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Seawater intrusion into groundwater aquifer through a coastal lake complex interaction characterised by 2

water isotopes H and

18

a

O b

b

Alexandra Gemitzi , Kyriakos Stefanopoulos , Marie Schmidt & Hans H. Richnow

b

a

Department of Environmental Engineering, School of Engineering, Democritus University of Thrace, Xanthi, Greece

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b

Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research – UFZ, Leipzig, Germany Published online: 20 Jan 2014.

To cite this article: Alexandra Gemitzi, Kyriakos Stefanopoulos, Marie Schmidt & Hans H. Richnow (2014) Seawater intrusion into groundwater aquifer through a coastal lake - complex interaction 2

18

characterised by water isotopes H and O, Isotopes in Environmental and Health Studies, 50:1, 74-87, DOI: 10.1080/10256016.2013.823960 To link to this article: http://dx.doi.org/10.1080/10256016.2013.823960

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Isotopes in Environmental and Health Studies, 2014 Vol. 50, No. 1, 74–87, http://dx.doi.org/10.1080/10256016.2013.823960

Seawater intrusion into groundwater aquifer through a coastal lake - complex interaction characterised by water isotopes 2 H and 18 O Downloaded by [Tulane University] at 10:19 31 December 2014

Alexandra Gemitzia *, Kyriakos Stefanopoulosa , Marie Schmidtb and Hans H. Richnowb a Department

of Environmental Engineering, School of Engineering, Democritus University of Thrace, Xanthi, Greece; b Department of Isotope Biogeochemistry, Helmholtz Centre for Environmental Research – UFZ, Leipzig, Germany (Received 27 March 2013; final version received 27 June 2013) The present study investigates the complex interactions among surface waters, groundwaters and a coastal lake in northeastern Greece, using their stable isotopic composition (δ 18 O, δ 2 H) in combination with hydrogeological and hydrochemical data. Seasonal and spatial trends of water isotopes were studied and revealed that all water bodies in the study area interact. It was also shown that the aquifer’s increased salinity is not due to fossil water from past geological periods, but is attributed to brackish lake water intrusion into the aquifer induced by the extensive groundwater pumping for irrigation purposes. Quantification of the contribution of the lake to the aquifer was achieved using the simple dilution formula. The isotopic signatures of the seawater and the groundwaters are considerably different, so there is a very little possibility of direct seawater intrusion into the aquifer. Keywords: coastal aquifers; groundwater/surface–water relations; hydrogen-2; oxygen-18; Vosvozis catchment area; water cycle

1.

Introduction

It is commonly accepted that world groundwater resources are facing various threats both from the quantitative as well as from the qualitative point of view [1,2]. Impacts on groundwater are directly caused by human activities [3] due to agriculture [4], urbanisation [5,6] and industrialisation [7,8]. During the last 20 years, groundwater modelling has been used as the main tool to model and predict the behaviour of groundwater systems under different scenarios of land uses and/or climate conditions. Two major types of groundwater models are used, i.e. deterministic or physically based models [9,10] and stochastic or non-physically based models [11,12]. Deterministic models require information on the hydraulic and geometric properties of the system in order to capture the conceptual character of the aquifer and to use it as a base for the deterministic model. In contrast, non-physically based models do not require the physical description of the system, and their results should be carefully analysed based on the knowledge of the physical system. This limits the robustness of stochastic models as physical processes are in most cases not integrated in computations. Hence, the knowledge of the conceptual character of an aquifer system is required prior to groundwater modelling. *Corresponding author. Email: [email protected] © 2013 Taylor & Francis

