Origin of shallow groundwater of Csepel Island (south of Budapest, Hungary, River Danube): isotopic and chemical approach

HYDROLOGICAL PROCESSESHydrol. Process. 19, 3299–3312 (2005)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5971

Origin of shallow groundwater of Csepel Island (south ofBudapest, Hungary, River Danube): isotopic and

chemical approach

I. Forizs,1* T. Berecz,2 Z. Molnar3 and M. Suveges4

1 Institute for Geochemical Research, Hungarian Academy of Sciences, Budaorsi ut 45, H-1112 Budapest, Hungary2 Department of Applied and Environmental Geology, Eotvos University, Budapest, Hungary

3 Vizimolnar Ltd, Pazmand, Hungary4 Water Resources Research Centre (VITUKI) plc., Budapest, Hungary

Abstract:

The shallowest aquifer of the alluvial Csepel Island is used for drinking water supply for Budapest and thesurrounding villages. A previous two-dimensional hydraulic model failed to explain many observations; therefore,a three-dimensional hydraulic model was constructed using the software application MODFLOW. Isotopic andhydrogeochemical investigations were carried out during the period 1998 to 2002 in order to provide model data.Four water sources have been identified, having different isotopic (and chemical) characteristics. Danube water ischaracterized by seasonally varying υ18O values: its unweighted mean υ18O value is �10Ð9‰ for the years 1998 to2002. The most negative υ18O values occur in late spring and summertime. Average tritium content is 20 TU. Infiltratedprecipitation has υ18O values between �9Ð0 and �9Ð5‰. The average tritium content of the precipitation is 10 TU,and that of infiltrated precipitation must be close to this value and about half that of the Danube. The υ18O value oflake water is > � 9Ð0‰. Its stable isotope composition plots on the right side of the local meteoric water line on theυD–υ18O diagram. Ascending deep groundwater (sampled depth of 20–140 m) has υ18O values between �12Ð0 and�13Ð5‰. Its radiocarbon age is between 16 000 and 30 000 years BP, probably infiltrated during and around the lastglacial maximum (Wurm III) period of the latest glaciation. Copyright 2005 John Wiley & Sons, Ltd.

KEY WORDS Danube; Csepel Island; bank filtration; stable and radioisotopes; infiltration; groundwater; lake water

INTRODUCTION

The drinking water for the Hungarian capital, Budapest, is provided by bank filtering wells of riparianaquifer systems of the River Danube. The majority of drinking water comes from Szentendre Island (northof Budapest) because of its high water quality (Figure 1), but, for a strategic reason, a small amount comesfrom Csepel Island (south of Budapest, Figure 1).

The aquifer system on Csepel Island is highly vulnerable to pollution coming either from the Danubeor from shallow groundwater. Budapest contaminates the Danube water continuously with communal andindustrial wastewater (about 0Ð5 ð 106 m3 day�1). The study area is industrialized: a highway with veryheavy traffic crosses the island and there is also agricultural activity. The contamination from these sourcescan have an effect on the quality of bank-filtered water. In the 1990s, a (horizontal) two-dimensional (2D)hydraulic model of the flow system of the area of interest was constructed (Molnar, 1997). The conditionsused for this 2D model were: hydraulic data (water level of the River Danube and in some monitoringwells) and multi-annual average of infiltration from precipitation; no upward flux from deep aquifers, nohorizontal flux beneath the Rivers Danube and Rackeve–Danube. Although this model described the majority

* Correspondence to: I. Forizs, Institute for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, Budaorsi ut 45,Hungary. E-mail: [email protected]

Received 19 July 2003Copyright 2005 John Wiley & Sons, Ltd. Accepted 19 December 2004

3300 I. FORIZS ET AL.

BUDAPEST

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pel I

slan

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0 5 10 15 km

SzentendreIsland

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A

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keve

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Figure 1. Map of Hungary. The area of Budapest and the two alluvial islands is magnified

of observed hydraulic and chemical phenomena well, in some cases it failed to explain the movement ofnitrate or chloride concentrations, and in one case (Forizs et al., 1999) there was a discrepancy between theorigin of water inferred from υ18O values and from the direction of water movement determined by this 2Dhydraulic model. For a better reconstruction of the water flow system, a three-dimensional hydraulic modelis planned using the groundwater modelling application MODFLOW. Hydraulic, water chemical and isotopicdata obtained over a 4 or 5 year period will be incorporated into the model or will be used for verification ofthe model.

