Isotopic and chemical investigation of water samples in Argolis Peninsula (Greece)

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1 Isotopic and chemical investigation of water samples in Argolis Peninsula (Greece) I.Matiatos, A.Alexopoulos National & Kapodistrian University of Athens, Faculty of Geology and Geoenvironment, Athens, Greece Abstract. The area under study, located in the northern-eastern part of Peloponnesus (Greece), is characterized by three major mountains (Arachnaio, Didima, Aderes), which constitute a complex orography and hydrogeology. Although the groundwater circulation in the peninsula is not very distinct due to the geological diversity, the objective of the study was to gain the first regional picture of groundwater recharge in the area. Samples were collected from springs, wells and rain collectors and analyzed for environmental isotopes as well as major ions. The interpretation of the results was carried out in the light of other geological and hydrogeological information in order to determine the sources and mechanisms of recharge in the area. Arachnaio Mountain, which shows the highest altitude and extent, appears to be the main recharge zone which replenishes a number of springs in the peninsula. 1. Introduction The area studied (Fig.1) is located in southern Greece, covers approximately 1300Km 2 and is bounded to the northeast by the Saronic Gulf and to the southwest by the Argolic Gulf, while it is limited to the north by the east-west trending ridge of Mt. Arachnaio. The aim of the study was to delineate the groundwater recharge in the area, based on the preliminary results obtained from the groundwater and rainwater isotopic and chemical data in Argolis Peninsula. In several studies hydrochemistry, stable and radioactive isotopes have been extensively used as an important tool to overcome problems related to altitude effect, evaporation conditions and residence time and consequently give answers to issues that concern the comprehension of groundwater flow patterns ([2][3][5]). 2. Hydrogeological setting Argolis Peninsula is characterized by dissimilar lithological formations with different permeabilities which contribute in the appearance of numerous springs. Actually, the studied area presents the following complex geological setting [4], as it is portrayed in Fig.1: The Quaternary sediments include alluvial deposits (G1), fans of clastic debris (G2) and volcanic sediments (G3), while the sequence of conglomerates, sandstones and marls is of Plio-Pleistocene age (G4). The Upper Cretaceous is represented by flysh sediments built up by sandstones, conglomerates and shales together with a series of limestones (G5). Another flysch formation, consisting of marls, sandstones, breccias, conglomerates and limestone bodies (G6), is also present. Limestones and dolomites of L.Triassic Jurassic age (G7) appear in a large extent, while there are occurences of basic and ultrabasic rocks (G8) together with part of an ophiolithic sequence of diabase, peridotite, serpentinite and pyroxenite (G9). International Atomic Energy Agency 1

2 FIG. 1. A simplified geological map of the studied area [4] and the location of the sampling sites. 3. Methodology We located and accurately determined the coordinates of fifteen sites which include ten springs, three wells/ boreholes and two rain tanks, presented in Fig.1. The samples were collected in polyethylene containers which were firstly rinsed in clear water several times and then filled to the top to minimize air entrapment. Most of the sites were sampled repeatedly during the period October 2005 January Groundwater and rainwater quality characteristics were defined by field measurements which included temperature, electrical conductivity (EC), ph and TDS using WTW 350i meters. Bicarbonate, chloride, calcium and magnesium were measured by titration using a Hach titrator. Sulphate and nitrate contents were measured photometrically with a HACH DR/2010 apparatus. Sodium and potassium analyses were perfomed with a flame photometer (JENWAY PFP7). Chemical analysis was routinely carried out at the University of Athens, Faculty of Geology and Geoenvironment. The stable isotope ratios and Tritium concentrations were measured at the Isotope Hydrology Laboratory of N.C.S.R. Demokritos, Athens, Greece. The 18 O/ 16 O ratios were defined with the help of an Isotope Ratio Mass Spectrometer (SIRA Series II), while Tritium analyses were performed with a low radiation counting device using the technique of Liquid Scintillation. Before the LSC counting the samples were electrolytically enriched. The 18 O/ 16 O ratios are expressed in δ, i.e. per mille deviation vs. the reference V-SMOW. The measurements standard uncertainties are 0.1 for δ 18 O. The Tritium concentration is expressed in Tritium units (1 TU = 1 tritium atom per H atoms) and the uncertainty is defined as ±10% of the tritium value. 2