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Several studies have dealt with the determination of the conceptual models of complex aquifer systems and their interactions with surface waters using stable isotope data combined with hydrogeological information. Banks et al. [13] conducted a thorough study on surface water/groundwater interactions of a regional river catchment in South Australia. They showed that the river receives water not from the regional-fractured rock aquifer, but mainly from the shallow sedimentary perched aquifer. Seebach et al. [14] used stable isotopes to study the groundwater/lake water interactions at two sites located in Lusatia, Germany. Subyani [15] combined a chloridemass balance method with the isotopic compositions of oxygen and hydrogen in an alluvial aquifer in western Saudi Arabia to show that the groundwater is mainly of meteoric origin estimating a recharge rate of 11 % of the effective annual rainfall. Schmidt et al. [16] studied a coastal lagoon system in southern Brazil using stable isotopes and defined that precipitation is the main groundwater source in the permeable coastal barrier. Stable isotopes of hydrogen (2 H) and oxygen (18 O) have been widely used in groundwater research in order to investigate groundwater origin, flow, interactions with other water bodies and sources of groundwater salinity [15]. Girard et al. [17] focussed on the determination of the recharge mode of Sahelian aquifers using water isotopes. In their work, they used stable (2 H, 18 O) and radioactive (3 H) isotopes in two different basins to prove that the aquifers were recharged from nearby rivers and the flow regime of surface waters controls the recharge mode. In order to examine the groundwater origin and the circulation pattern of a multi-layer aquifer, Dassi [18] applied a multi-tracer approach including hydrochemical and isotopic signatures of the different aquifer layers. The results provided evidence of mixing between the upward-moving ‘old’ groundwater and the downward-moving ‘present-day’ groundwater. Wassenaar et al. [19] examined the isotope hydrology of precipitation, surface and groundwaters in the Okanagan valley in Canada. The stable isotopes 2 H and 18 O were used to assess the rainfall importance, the sources of water into and from the rivers and lakes in the basin and finally to evaluate the origin of the groundwater resources. Cartwright et al. [20] focussed on the interaction between saline lakes and shallow groundwater in southeast Australia using hydrochemical and isotopic analysis. They concluded that leakage from lakes is responsible for high levels of salinity in the groundwater. The basic aim of the present study is to elucidate the complex interactions among various water bodies in a Mediterranean coastal aquifer, i.e. the Neon Sidirochorion aquifer, through the determination of the stable isotope composition (δ 2 H and δ 18 O) of respective water samples. Moreover, the combination of the isotopic data with the existing hydrogeological and hydrochemical information provided a sound basis for the verification of the proposed conceptual model of the physical system. The present study builds on a previous work by Gemitzi and Stefanopoulos [21] concerning the interactions of the Neon Sidirochorion coastal aquifer (Figure 1) with surface water bodies and the sea. Therein, the authors applied time series analyses with external predictor variables and determined the parts of the aquifer that interact with external water bodies. In the present work, the results of the isotopic analyses in water samples from the water bodies occurring in the study area were used for the verification of the conceptual model for the Neon Sidirochorion coastal aquifer. Furthermore, quantification of the leakages of Ismarida coastal lake was achieved using the simple dilution formula.

2. 2.1.

Methods and materials Study area description

The study area (Figure 1) is located in northeastern Greece, in the coastal part of Rhodope regional unit, where Vosvozis River and Ismarida Lake are the main surface water resources. Ismarida Lake and its surrounding wetland area, located in the southern part of the study area, are protected by

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Figure 1.

Alexandra Gemitzi et al.

Location map of the study area with monitoring locations.

national and international treaties such as the Ramsar Convention of 1971 and the Natura 2000 network [22,23]. The Philiouris river (Figure 1) is another important river that has its estuaries in this protected wetland area, forming thus a deltaic system. However, Ismarida Lake is not connected to Philiouris river and they are regarded as two independent ecosystems. Ismarida Lake is artificially connected with a channel to the sea. This channel was constructed during the 1970s in order to drain the wetland area, as this was a common practice during those times. However, due to malfunction of the channel, sea water enters the lake during low flow periods, i.e. summertime, converting thus periodically an initially freshwater ecosystem to a brackish one. Therefore at the present times, Ismarida serves as a coastal lagoon, rather than a freshwater lake. The climate of the area is typically Mediterranean with hot dry summers. The mean annual precipitation is 628 mm, whereas the mean monthly temperatures range from 1◦ C (January) to 25◦ C (August) [21]. Cotton, corn, wheat and sugar beetroots are the main agricultural land-use types in the study area. Groundwater abstractions for irrigation are estimated in the range of 5 × 106 −8 × 106 m3 , taking place from May to September [21]. From the hydrogeological point of view, the area of interest is occupied by an aquifer formed within Quaternary deposits known as Neon Sidirochorion aquifer, of 50 – 100 m thickness (Figure 1). The aquifer is composed of intercalations of gravels and fine to coarse sands, overlain by a thin semi-confining silt layer of 2 – 4 m thickness [21]. Originally, the aquifer was regarded as a semi-confined one, nowadays, however, due to the excess pumping for irrigation, the groundwater level in most parts of the aquifer has dropped below the semi-confining unit, thus leading to the establishment of phreatic conditions. The coastal deltaic area, south of Ismarida Lake, is occupied by fine-grained alluvial deposits, whereas no important aquifer is found there. That is why any direct connection of the Neon Sidirochorion aquifer with the sea is doubtful and the main aquifer body is located to the north of Ismarida Lake (Figure 1). Detailed description