As part of this project, the origin of all water sources has been identified by means of isotope hydrogeo-chemical research and these results are presented in this paper.

HYDROGEOLOGY OF CSEPEL ISLAND

Csepel Island is part of the modern Danube valley and located in the northwestern corner of the GreatHungarian Plain, where it meets with the karst reservoir of the Transdanubian Central Range. The island ischaracterized by a rather flat floodplain landscape with an average elevation of about 100 m above BalticSea level. The base of the aquitard of the Quaternary aquifer consists of alternating layers of Pliocene andUpper Pannonian clay, silt and sand. On the western bank of the Danube, Eocene limestone deposits formsteep slopes. This area was formed and banked up by the Danube during the Quaternary (in the Pleistoceneand Holocene). The direction of flow of the river continuously shifted from the northwest–southeast to thepresent north–south direction. Aquifers are located in Quaternary deposits, which consist of Pleistocene andHolocene gravel and sand, silt and clay. The total thickness of these formations is 10–20 m in the north, butit is greater towards the east and south. The aquifer consists of coarse-grained sediments (gravel and sand)with a thickness of 5 to 20 m. The horizontal conductivity of gravel aquifer ranges from 10 m day�1 fordeposits made up of moderately coarse sands to more than 200 m day�1 for some gravelly deposits.

The shallow riparian flowing system is under the influence of the water level of both branches of theDanube (the water level is continuously changing in the main branch of the river, but close to constant in theRackeve–Danube), the background shallow groundwater of the island and the rate of water exploitation.

Copyright 2005 John Wiley & Sons, Ltd. Hydrol. Process. 19, 3299–3312 (2005)

ORIGIN OF SHALLOW GROUNDWATER: ISOTOPIC STUDY 3301

Based on some earlier hydrogeological papers (Erdelyi, 1979; Ronai, 1985), this area can be part of thedischarge area of an intermediate flow system (Toth, 1963) with recharge area on the land of the sand-ridge inthe Danube–Tisza interfluve. At the same time, it can also be a discharge area of some karstic flow systems ofthe Transdanubian Central Range (Vendel and Kishazi, 1963; Lorberer, 1997; or see cover page of Episodes17(4), 1994) as well.

During the recent 20–30 years three main elements, i.e. shortage of rain, increased water exploitation andchanges in land use, have caused a decrease in the groundwater level. On some areas this decrease is as greatas 10 m.

Both the Danube water and the shallow groundwater is generally of the Ca–Mg–HCO3 type.

SAMPLING AND METHODS APPLIED

Sampling

The River Danube at the northern edge of Budapest was sampled continuously during the 5 year periodwith varying frequency for stable oxygen isotope measurements. In wintertime, when the υ18O value varieslittle, the Danube was sampled one to three times a week. In other parts of the year it was sampled threetimes a week or at a certain period of time every day.

Selected production wells next to Halasztelek (Figure 2: H4, H5, H13, H14, H15) were sampled fordetermination of the υ18O value at the beginning of the study once every 2 weeks, and subsequently once aweek.

On the northern part of Csepel Island (Figure 2) the monitoring wells were sampled once every quarter yearduring a 3 week campaign, sampling primarily for oxygen isotope and water quality measurements. A fewmonitoring wells were also sampled for tritium content. Monitoring wells along the M0 highway (M01–M09)and Kavicsos Lake (an artificial lake created by mining) were also sampled for stable oxygen and hydrogenisotope measurements in an extra campaign.

Deep groundwater from the depth range of 30–400 m was sampled for oxygen isotopic composition andchemistry; additionally, five wells were sampled for radiocarbon age determination (14C, υ13C).