3 4. Results and discussion 4.1. Isotopic results and interpretation The springs showed minor significant temporal variation in stable isotopic composition, while the wells/boreholes and rain tanks presented important variation. The Tritium content fluctuated in most cases. In Table 1, we present the average stable isotopic values and the tritium variation range as well as its average concentration. Additionally, we classified the sampling sites in eight groups according to their mean isotopic concentration in 18 O (Table 2). Table 1.Sampling sites, number of analytical measurements (n) and isotopic concentrations for the period October 2005 January No. Sampling site Type Altitude (m) n oxygen-18 δ 18 O ( ) n tritium 3 H (TU) Mean TU 1 Ag. Marina Spring Ag. Andreas Spring Ag. Anna Spring Ag. Paraskevi Spring Epidavros Spring Kantia Borehole Pelei Spring Loukaiti Well Tsoukalia Spring Monastery Spring Fournoi Spring Chiafa Spring Arachnaio_1 Well Arachnaio_2 Rain tank Arachnaio_3 Rain tank The mean tritium composition of precipitation, as observed in sampling sites 14-15, indicates relatively high values in the area of 10TU. The tritium range in groundwater outflows points out that the source supply in most of the springs originates from a mixture between modern precipitation and low-tritium older groundwater [1]. The sampling sites of Group C and Group E are located around the foot of Mt. Arachnaio. Their stable isotopic content is different and is attributed to the altitude effect [5], so that Group E is recharged from higher altitudes than Group C. In addition, their tritium values confirm the short residence time that characterizes the area [6]. However, although sampling site 5 (Group B) lies also close to the 3

4 previous Groups, its stable isotopic composition is heavier than that of the adjacent sites, and may be attributed to local recharge from lower altitudes or to evaporation effect. Unlike the stable isotopic values in site 5, its tritium measurements follow those of both Groups C and E. Additionaly, the sampling sites 5 and 6 of Group B have similar δ 18 O values, even though they are found in very different locations. Sampling site 6 represents a borehole very close to the coast, but the chemical analysis (Table 3) shows no sign of sea water intrusion. This could mean that the heavy δ 18 O signatures of Group B are due to low height of recharge. Table 2.Classification of the samples according to their mean stable isotopic composition. Group A B C D E F G H Sites 8,11 5,6 1,2 7,9 3,4 10, ,15 Mean δ 18 O SD In the uplands of Mt. Arachnaio we find Groups G and H. Group H is represented by two sealed large manmade rain tanks and their isotopic composition approximates the mean annual stable isotopic values, which are the lowest in the studied area. At the same time, the tritium content shows the highest values and defines the mean tritium composition of the precipitation. In the vicinity of these rain tanks Group G is located, which is a shallow well and the δ 18 O measurements show a slight difference from those of the rain tanks. A probable cause could be the evaporation effect [2], which increases the stable isotopic concentration during the summer. On the other hand, tritium in this site lies within the value range of the rain tanks, enhancing the assumption that the shallow groundwater in the well is replenished mainly from the precipitation in Mt. Arachnaio. The Groups D and F present different stable isotopic values, with Group F values being more depleted than those from Group D, indicating again the influence of altitude effect [5]. However, the samples of Group D do not show similar tritium content, since site 7 contains tritium which oscillates between TU, while site 9 ranges between TU indicating that the groundwater residence time of the former is probably longer than the latter. Group s D tritium content is not comparable to the one from Group F that ranges between TU. Group A has the most positive stable isotopic content in the area, in contrast to the expected values based on the morphological structure of the surrounding environment. This effect in sampling site 11 can be attributed to local recharge, while in shallow well 8, it could be due to the evaporation effect. The tritium value range of site 11 could be attributed to the modern recharge, while this from site 8 reflects a mixture between old groundwater component and modern precipitation. The well is regularly pumped and the water is mainly used for the supply of local agricultural activities, especially during the summer season, where we observed a decrease in tritium content. It is likely that, due to the intense pumping, there is a significant runoff of stored water in the subterranean reservoir. Additional information on the isotopic precipitation content are required in order to determine the stable isotopic altitude gradient and to verify the observed tritium levels. Moreover, we consider that supplementary environmental isotopes (i.e. Deuterium) are necessary to comprehend more precisely the groundwater flow patterns in the peninsula Hydrochemistry Water chemistry data (Table 3) were processed using the program code AQUACHEM in order to determine properly the water types of the studied groups and we also constructed the representative Piper graph for January 2007 [Fig.2]. 4