Isotopes in Environmental and Health Studies

77

Geological map of the study area Legend Quaternary al Plst

Holocene sediments Pleistocene sediments

Tertiary pl

Pliocene sediments

Ec-m

Tertiary molasic rocks

ign

Tertiary igneous rocks

Mesozoic

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Mz

Marbles and phyllites

Major faults

Figure 2.

Geological map of the study area.

of the hydrogeology of this aquifer can be found in previous works [21] (Figure 2). The main hydrochemical feature that characterises groundwaters in the study area is the increased electrical conductivity (EC), which demonstrates spatial and temporal variation. In the southern part of the aquifer and close to Ismarida Lake, EC values show high temporal variability and are in good correlation with the salinity levels observed in Ismarida Lake [21]. The central part of the aquifer shows constantly increased EC values, ranging from 2500 to 3000 μS cm−1 or even higher throughout the year. In the northernmost part of the aquifer, the EC values remain low, reaching up to 1025 μS cm−1 . In order to clarify interactions between various water bodies with the Neon Sidirochorion aquifer, Gemitzi and Stefanopoulos [21] applied time series analyses with external predictors. They assumed that the groundwater level is influenced by both groundwater abstractions (human intervention) and meteorological conditions. Thus, they simulated groundwater level in eight groundwater monitoring locations, while external predictors were incorporated in the form of abstractions for irrigation, precipitation, temperature and evapotranspiration. Their results showed that the southern part of the aquifer clearly interacts with the surface waters of the coastal wetland area and is less influenced by abstractions. The central part of the aquifer shows a mixed dependence both on groundwater abstractions and meteorological conditions, whereas the northern part is mostly dependent on human abstractions. As Ismarida Lake is an important ecosystem constantly decreasing both in volume and in extent [21], it is considered of great importance to clarify its dependence on groundwaters. Moreover, although previous studies [24] attribute the increased salinity of the aquifer to perched water from past marine transgressions, it was necessary to examine possible interactions with the wetland area of Ismarida Lake by conducting isotopic analyses of all water bodies present in the area. 2.2. Sampling and analytical procedures Sampling for isotopic determination was performed during 2010 on a seasonal basis. In order to examine seasonal differences between winter and summer periods, samples collected from December to May were classified as representative of the winter isotopic composition, whereas samples from June to November were those representing summer isotopic composition. Fifteen sampling locations were selected, i.e. nine groundwater sampling locations, two locations in Vosvozis River, one seawater sampling point and three sampling points in Ismarida Lake, as shown

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Alexandra Gemitzi et al.