Methods

Stable oxygen and hydrogen isotope measurements were carried out in the Institute for GeochemicalResearch of the Hungarian Academy of Sciences (IGCR-HAS) using the principle for sample preparationdescribed by Epstein and Mayeda (1953) in a water–CO2 equilibration device described by Roether (1970)for oxygen and the conventional zinc reduction method using ampoules (Coleman et al., 1982). Measurementswere made on a Finnigan MAT delta S gas-isotope-ratio mass spectrometer. Isotope ratios of oxygen andhydrogen are expressed by the conventional υ notation:

υ18O�or υD��‰� D �Rsample/Rstandard � 1� ð 1000

where Rsample and Rstandard are isotope ratios (18O/16O or 2H/1H) in the sample and standard (Vienna standardmean ocean water, VSMOW) respectively. Uncertainties are š0Ð1‰ for υ18O and š1Ð0‰ for υD.

In five samples the total dissolved carbonate was precipitated as barium carbonate. The 14C concentration ofthe precipitated barium carbonate was determined in the Environmental Radioactivity Laboratory of the WaterResources Research Centre plc. (Budapest) by means of scintillation counting, and the results are expressed asper cent modern carbon (pmC). For the determination of groundwater age, the 14C concentration of dissolvedinorganic carbon precipitated as barium carbonate was determined. A correction for the dissolution of carbonatewith no 14C was made using the 13C/12C of barium carbonate, which was measured at IGCR-HAS usingconventional phosphoric acid digestion to liberate CO2 from carbonate (Demeny and Forizs, 1991).



Figure 2. The study area: northern part of Csepel Island

Tritium measurements were made in the above-mentioned Environmental Radioactivity Laboratory byscintillation counting (after enrichment), and data are expressed as tritium units (1 TU is one tritium atomamong 1018 hydrogen atoms).

RESULTS

Using mostly isotope data, and less frequently water chemical data, four sources of groundwater have beenidentified in the aquifers of Csepel Island. In the following, these water types are characterized and theirdistribution is described.

Danube water

The River Danube splits into two branches around the north–south-extending Csepel Island (Figures 1and 2): the western branch is the main branch, carrying the majority of the river water, and is called the‘Danube’; the eastern branch is a small-capacity branch called the ‘Soroksar–Danube’ or ‘Rackeve–Danube’(Figure 2).

The υ18O time series of the Danube water at Budapest (at the northern edge of the city) is shown in Figure 3.The Danube water, as with other river waters, comes from surface runoff and discharging groundwater. Duringwinter, the daily mean air temperature on the watershed up-stream from Budapest is below 0 °C and Danube



-12.5

-12.0

-11.5

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-10.0

1997Dec

1998Jun

1998Dec

1999Jun

1999Dec

2000Jun

2000Dec

2001Jun

2001Dec

2002Jun

2002Dec

Date

δ18O

[‰] V

SM

OW

DanubeH5

Figure 3. Oxygen isotope time series of the River Danube and the H5 production well from 1998 to 2002

water comes only from groundwater discharge. It has a υ18O value of about �10Ð4 to �10Ð8‰ (Figure 3).During winter, the υ18O of Danube water varies little. At the end of spring and during summer, when winter-precipitated snow in the high Alps melts, the υ18O value of the Danube decreases substantially (Figure 3),with υ18O values reaching �11Ð5 to �12Ð5‰. These low υ18O values are not simply due to water from a singlesource, but are comprised of water from at least two sources. The υ18O value of spring or summer rain isconsiderably more positive than that of the Danube during the spring or summer. If precipitation is heavy andthe contribution of this rain is comparable to the amount of Danube water, then the Danube water becomesenriched in 18O, e.g. see the superimposed increase in υ18O (positive peak) at the May–June boundary in1999 on the middle of the negative peak (Figure 3).

When Danube water infiltrates into the gravel aquifers, this seasonal variation of the υ18O time seriesbecomes less pronounced because of dispersion (Małoszewski et al., 1990). Peaks become wider and theiramplitudes decrease. In Figure 3 one can compare the υ18O time series of the Danube water at Budapestand at the Halasztelek-5 (H5) production well, a well that is rather close to the riverbank (Figure 2). Thenegative peaks on curve H5 correlate well with those on the Danube curve, but the amplitudes of thesepeaks are smaller and the tops of these peaks are delayed in time by varying time spans. In 1999, when thewater level of the Danube was extremely high (this year was an exceptionally rainy year), the time differencebetween the negative peaks on the Danube curve and the H5 curve (Figure 3) is about one-third of that inthe following year (2000 was an exceptionally dry year), when the Danube water level was extremely low.Comparing these two curves and applying dispersion modelling, one can calculate the transit time of riverwater to the production well with an uncertainty of 1–2 days. As one can see from the above statements, thistransit time depends upon the Danube water level and also upon other parameters, such as the backgroundlevel of shallow groundwater or pumping rate. The transit time information is very important in cases whenriver water becomes seriously polluted.

