries and fish-farms operate in the same region.
Karst aquifers, which supply drinking water to an estimated 25% of the global population (Pulido-Bosch 1999), are very vulnerable to contamination because of their hydrogeologic characteristics (Escolero et al. 2002) and display properties that are directly related to the investigation of microbiological contamination. Mahler et al. (2000) have reported four reasons that justify the above considerations for karst aquifers: (1) the direct and rapid connection between the surface water and groundwater systems, (2) the sediment mobility, (3) the karst heterogeneity and (4) the possibility of sudden water quality variations (episodic contamination).
The study emphasizes on karstic aquifers, hosted in the carbonate formations of Louros drainage basin, aiming to elucidate the major hydrochemical processes, which control the chemical composition of the groundwater, but also to estimate potential anthropogenic impacts on groundwater quality.
2 Climate and morphology
Evaporation and humidity are relatively high throughout the year. Climate can be characterized as mild temperate, to continental, changing by the influence of geographical position and relief (Boltsis 1986).
The hydrological regime of northern Epirus is characterized by uneven seasonal and regional distribution of precipitation. An average precipitation for Louros drainage basin is 1150-1800 mm. Out of them 50% is evapotranspiration, 35% is recharge and 15% is discharge (Fig. 1; Nikolaou 2005).
Figure 1 The distribution of precipitation in Louros drainage basin
Morphologically, Louros karstic system is characterized of high elongated mountain ranges and narrow valleys, due to the tectonic (anticline structures) and geologic conditions of the region and the lithologic alternation between limestones and flysch.
Louros basin is also characterized by numerous exokarstic and endokarstic features such as caves, sinkholes, karst lakes and springs that make the system vulnerable to groundwater pollutants.
3.1 Geology
Geologically, the basin is hosted in the formations of the Ionian geotectonic zone, being the western part of the External Hellenides, which constitute the extension of the Dinarides towards the South.
The Ionian zone was first described by Philippson (1896), who also included in it the Pre-Apulian and Gavrovo zones. However, the main stratigraphic features and the geotectonic position were presented by Renz (1955), who named it "Adriatsche- Ionische Zone".
Aubouin (1959) discussed the stratigraphic and geotectonic evolution of the basin, subdividing it into Internal, Central and External. The subsequent investigations made by IGRS-IFP (1966) in Epirus, Corfu and Lefkas, and by British Petroleum (BP) in Central Greece and the neighbouring Ionian Islands, provided further information on the geology of the zone.
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The older formations of the study area consist of evaporitic layers of Early to Middle Triassic age, which outcrop at the western and southern part of Ziros Lake (Fig. 2). The Triassic evaporites were deposited in a shallow supersaline basin and they are locally overlain by breccias derived from calcification processes (Karakitsios and Pomoni-Papaioannou 1998).
These formations are overlain by a thick sequence of carbonate and clastic rocks reflecting a continuous sedimentation from Late Triassic to Upper Eocene.
Until the Middle Liassic the Ionian Zone formed part of the Apulian Platform where thick, neritic limestones were deposited. From bottom to top this sequence includes Foustapidima formation that consists of black, hypolithographic limestones and dolomites of Carnian age. They are overlain by massive Early to Middle Liassic neritic limestones, the so-called Pantokrator Limestones (Karakitsios and Tsaila- Monopolis 1988). In Louros drainage basin their thickness is estimated to range from 1000 to 1500 m (Leontiadis & Smyrniotis 1986). A transition to pelagic sedimentation prevailed within the basin during the late Liassic giving rise to the following formations: The Siniais Limestone composed of thin-bedded limestones with chert nodules and cherty interbeds occurring locally on top of the Pantokrator Limestone (IGRS-IFP, 1966; Skourtsis-Coroneou et al., 1995), the Lower Posidonia Shales or their lateral equivalent, the marly Ammonitico Rosso, both of Toarcian age, Middle Jurassic limestones with filaments, and the Upper Posidonia Shales of Callovian-Kimmeridgian age, that consist of very thin-bedded, brown, green and black cherts alternating with thin beds of shales and marls (Fig. 2).