in Figure 1. Additionally, one rainwater sample was collected in October 2010 from the area and was analysed for stable isotopes. All samples were collected in clean 100-ml polyethylene bottles first rinsed with water from the sampling point and then tightly capped. EC was measured in the field using a portable HQ40d multimeter (Hach Lange, Shalford, England). Samples were stored at 4◦ C prior to analysis. Lake and seawater samples were collected using an Eijkelkamp 250 ml stainless steel sampling bailer. Seawater was sampled approximately 100 m from the coastline and at a depth of 50 cm below surface. Since the maximum depth of Ismarida Lake is always less than 1.5 m, samples were collected at a depth of 50 cm below surface. The specified sampling locations within Ismarida Lake, i.e. L1 (inlet), L2 (centre) and L3 (oulet) were selected so as to study the mixing process between seawater and fresh water within the lake, and they were based on the EC variations observed especially during the summer period at those locations. Groundwater samples were collected from irrigation boreholes, screened in multiple depths. Therefore, samples represent a mixture of groundwater over the entire aquifer depth. Thus, no analysis can be achieved related to the variations of isotopic composition with depth. Groundwater samples were collected after purging each well so that at least three well volumes of water were removed from the wells prior to sampling. Stable isotope ratios (2 H/H and 18 O/16 O) were determined by isotope ratio mass spectrometry according to the following procedure. Briefly, for the analysis of the stable isotopes of water (2 H,18 O), 1 mg of activated carbon was added to the water sample and the dissolved organic material was removed while shaking (10 min). Then the activated carbon was removed by sedimentation. Hydrogen and oxygen isotope ratios were analysed using an elemental analyser (EA1108, CE Instruments, Italy) with a high-temperature furnace (Hekatech, Wegberg, Germany) allowing a pyrolysis at a temperature of 1550◦ C. The EA unit was coupled to a Finnigan MAT 253 Stable Isotope Ratio Mass Spectrometer (Thermo Fisher Scientific, Bremen, Germany). Samples were pyrolysed in the presence of nickelised graphite (approximately 10 % Ni, carbon powder, Merck, Darmstadt, Germany, mixed with nickel powder, 100 mesh, Aldrich, Steinheim, Germany). The method is described in detail by Kornexl et al., [25] Gehre and Strauch [26], and Gehre et al. [27]. The hydrogen and oxygen isotope ratios of the water were reported in the delta notation (δ 2 H, δ 18 O) in per mil(‰) units according to Equation (1) calibrated to Vienna Standard Mean Ocean Water (VSMOW) as a reference [28].   Rsample − Rstandard 18 2 δ Osample or δ Hsample [‰] = 1000. (1) Rstandard The analytical precision is ±2 ‰ for δ 2 H and ±0.3 ‰ for δ 18 O.

3.

Results and discussion

3.1. Origin of the water bodies The isotopic values of δ 18 O and δ 2 H measured in the study are presented in Table 1. Figure 3(a) and 3(b) shows scatter plots of δ 18 O versus δ 2 H of all samples during winter and summer, respectively, compared with the Local Meteoric Water Line (LMWL) and Vienna Global Meteoric Water Line (VGMWL). As only one measurement of 18 O and 2 H in precipitation was available, the estimated LMWL for Greece defined as δ 2 H = 8.7δ 18 O + 19.5 was used [29]. TheVGMWL is the one provided by the International Atomic Energy Agency [30] and is defined as δ 2 H = 8δ 18 O + 10. In general, all samples seem to have a better fit to the VGMWL than to the LMWL. This can be explained by fact that the LMWL used herein [29] is based on data for the whole Greece, whereas no precipitation data from the Thrace area were incorporated. Second, our area is highly

Table 1.

Stable isotopes, salinity and EC in water samples from the study area during 2010.

Vosvozis River

Ismarida Lake

Date

(‰ VSMOW)

B3

–

B5

L1 L2 L3

Groundwater samples

A1 A2 A6 A9 A10 A20 GA GE AB

(‰ VSMOW)

Salinity (mg l−1 )

Conductivity (μS cm−1 )

–

–

–

–

17/02/2010 29/03/2010 29/04/2010 05/05/2010 10/05/2010

−6.5 −6.5 −6.4 −6.6 −6.4

−40.1 −40.2 −39.2 −39.9 −39.3

141.1 208.0 328.3 361.4 383.5

217 320 505 556 590

30/03/2010 30/04/2010 30/03/2010 29/03/2010 30/03/2010 29/04/2010 05/05/2010 10/05/2010

−6.6 −6.1 −6.6 −5.2 −6.5 −6.2 −5.9 −5.4

−40.0 −37.9 −40.0 −33.4 −39.7 −38.9 −37.0 −35.1

257.4 324.4 223.0 215.2 215.2 328.9 335.4 394.6

396 499 343 331 331 506 516 607

– 14/05/2010 14/05/2010 14/05/2010 14/05/2010 14/05/2010 – – – –

– −6.2 −6.2 −6.4 −5.9 −6.5 – – – –

– −40.7 −40.7 −39.4 −37.4 −40.4 – – – –

– 651.3 651.3 1586.0 2548.0 685.8 – – – –

– 1002 1002 2440 3920 1055 – – – –

δ 18 O Date

(‰ VSMOW)

δ 18 O (‰ VSMOW)