As Danube water infiltrates and flows through the gravel aquifer, the seasonal variation of υ18O of waterdecreases because of dispersion. The result of this phenomenon is even visible on the υ18O curve of well H5,which is close to the riverbank. When the water flows further into the inner part of the island, the amplitude of



the seasonal variation decreases and eventually disappears (the υ18O value becomes constant). Of course, thisprocess is not so simple, because as the river water flows into the inner part of the island it mixes with locallyinfiltrated meteoric water, which has a substantially different oxygen isotopic composition (see ‘Infiltrationfrom precipitation’ section). If one wants to calculate the mixing ratio, then one needs to know the annualor multi-annual mean υ18O value of Danube water. From our data for the 5-year period of 1998–2002, wehave calculated the weekly mean υ18O values, and from these values the annual mean υ18O value has beendetermined for every year. There were a few weeks with no data; in such cases the mean of the υ18O valuesof the previous and the next week was assigned. The annual mean υ18O values are: �10Ð67‰ for 1998;�11Ð00‰ for 1999; �10Ð93‰ for 2000; �10Ð97‰ for 2001; �10Ð75‰ for 2002. The year-to-year variationin the annual mean υ18O is smaller than š0Ð3‰.

From these annual mean values one can calculate a multi-annual average υ18O value for Danube water atBudapest of �10Ð86‰. Of course, this is only an approximation, because the rate of infiltration of river wateris not constant during the year, this depending upon the difference between the background water level andthe water level of the river. The background water level varies little during the year compared with that of theriver; thus, the main factor is the water level of the river. Usually, the water level of the river is substantiallylower during the winter than in other seasons, and the winter period should be given less weight. Becausethe river water is most depleted during winters, we can possibly infer that the multi-annual average υ18O ofDanube water is more negative than calculated above. It is probably somewhere between �10Ð9 and �11Ð0‰.(Forizs et al. 2006)

The Rackeve–Danube branch (Figure 2, along the eastern shore of Csepel Island) is characterized by asmall rate of discharge and a slow flow velocity, because the water level is kept constant (š20 cm) for usein sport fishing. Because it was not obvious that the average υ18O value of the Rackeve–Danube is identicalto that of the Danube, we sampled the Rackeve–Danube 40 times on the same day when the Danube wassampled during a period of about 2 years. The υ18O values of the Rackeve–Danube scatter around the Danubeυ18O values. The average of the differences between the 40 υ18O pairs is 0Ð03‰, which means that the υ18Ovalues of the Danube and the Rackeve–Danube are statistically indistinguishable.

The average tritium content of the Danube water at Budapest for the period 1998–2002 is 20 TU; itgenerally varied between 12 and 30 TU, with some exceptionally high values (50 TU). The annual averagetritium content decreased year by year during the above period.

Infiltration from precipitation

Deak and Coplen (1996) determined the average υ18O value of the shallowest groundwater (<20 m) inHungary using the VITUKI (Water Resources Research Centre plc., Budapest, Hungary) database and theydetermined a value of �9Ð3 š 0Ð3‰ based on 140 samples. This value is rather characteristic for the lowland,including Csepel Island. Berecz et al. (2001) published υ18O data for groundwater infiltrated during Holoceneand Pleistocene epochs along a west–east profile in the area of the Danube–Tisza interfluve, 100–150 kmsouth of Csepel Island (another part of the same lowland). For Holocene water they reported υ18O valuesbetween �9Ð2 and �9Ð7‰, mostly around �9Ð4‰, which is in good accordance with data published by Deakand Coplen (1996). Forizs and Deak (1998) reported υ18O values of �9Ð3‰ and �8Ð9‰ for two observationwells on the left bank of the Rackeve–Danube (one of them is the F42 observation well in Figure 2).Summarizing the above data, one can conclude that groundwater with υ18O values between �9Ð0 and �9Ð5‰from Csepel Island can be regarded as having been infiltrated from local precipitation only. Groundwaterswith more negative υ18O values (<�9Ð5‰) contain some Danube water component.