Vigla Limestone, which was deposited during the latest Kimmeridgian-Santonian (Skourtsis-Coroneou and Manacos 1995) over the Upper Posidonia Shales, is the first pure pelagic phase in the zone’s sequence. This consists of light-coloured, thin-bedded limestones with abundant calpionellids, foraminifera and radiolaria, and cherty lenses and interbeds, which become more common in the upper parts, locally forming the ‘upper chert horizon’.
‘Clastic’ limestones, overlying the Vigla Limestone, were deposited during the late Senonian–Eocene interval (Aubouin 1959). They include microbrecciated limestones and breccias with clastic materials that invaded the basin from its marginal areas and from the neighbouring zones, as well as numerous intercalations of pelagic, micritic limestones towards the top (Skourtsis-Coroneou et al. 1995).
A Flysch unit was deposited during the Oligocene on top of the ‘Clastic’ limestones.
The transition is characterized by marly limestones and calcareous marls (Skourtsis-Coroneou et al. 1995, 1999).
Unconsolidated deposits outcrop at the lowlands. These consist mostly of lacustrine deposits of Pliocene age, glacial drift, siliceous deposits, riparian terrace deposits, talus cones, and alluvial sediments.
3.2 Tectonics
Within the Ionian Zone several major thrusts have been described as well as other important tectonic features like great E-W trending strike-slip fault systems (IGRS-IFP 1966).
Most of the thrusts show a "normal" westward dip (Louros, Paramithia, Margariti and Parga Thrusts) and similarly, most of the folds are asymmetric with east-dipping axial planes.
Nevertheless important tectonic structures with an opposite dip also exist (Xerovouni and Tomaros back-thrusts). Since all the Alpide edifice of the Hellenides was created by west-dipping thrusting, we refer here to the east- west-dipping thrusts as back-thrusts.
Moreover, the large E-W fault zones, like the Souli strike-slip fault system, also known in the literature as Petoussi Fault, play an important role in the Alpide tectonic evolution of the area. Indeed, on the two sides of these faults the amount of displacement and the style of the NNW-SSE trending shortening structures often differ (IGRS-IFP 1966).
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STRATIGRAPHIC COLUMN OF THE IONIAN ZONE
Upper
Chert, shale with PosidoniaLimestone with Filaments Up. Liassic (Toarcian)
Figure 2 Geographic position, geological map and lithostratigraphical column of Louros drainage basin
101 4 Hydrogeological Conditions
The study area is structured from geological formations of different hydrogeological properties. The geological conditions, the lithostratigraphical diversity, and the complicated tectonics in combination with the geomorphological conditions have resulted in the formation of independent or semi-independent hydrogeological units.
The geological setting of Epirus illustrates characteristic alternant limestone anticlines and flysch syncline structures with general axis direction of NW-SE. Groundwater circulates following the same directions resulting in the formation of extensive hydrolithological units hosted in the karstified calcareous anticlines separated by the impermeable flysch synclines (Leontiadis and Smyrniotis 1986)
Carbonate formations are the major units constituting the highlands of the study area.
These units include dolomites, the Pantokrator Limestone, the Vigla Limestone, Late Senonian limestones, Paleocene–Eocene limestones, and Posidonian chert.
The most important aquifers in the broader area have been developed in carbonate rocks (mainly Pantokrator and Upper Senonian limestones). These formations show high permeability due to their intense karstification and fracture porosity and their occurrence is extended especially in the upper parts of Louros draining basin.
The flow rates of the major karstic springs in the region (Fig. 3) are higher than those expected by the hydraulic properties of individual units. This suggests the existence of hydraulic interconnections among carbonate formations of various ages, as well as replenishment of groundwater from rivers and lakes.
Figure 3 Discharge (m3/s) of the major springs of the study area (Nikolaou 1991)
According to Leontiadis and Smyrniotis (1986) Louros hydrogeological basin extends beyond the surface of the hydrological one. From the hydrogeological point of view the basin can be separated into five different hydrogeological systems. The first consists of the sub-basin of Thesprotika mountains, which outcrop in the western part of Louros sub-basin, and covers an area of 120 km2. The second is the Priala-Rizovouni system, the third is Chanopoulo springs system being in the eastern part of the study area and the other two are the upstream and downstream part of Louros River and cover 195 and 53 km2, respectively.