01/06/2010 30/06/2010 21/07/2010 01/06/2010 14/06/2010 30/06/2010 21/07/2010 25/08/2010 21/09/2010 21/10/2010 03/11/2010 28/07/2010 02/09/2010 02/09/2010 01/06/2010 14/06/2010 21/07/2010 28/07/2010 25/08/2010 02/09/2010 21/09/2010 21/10/2010 03/11/2010 08/11/2010 08/11/2010 – – – – – 13/06/2010 13/06/2010 03/11/2010 18/08/2010 27/10/2010

−6.5 −6.3 −6.2 −6.3 −6.3 −6.1 −6.1 −5.4 −5.9 −7.1 −7.1 −4.3 −2.3 −2.2 −5.2 −4.4 −4.2 −4.3 −2.7 −1.6 −0.4 −3.1 −4.8 −4.9 −6.2 – – – – – −5.3 −6.2 −6.6 0.4 −4.9

−39.8 −38.3 −38.6 −38.5 −39.2 −37.5 −37.6 −35.2 −36.8 −44.3 −43.6 −29.3 −19.7 −19.6 −33.3 −30.3 −29.5 −29.9 −22.0 −15.5 −8.6 −23.2 −32.2 −32.6 −38.5 – – – – – −36.0 −39.6 −40.3 −0.1 −32.6

Salinity (mg l−1 )

Conductivity (μS cm−1 )

486.9 384.8 416.7 471.9 459.6 444.6 437.5 496.0 529.1 210.0 243.1 285.4 643.5 1027.0 438.8 384.8 392.0 272.4 1170.0 884.0 14495.0 6435.0 2990.0 2730.0 1378.0 – – – – – 1119.3 709.8 529.8 27040.0

749 592 641 726 707 684 673 763 814 323 374 439 990 1580 675 592 603 419 1800 1360 22300 9900 4600 4200 2120 – – – – – 1722 1092 815 41600

79

Seawater Rainwater

Summer δ2 H

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Winter δ 18 O

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Alexandra Gemitzi et al.

10

δ2H (‰VSMOW)

(b) 20

10

δ2H (‰VSMOW)

(a) 20 0 -10 -20 -30 -40 -50

-20 -30 -40 -50

-60 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 δ18Ο (‰VSMOW)

0.0

1.0

-60 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 δ18Ο (‰VSMOW)

GMWL

Vosvozis River

Groundwater Samles

Rain Water

LMWL

Ismarida Lake

Seawater

Mixing lines

Figure 3. Scatter plot between summer 2010.

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0 -10

δ2 H

and

δ 18 O

1.0

in various water bodies of the study area during (a) winter 2010, and (b)

influenced by the Rhodope mountain chain (which forms the Greek and Bulgarian border), and therefore some deviations from the Greek LMWL are expected due to the local climate, and this is also evidenced by the plot of rain water sample (Figure 3) much closer to the VGMWL than to the LMWL. Differences in the isotopic composition of the samples from the different water bodies are observed between winter and summer. During winter, surface and groundwater samples are plotted very close to the VGMWL, indicating thus a common isotopic composition of both surface and groundwaters. The deviation of seawater from the VGMWL during summer indicates that seawater is enriched in heavy isotopes due to evaporation processes. At the same period, Ismarida’s Lake water moves clearly towards seawater composition below the VGMWL, indicating seawater intrusion that is known to take place through the artificial channel as well as the evaporation effects. On the contrary, relative to the VGMWL, both Vosvozis River water as well as groundwater do not alter significantly their position throughout the year. The isotopic composition of Vosvozis River water is characteristic of a freshwater body [31]. Regarding the isotopic composition of groundwaters, it can be explained as a result of mixing processes that are taking place, considering also the alterations of EC values throughout the year. Figure 4(a) and 4(b) shows the scatter plots of salinity versus δ 18 O for winter and summer, respectively. The salinity of water of Ismarida Lake during summer increases clearly towards seawater (Figure 4(b)), suggesting mixing with seawater in the lake during this period. Mixing of seawater with Ismarida Lake is also suggested by the mixing line shown in Figure 3(b). In Figures 3(a) and 4(a) it is evident that Ismarida Lake, Vosvozis River and the Neon Sidirochorion aquifer demonstrate a similar isotopic composition, indicating the possibility of mixing between them. Taking also into account the hydrogeological information acquired by previous works [21] showing that the piezometric level of the Neon Sidirochorion aquifer is always below the water level of Ismarida Lake and that groundwater salinity variations are in correlation with those observed in Ismarida Lake, it is concluded that Ismarida Lake recharges the adjacent aquifer. Additionally, the hypothesis that is expressed in previous works for the study area [24] that groundwater salinity due to fossil water from past geological transgressions seems to be invalid as the isotopic composition of groundwaters does not correspond to fossil water. Furthermore, river flow measurements conducted in Vosvozis River during 2010 indicated river discharge to the aquifer. This is also evidenced by the similar isotopic composition of river water and groundwater. Finally, another source of water seems to be precipitation, which is implied by the clustering of river water and groundwater almost on the VGMWL (Figure 3(a) and 3(b)). This is also demonstrated in Figure 4(c) where the deuterium excess (d-excess) for the various water bodies is plotted. In those figures besides the similar d-excess values with those reported in precipitation values [32], the characteristic decrease in dexcess values during the summer in Vosvozis River, groundwater and Ismarida Lake implies that