The average water table (metres above Baltic Sea level) is shown on Figure 4. Water levels of the river andin the observation wells close to the river vary substantially. During floods (mostly during May and June), asriver water infiltrates into the aquifer, the water level of the river increases and is higher than the water tableat the middle part of the island, which varies little. At low flux, groundwater moves in the direction of theriver. In spite of the large and fast changes of water levels, this map of average values characterizes well themean water flow directions and provides a good reference for the evaluation of the υ18O values.



Figure 4. Hydraulic contour map of the study area created by the SURFER software (Golden Software)

Figure 5. The υ18O contour map of the study area created by the SURFER software (Golden Software)

Figure 5 shows the υ18O contour map of the study area constructed using the SURFER software (GoldenSoftware). It can be seen that groundwater in the middle part of the island is comprised of infiltratedprecipitation, whereas groundwater near the banks on both sides of the island originates from the river.Between these two end members there is a gradual change in isotopic composition (the two end membersmix with varying ratios).

On the west side of the island near Halasztelek (see Figure 2), river water intrudes inward into the island(indicated by the arrow in Figure 5). Along the bank there are production wells; so, it is improbable that



river water can flow through the chain of these wells. In the ‘Deep groundwater’ section it will be shown thatdeep groundwater with very negative υ18O values is ascending between the river bank and the Halaszteleksettlement and it mixes with the shallowest groundwater, which is a mixture of river water and infiltratedprecipitation. Thus, in this location the two-component mixing model is not valid because waters from threesources mix.

Comparing the hydraulic (Figure 4) and the υ18O (Figure 5) contour maps, one can find several placeswhere the maps do not fit each other. One of these locations was mentioned above and is indicated by anarrow in Figure 5. Another location is the surroundings of the lake, where the υ18O contour lines are theresult of automatic interpolation, and they do not show the real situation. In the area south of the lake thetwo contour maps fit each other well, showing the infiltration of the lake water. Along the banks of the bigbranch of the River Danube, both maps show the infiltration of the river water.

The average tritium content of the precipitation at Budapest for the period 1997–2002 is 10 TU; it variedbetween 5 and 20 TU (values between 12 and 20 TU were observed in summer).

Infiltration from the lake

The stable isotope composition of meteoric water plotted on a υD–υ18O diagram scatters along a line calledthe global meteoric water line (MWL; or local MWL (LMWL)). When some of the water evaporates, as oftenhappens in lakes, the residual water becomes enriched in D and 18O. In this manner, the isotopic compositionnot only changes, but also deviates to the right of the MWL.

There are observation wells with significant chloride concentrations. These wells are located mostly near theM0 highway; thus, it seemed reasonable that this chloride originates from the sodium and calcium chloridesused for de-icing the M0 highway during wintertime. The water from the road surface is drained into pools,where it evaporates. If these artificial pools are not isolated well, then the water with high chloride contentand enriched isotopic composition may infiltrate into the surrounding shallow aquifer and mix with thegroundwater. To check this possibility, samples for stable hydrogen and oxygen isotope ratio and for tritiumcontent were taken from the observation wells along the M0 highway and from different parts of KavicsosLake and lake sediments. (Pools along the highway were dry during the sampling campaign.)

The stable isotope results are shown in Figure 6. Lake water samples and water extracted from lakesediment plot on the ‘evaporation line’ (Figure 6). Among the observation wells, M06 and M07 also plot on

M08M01 M03

M02 M05

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M07

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lake sediment

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-13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1

δ18O [‰]VSMOW

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] VS

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Evaporation line

Local meteoric waterline

Figure 6. The υD–υ18O diagram of the samples from monitoring wells along the M0 highway and Kavicsos Lake and its sediment



the ‘evaporation line’; these are not far from the LMWL. This suggests that a smaller part of the water inwells M06 and M07 originates from the lake. The other wells (M01, M02, M03, M04, M05, M08 and M09)plot on the LMWL. Of these wells, M04, M05 and M09 have water infiltrated from local precipitation only,and well M01 has water infiltrated from the river only. Waters in the wells M02, M03, M08 are mixtures ofriver water and infiltrated precipitation.