5 Groundwater chemistry
5.1 Sampling and analytical procedures
In order to define the hydrochemical composition of the karstic aquifer in the frames of the present study a total number of 108 samples were collected during October 2008 and May
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2009 from springs and boreholes along Louros drainage basin according to US EPA (1976) procedures.
Two polyethylene bottles of 1 and 0.1 L volume, respectively, were collected for each sampling site. The first bottle contained bulk water sample proper for the anion concentrations analyses. The second bottle contained water filtered through a Whatman 0.45 m cellulose membrane and acidified with 0.5 mL ultra pure HNO3.
The unstable physicochemical parameters including temperature, pH, dissolved oxygen, electric conductivity and redox potential were measured in situ using Hana® HI 9828 portable equipment. Additionally, alkalinity was also determined on the field using Hach® Digital Titrator.
All chemical analyses were performed in the Laboratory of Hydrogeology, University of Patras. Anion (NO3-, NO2-, PO43-, SO42-, and F-), NH4+ and SiO2 concentrations were measured in a Hach® DR 4000 spectrophotometer. For the determination of Cl- content titration techniques were applied by using AgNO3 0.1 N.
Major cation (Ca2+, K+, Mg2+, Na+) concentrations were determined in a GBC® Avanta flame atomic absorption spectrophotometer.
Trace element concentrations were measured using inductively coupled plasma-mass spectrometry (ICP-MS) in an ELAN 6100 Perkin-Elmer. It should be noted that the percentage error of the chemical analyses results calculated according to ion mass balance does not exceed 5%.
5.2 Descriptive statistics and groundwater classification
The samples are classified into two water types according to the Piper classification (Fig. 4). In the first water type the samples are characterized by the Ca2+ and HCO3
-predominance, whereas in the second, the anion site is shared between HCO3
and SO4
2-. Samples of the second type display higher salt content, especially sulphates, compared to the first. This could be attributed to the presence of a sulphate-rich source, like evaporite minerals that outcrop on the south-western part of the study area.
Mg Ca Na+K Cl SO4 HCO3
Figure 4 Schoeller and Piper diagrams of the analyzed samples. WS: groundwater sample, SW: seawater, RW: rainwater
The descriptive statistics of the two main water types of the study area are shown in Table 1.
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The Scholler diagram (Fig. 4) demonstrates that rCl/rNa and rNa/rCa ratios reveal values similar to those of rainwater. On the contrary, rCl/rSO4 ratio is diverse, probably due to the interaction of water with the evaporites. Samples R3, LP5 and LP11 represent hydrogen sulphide-rich waters, occurring at the southern parts of the study area.
Table 1 Descriptive statistics for the analyzed water samples. N: number of samples
Ca-HCO3 Ca-HCO3-SO4
N Min Max Mean Std.Dev. N Min Max Mean Std. Dev.
Twa (oC) 26 7.72 17.97 14.59 2.50 22 13.63 18.70 15.54 1.47
Eh (mv) 27 -20.00 277.00 104.54 82.52 25 -290.00 170.00 84.45 96.70
pH 27 7.00 8.21 7.53 0.34 25 6.99 8.85 7.47 0.39
Cond 27 200.00 604.00 349.41 93.97 24 338.00 3920.00 753.96 847.54 Alk (mg/L) 33 95.00 276.00 161.41 41.85 25 92.00 246.00 164.32 26.83
HCO3 33 115.90 336.72 196.92 51.06 25 112.24 300.12 200.47 32.73
K (mg/L) 33 0.15 8.84 1.23 1.60 25 0.38 44.00 2.93 8.94
Na (mg/L) 33 1.64 19.12 5.71 3.23 25 3.36 733.00 59.55 167.69
Mg (mg/L) 33 0.72 10.11 3.43 2.45 25 3.66 80.80 12.55 17.02
Ca (mg/L) 33 40.00 117.50 69.08 17.14 25 52.50 414.00 106.80 68.02
NH4(mg/L) 33 0.00 1.47 0.08 0.26 25 0.00 0.52 0.04 0.10
NO3(mg/L) 33 0.00 21.00 7.15 4.97 25 0.00 26.00 7.08 6.55
NO2(mg/L) 33 0.00 0.47 0.03 0.09 25 0.00 0.34 0.03 0.07
SO4(mg/L) 33 0.00 50.90 17.19 15.42 25 42.50 1120.00 172.00 238.09
F (mg/L) 33 0.00 1.48 0.15 0.27 25 0.08 1.38 0.34 0.28
Cl (mg/L) 33 0.90 30.00 8.84 6.60 25 3.40 830.00 69.33 189.34