Isotopes in Environmental and Health Studies

(b)

(a) 10000

10000 Salinity(mg-l)

100000

Salinity(mg-l)

100000

1000 100

1000 100

10

10

1 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 δ18O

d-excess(‰)

(c)

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16 14 12 10 8 6 4 2 0 -2 -4

0.0

1 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0

1.0

1.0

δ18O

Vosvozis River Ismarida Lake

GW samples Seawater

Feb

Mar

April

May

Jun

Jul

Aug

Sept

Oct

Nov

Month

Figure 4. Scatter plots of salinity versus δ 18 O from various water bodies for (a) winter 2010, (b) summer 2010 and (c) the d-excess.

(a)

Figure 5.

(b)

Spatial distribution of (a) δ 18 O (‰ VSMOW) and (b) δ 2 H (‰ VSMOW) in groundwaters during 2010.

those water bodies are also recharged by precipitation. However, during summertime, Ismarida Lake demonstrates the effects of evaporation which are also observed in the seawater. In order to identify the spatial distribution of groundwater isotopic composition in the study area, δ 2 H, δ 18 O and the calculated d-excess values are plotted in Figures 5(a) and 5(b) and 6, respectively. Winter isotopic parameters are shown as contour lines produced after interpolating the point monitoring data in order to produce a grid, where contours were extracted from. All spatial calculations were performed using the Geographical Information System (GIS) software Mapinfo Professional v. 10. Interpolation was conducted using kriging interpolation formula.

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Figure 6.

Alexandra Gemitzi et al.

Spatial distribution of the d-excess in groundwaters during 2010.

Different models were fitted on the experimental variograms for each interpolated parameter. Regarding δ 2 H, a quadratic model was used a range at 2942 m and sill at 0.2. δ 18 O was interpolated using a power model with range at 0.5938 m and a sill at 0.01. An exponential model was used for d-excess interpolation with range at 2.604 m and sill at 1. As there were only two groundwater monitoring locations during summer, no such processing could take place, thus those locations and their corresponding values are plotted on Figures 5(a) and 5(b) and 6 as black triangles, in order to compare their values to the winter contours. In general, during winter season isotopic composition is enriched close to the Neon Sidirochorion, i.e. increased values of δ 2 H and δ 18 O were observed in that area. Decreased values are found close to Vosvozis River. This pattern of increased stable isotopes during winter observed close to Neon Sidirochorion is in contrast to the general pattern observed in precipitation in Eurasia [32] and with the observed values of surface waters in the study area (Table 1). It may be attributed to the fact that close to Vosvozis River the aquifer is recharged by river water and precipitation during this period, whereas close to Neon Sidirochorion the aquifer seems to have lower inputs from the river or from precipitation leading to the conclusion that is very probable that intruded lake water reaches this part of the aquifer during winter. With regard to the d-excess (Figure 6), lower values were determined during winter in the area close to Neon Sidirochorion and higher values close to Vosvozis River. Regarding EC distribution in the groundwaters, increased values were observed close to the Neon Sidirochorion area during December and sufficiently lower values were determined during May (Figure 7). Taking into account the fact that in Ismarida Lake extremely high EC values (up to 22,300 μS cm−1 ) were measured in late August and September due to seawater intrusion, it can be concluded that increased EC values in the Neon Sidirochorion groundwaters were determined

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Figure 7.