Infiltrated water enriched in D and 18O has been identified in wells M06 and M07 only, which also containmoderate chloride. From this observation, one can infer that chloride pollution and infiltration of water enrichedin D and 18O are not related to each other.

Thus, the groundwater south of Kavicsos Lake is a mixture of infiltrated precipitation and the lake waterenriched in D and 18O. North and west of Kavicsos Lake data are available from only a few observationwells; thus, contours on Figure 5 around the lake are mostly caused by automatic interpolation and rarelyshow the real situation.

The tritium contents of some wells and one of the lake water samples have been determined (M01 is 12Ð9TU; M06 is 9Ð9 TU; M07 is 11Ð3 TU; M09 is 11Ð0 TU; lake water is 12Ð6 TU). The tritium contents of wellsclose to the lake are slightly lower than those of the lake water (12Ð6 TU) and slightly higher or equal tothose of the multi-annual means of precipitation (10 TU). The well close to the river (M01) has the highesttritium content among the wells because its water comes almost entirely from the river. Nevertheless, it isconsiderably lower than the average tritium content of the river in the year preceding the sampling date,because the tritium content of the M01 observation well represents a short period of time when the tritiumcontent of the River Danube was lower than the average.

Deep groundwater

Although local precipitation and river water infiltrates into the shallowest (exploited) aquifer, Csepel Islandis also a regional discharge area. For instance, hydrogeological papers by Erdelyi (1979) and Ronai (1985)imply that Csepel Island can be reached by the upwelling water of an intermediate flow system with a rechargearea on the land of the sand-ridge in the Danube–Tisza interfluve. At the same time, Csepel Island can alsobe a discharge area of some karstic flow systems of the Transdanubian Central Range (e.g. Lorberer, 1997).

Stable oxygen isotope characteristics of thermal waters in Budapest and its surroundings (originating fromthe Transdanubian Central Range) indicate ice age infiltration, and in a few cases indicate some contributionof Holocene infiltrated cold karstic water (Babidorics et al., 1998). The upwelling water from the flow systemof the Danube–Tisza interfluve could infiltrate during the Holocene (υ18O is between �9Ð2 and �9Ð7‰) orthe Pleistocene (υ18O is ��11‰) (Deak et al., 1987; Marton et al., 1991; Deak and Coplen, 1996; Bereczet al., 2001).

Summarizing the above statements, there is a chance that deeper groundwater with stable isotopecompositions differing from the previously described sources (river, precipitation, and lake) can reach theshallowest aquifer and mix with its water. Since thus far no isotope hydrological investigations have beenmade on the deeper groundwater of Csepel Island, samples from wells collecting water from depth >20 mwere taken for stable oxygen isotope ratio measurements and for determination of water chemistry. In fivecases, radiocarbon and stable carbon isotope measurements have been carried out on total dissolved carbonate.These results are shown in Table I.

The oxygen isotopic compositions vary between �11Ð95 and �13Ð50‰, suggesting that those waters wereinfiltrated during the last glaciation. The majority of the υ18O values are around �13‰ and appear to representprecipitation from a cooler period of the last glaciation (Wurm) in the Carpathian basin. The five radiocarbonages (corrected with υ13C data) support this observation. If one compares the υ18O values with the υ13Ccorrected radiocarbon ages, then one finds that the oldest (30 200 year BP) and the youngest (16 600 year BP)waters are the least depleted in 18O, whereas others in between are more depleted in 18O. It is well knownthat there is an indirect correlation between the mean air temperature and the υ18O value of meteoric water(e.g. Clark and Fritz, 1997); thus, a negative excursion of the υ18O value indicates a cooler period. Taking into



Table I. Isotope and chemical data of deep groundwater samples taken in 2001–2002 on Csepel Island, Hungary