B (mg/L) 33 0.00 0.03 0.01 0.01 25 0.01 0.41 0.04 0.10
Mn (mg/L) 33 0.00 0.19 0.01 0.03 25 0.00 0.07 0.01 0.01
Fe (mg/L) 33 0.01 0.12 0.03 0.03 25 0.00 2.00 0.12 0.41
Sr (mg/L) 33 0.03 0.24 0.11 0.06 25 0.11 3.84 0.55 0.73
5.3 Factor analysis
R-mode factor analysis was applied, according to the steps described by Davis (1987), to study the interrelations among 19 selected variables measured in the Louros basin water samples. The aim of this analysis is to reduce a large number of variables in the original data to a significantly smaller number of ‘factors’, each of which is a linear function of the original variables (Ashley and Lloyd, 1978; Adams et al, 2001). The selection of the four factors in Table 2, was based on the criterion that eigenvalues must be higher than 1. The chosen four factor model explains more than 77% of the total variance. All variables display very high communalities (1.000), indicating that the 4-Factor model describes them very well. In a next step, the contribution of each factor at every site (factor scores) was calculated (Fig. 5).
The first factor explains 45.7% of the total variance, and shows that most of the covariance in the properties of the system may be represented by variances of Na+, Mg2+, Ca2+, SO4
2-, F-, Cl-, B, Fe, and Sr2+. This indicates the principal role of these elements in the chemical composition of the groundwater. Figure 5 shows that samples LP11 and LP6, which are chemically affected by the presence of underlain evaporites, have a major contribution to the formation of factor 1.
Samples LG3, LG47 and LG48 from the east-central part of Louros basin, with high positive scores on this factor indicate a possible extension of the same geological conditions in this area. The first factor also highlights the effect of redox potential in the groundwater chemical composition.
The second factor accounts for 13.84% of the total variance and shows a negative relationship between two redox sensitive constituents of groundwater (NH4+
, Mn). Samples LP23, LP27, LP29, LP5 and LP9 showing high positive scores on this factor (Fig. 5) delimit an area in the western part of Louros basin. This area is characterized by outcrops of Triassic breccias.
104 Table 2 Factor loadings of the selected variables
Factor 1 Factor 2 Factor 3 Factor 4 Communalities
T(wa) 1.000
Eh -.689 1.000
pH -.679 1.000
HCO3 .730 1.000
K 1.000
Na .969 1.000
Mg .812 1.000
Ca .883 1.000
NH4 .963 1.000
NO3 -.788 1.000
NO2 1.000
SO4 .968 1.000
F .713 1.000
Cl .968 1.000
B .849 1.000
Mn .924 1.000
Fe .943 1.000
Sr .937 1.000
Rotation Sums of Squared Loadings
Total 8.238 2.484 1.958 1.353
% of Variance 45.768 13.800 10.878 7.519
Cumulative % 45.768 59.568 70.445 77.964
Extraction Method: Principal Component Analysis. Rotation Method: Varimax with Kaiser Normalization.
Figure 5 Factor 1 and 2 scores of the analyzed samples
The third factor, which accounts for 10.9% of the total variance, is characterized by the well-known relationship between pH and HCO3 ion, whereas the fourth factor accounting for 7.5% of the total variance is related to NO3- and indicates a probable anthropogenic impact.
105 6 Conclusions
The chemical composition of the groundwater in Louros drainage basin showed the prevalence of Ca-HCO3 and Ca-HCO3-SO4 water types. The presence of SO4 is attributed to the Triassic evaporites and the overlying breccias, which also affect the chemical composition of groundwater in general, as it is suggested by Schoeller and Piper diagrams. In the southern part, the reduction of sulphates, contributes to the rise of hydrogen sulphide-rich waters.
R-mode factor analysis reveals that the most important process that controls groundwater chemistry is the dissolution of minerals along with redox environment. Reducing conditions are predominant in the periphery of the basin, mainly to the western and southern parts. Nitrates, which derive from agricultural activities, affect groundwater quality.
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