83

EC of groundwaters during (a) May 2010 and (b) December 2010.

North South Neon Sidirochorion

Ismarida lake

Sea

Aquifer

Semi confining unit Piezometric level

Figure 8.

Fresh water / Salt water interface

Conceptual model of the Neon Sidirochorion aquifer.

due to lake water intrusion. The water intrudes the aquifer during summer as a result of the increased pumping in the aquifer and reaches the Neon Sidirochorion area with a lag of 2–3 months, based on the peak EC values observed at this location during December. The fact that Ismarida Lake recharges the aquifer is in agreement with the findings of the work previously conducted by Gemitzi and Stefanopoulos [21] and is also supported by the findings of the present work. Therefore, salt water in the study aquifer cannot be regarded as fossil water from past geological periods. Close to Vosvozis River, the isotopic composition of groundwaters as well as the low EC values imply interaction of river waters with groundwaters. Direct interaction of the aquifer with the sea, although it cannot be excluded, seems quite improbable, as the isotopic composition of groundwaters compared with seawater is different, and the most likely scenario is that of seawater contributing to the aquifer indirectly through Ismarida Lake. The conceptual model indicated by the results of the present study is shown in Figure 8. 3.2. Quantification of Ismarida Lake’s losses to the groundwater system In order to quantify the contribution of Ismarida Lake to the aquifer, two possible options were examined. The first one involves the application of end-member mixing analysis (EMMA) [33,34]. In the present case, however, the application of EMMA could lead to peculiar results, since one

84

Alexandra Gemitzi et al.

of the end-members, i.e. Ismarida Lake, shows great seasonal variability regarding the isotopic composition of its waters. The second option involves the determination of a dilution formula, concerning the salinity of the various interacting water bodies. For this purpose, the aquifer body is considered as a reservoir of known salinity throughout the year, which is recharged by water of various sources and provides water for abstraction. Since the salinity of all other water bodies is also measured throughout the year, the dilution equation formed for a whole year is provided as follows. Here, the annual volume of water which Ismarida Lake contributes to the Neon Sidirochorion aquifer is calculated: Vlake =

CFinal V Final θ + CAbstr VAbstr − CRiver VRiver − CRain VRain − CInitial VInitial θ , CLake

(2)

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where VLake CInitial VInitial  CFinal VFinal CRain VRain CRiver VRiver CAbstr VAbstr CLake

(m3 ) (μS cm−1 ) (m3 ) (%) (μS cm−1 ) (m3 ) (μS cm−1 ) (m3 ) (μS cm−1 ) (m3 ) (μS cm−1 ) (m3 ) (μS cm−1 )

= = = = = = = = = = = = =

Volume of water which the lake contributes to the aquifer during 2010 EC of initial groundwater (January 2010) Volume of initial groundwater (January 2010) Aquifer porosity EC of final groundwater (January 2011) Volume of final groundwater (January 2011) Mean EC of rain Rain volume that contributes to the aquifer during 2010 Mean EC of river water during 2010 Volume of river water contributing to the aquifer during 2010 Mean EC of abstracted groundwater during 2010 Volume of groundwater abstractions during 2010 Mean EC of lake water during 2010

The various parameters in Equation (2) were either directly measured or implicitly estimated and are provided in Table 2. Due to the high uncertainty concerning the estimated parameters in our analysis, we used a range of values rather than a value. The initial and final groundwater volume were calculated from piezometric measurements conducted in January 2010 and January 2011 on 20 groundwater monitoring points. The groundwater volume was estimated from piezometric surface as in the majority of the 20 monitoring sites, phreatic conditions prevailed and the groundwater level was lower than the thin semi-confining unit described above. Initial and final EC of groundwater was measured directly in January 2010 and 2011 in five locations (Figure 7). Aquifer porosity was implicitly estimated from the literature [35–37] and was assigned a range of values between 0.1 and 0.25, typical for Quaternary sands and gravels. The mean EC of rainwater was measured directly quarterly during 2010 and its mean value was used, i.e. 35 μS cm−1 . The rain volume which contributes to the aquifer was assigned a range of 10 – 25 % of rain volume, which is a realistic range of percentages for the Mediterranean areas. In a previous work, Pisinaras et al. [24] assigned much lower values of rain contribution to the aquifer. However, assigning percolation values lower than 10 % leads to unrealistically high values of river and lake water contributions. The river contribution to the aquifer was estimated by river flow measurements in points B3 and B5 on a weekly basis throughout 2010 (Figure 1). River flow was measured using a mechanical flow metre OSS-B1 10-2 (Hydrological Services, Australia). The contribution of river water to the aquifer was estimated by the differences calculated in the discharges between points B3 and B5 (Table 2). The EC of river water and lake water was directly measured on a weekly basis, and the mean values of each of those two parameters during 2010 were used in the computations (Table 2). Groundwater abstractions were estimated in a previous study [21] and they were also used in the present work. In particular, 93 pumping boreholes were detected in the study area, with a mean discharge during summer up to 30 m3 h−1 .