5 Table 3.Mean chemical composition of groundwater and rainwater samples from Argolis peninsula. No. ph EC Ca 2+* Mg 2+* Na +* K +* HCO 3 * Cl * SO 4 2 * NO 3 * Values given in µs cm 1. * Values given in mg l 1. The majority of samples from the springs and wells/ boreholes contained a high proportion of HCO 3, reflecting mainly the weathering of the surrounding carbonate formations. Groups C, E and G showed a low concentration in magnesium leading to their classification in water type Ca-HCO 3. In Group B, site 5 is classified in water type Ca-Mg-HCO 3, while site 6 in water type Mg-Ca-HCO 3. Therefore, we presume that the borehole in sampling site 6 penetrates a different geological horizon than that of sampling site 5, richer in magnesium. Sampling sites 10 and 12 belong to the water type Ca-HCO 3, although site 10 was sometimes classified in water type Ca-Mg-HCO 3. Therefore, we can consider the same lithotype for Group F, but with higher magnesium content in water sample 12. The hydrochemistry of Group D is again dissimilar for the sites 7 and 9. The first has increased Mg concentration and low Ca value, while the second shows an increased Ca and a decreased Mg content. Consequently, the water types, Mg-Ca-HCO 3 and Ca-Mg-HCO 3 respectively, suggest a different water-rock interaction, with possible higher contribution of dolomite in the former. Moreover, the source of recharge may not be in the same region. The same goes for sites 8 and 11 of Group A. Groundwater of sample 11 seems to dissolve more magnesium carbonate minerals than sample 8. 5

6 FIG. 2. Representative Piper graph of the Argolis peninsula water sampling sites (January 2007). 5. Conclusions In the studied area, half of the springs are likely to be related to carbonate groundwater reservoirs around Arachnaio mountain range. The lack of significant stable isotopic seasonal variations in most of the springs indicates that precipitation flattens the isotopic composition of stored groundwater, while the different stable isotopic signatures between the sampling sites was attributed mainly to the altitude effect and the evaporation effect. Additionaly, the tritium content of the springs outlined a short residence time and a mixture between old groundwater component and modern precipitation. ACKNOWLEDGMENTS The authors would like to give special thanks to the Isotope Hydrology Laboratory, N.C.S.R. "DEMOKRITOS" (Athens, Greece), as well as to Greek State Scholarship Foundation (S.S.F.). REFERENCES [1] CELICO, P., et al, Environmental isotope studies of limestone aquifers in central Italy, Isotope Hydrology (Proc. Symp. Vienna 1983), IAEA (1984) [2] GONFIANTINI, R., et al., Hydrochemical and isotope study of lake Titicaca, Study of Environmental Change using Isotope Techniques (Proc. Conference C & S Papers Series 13/P Vienna, 2002), IAEA (2001) [3] LARSEN, D., et al., Hydrochemistry and isotope composition of springs in the Tecopa basin, southeastern California, USA, Chemical Geology including Isotope Geoscience 179 (2001) [4] IGME (Instiute of Geology and Mineral Exploration), Editing of geological map of scale 1: with data from geological maps of Greece, Athens. [5] SIEGENTHALER, U., et al., Isotopic and chemical investigations of springs from different karst zones in the Swiss Jura, Isotope Hydrology (Proc. Symp. Vienna 1983), IAEA (1984) [6] SUESS, H., E., Tritium Geophysics as an International Research Project, Science 163 (1969) [7] VASSILOPOULOU, S., Geodynamics of Argolis Peninsula with the G.I.S. development and use of remote sensing data, PhD Thesis, Univ. of Athens, Greece (1999). 6

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