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Table 2. Values of various parameters involved in the estimation of lake water contribution to the aquifer. Parameter

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VInitial VFinal Mean groundwater level Jan 2010 Mean groundwater level Jan 2011 CInitial CFinal  VRain CRain VRiver CRiver VAbstr CAbstr CLake

Value 9,494,760.5 m3 (at porosity of 20 %) 8,946,105.5 m3 (at porosity of 20 %) −7.3 m relative to mean sea level −8.1 m relative to mean sea level 2,185 μS cm−1 2,332 μS cm−1 10–25 % 2,260,800 m3 yr−1 (71.69 l s−1 ) (10 % percolation) −5, 652, 000 m3 yr−1 (79.22 l s−1 ) (25 % percolation) 35 μS cm−1 1,304,225 m3 yr−1 or 41.36 l s−1 564 μS cm−1 6,500,000 m3 yr−1 or 206.11 l s−1 2,787 μS cm−1 4,391.8 μS cm−1

Based on the computations performed using Equation (2), the quantity of Ismarida Lake’s water contributing to the aquifer (VLake ) is estimated to range between 3,912,355 and 3,939,360 m3 or from 124 to 125 l s−1 according to the range of values assigned for porosity and percolation. This quantity is much higher than all other sources of water for the Neon Sidirochorion aquifer. Moreover, the fact that Ismarida Lake and its surrounding wetland area are facing the threat of both quantitative and qualitative degradation [21] shows that Ismarida Lake cannot sustain much longer the pressures involved with the current groundwater abstractions. Furthermore, unlike previous studies that neglected the interaction of Ismarida Lake with the Neon Sidirochrion aquifer, the present work supports the assumption that the main source of salinity for groundwaters in the study aquifer is Ismarida Lake. 4.

Conclusions

Interactions among surface waters, groundwaters and seawater were examined by means of stable isotope analyses. The results showed that the study aquifer interacts with Ismarida Lake and Vosvozis River, whereas the hypothesis of direct interaction with the sea was not supported by the available data. The spatial and temporal trends of the isotopic composition and EC of the groundwaters revealed that lake water contributes to the Neon Sidirochorion aquifer. This process is mainly induced through intense pumping of aquifer water, leading thus to excessive groundwater level drawdown during summer and resulting in increased salinity of the aquifer. The fact that the groundwater-dependent ecosystem of Ismarida Lake decreases constantly indicates that it cannot sustain the provision of water to Neon Sidirochorion aquifer. Therefore, it is considered of urgent importance to control the pumping quantities from the aquifer minimising water losses from the lake. This may well be achieved by changing the irrigation systems from sprays and sprinklers currently used to more efficient ones such as drips. Additionally, the local authorities should provide all the necessary measures that will control seawater intrusion into the lake, redesigning the opening channel to the sea. In that way Ismarida Lake will regain its initial freshwater nature and the aquifer will not be threatened from salt water intrusion. Acknowledgements This work is developed within the frames of the European FP7 project GENESIS (Groundwater and Dependent Ecosystems: New Scientific and Technical Basis for Assessing Climate Change and Land-use Impacts on Groundwater Systems).

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Grant agreement No.: 226536–GENESIS CP-IP. The isotopic analyses were conducted in the Department of Isotope Biogeochemistry of Helmholtz Centre for Environmental Research – UFZ, Leipzig, Germany. We thank Falk Bratfisch and Ursula Günther from the Stable Isotope Laboratory of the UFZ for providing isotope analysis. We would also like to thank the two anonymous reviewers for their valuable comments.

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