T(°C)

Electricconductance(µS cm�1)

υ18O(‰ VSMOW)

υ13C(‰ VPDB)

14C(pmC)

Chloride(mg l�1)

Aquifer(m)

Calc.age (years

BP)

Szigethalom VM 1 18Ð8 910 �13Ð21 151 110–120Szigethalom VM 2 16Ð5 730 �13Ð50 79 130–140Szigethalom VM 3 14Ð6 680 �13Ð24 �11Ð31 1Ð66 š 0Ð32 53 50–60 27 300Szigethalom VM 4 17Ð1 1360 �13Ð17 �10Ð81 2Ð12 š 0Ð28 361 110–125 24 900Szigethalom VM 5 14Ð7 760 �12Ð94 88 60–70Szigethalom VM 6 15Ð9 780 �13Ð40 92 65–95Ð2Szigethalom VM 7 14Ð1 800 �13Ð48 120 35–50Szigethalom VM 8 15Ð9 925 �13Ð02 166 85–95Szigethalom VM 9 16Ð2 1010 �13Ð35 232Szigethalom, Jozsef

A. str. 6216Ð8 1300 �12Ð85 �12Ð48 4Ð53 š 0Ð58 290 45–60 19 800

Szigethalom, Nyar str.164

15Ð9 620 �12Ð58 31 85–95

Szigetszentmiklos,Rakoczi str. 51

14Ð8 830 �13Ð27 21 86–101Ð3

Szigetszentmiklos,Szegfu str. 11

15Ð2 490 �12Ð36 �12Ð64 6Ð75 š 0Ð65 2 89Ð5–98Ð8 16 600

Szigetcsep, Dunasorstr. 58

13Ð8 560 �13Ð20 14 33Ð5–45Ð5

A (Halasztelek-13) 12Ð3 4050 �12Ð25 �19Ð4 2Ð00 š 0Ð25 1340 13–27Ð2 30 200B (Halasztelek-14) 12Ð0 3380 �12Ð51 1130 7Ð8–21Ð2C (Szigetszent-miklos-

61)11Ð8 3810 �11Ð95 1350 10Ð8–26

consideration that the negative excursion of the υ18O values took place between ca 17 000 and 30 000 yearsBP according to our radiocarbon age determination, one finds that this period was the last glacial maximum(LGM, Wurm III period), when the temperature was even lower than the average Wurm temperature. Althoughthe calculated radiocarbon ages have an uncertainty of few thousand years because of the error in 14C contentdetermination (see Table I) and uncertainty of the assumed values for υ13Ccarb �0‰� and υ13CsoilCO2 ��25‰�,the υ18O values and water ages are in very good correlation in light of our knowledge about this period oftime (e.g. see Adams and Faure (1996: figure 1a)).

Owing to ion exchange between seeping water and clay minerals in sedimentary aquifers, water becomesmore alkaline (usually higher Na C K meq% defined as [�Na C K�/�Na C K C Ca C Mg�] ð 100, whereelements are given in equivalents per million) during its evolution (Forizs, 1998). Berecz et al. (2001) reporteda linear correlation between the corrected radiocarbon ages and the Na C K meq% parameter (see Figure 7)along a flow path on the southern part of the Danube–Tisza interfluve. If one assumes that deep groundwater onCsepel Island originates from the flow system in the Danube–Tisza interfluve, then Csepel Island groundwatersamples should plot on the above mentioned line, but this is not the case (see Figure 7). Their Na C K meq%parameter is very high, suggesting that this water is rather old, and this supports the observation that CsepelIsland samples do not lie near the line of Na C K meq% age as determined by Danube–Tisza interfluvesamples. From this observation one can infer that the groundwater of Csepel Island may not originate solelyfrom the flow system of the Danube–Tisza interfluve.

The chemistry of these deep groundwater samples shows high variations, e.g. electrical conductivity rangesfrom 620 to 4050 µS cm�1 (Table I) and chloride content ranges from 2 to 1350 mg l�1 (Table I). Althoughit is uncertain whether samples A, B and C have natural chemical composition or are polluted, the variationsare rather high among the rest of the samples. This may indicate different sources. Because this area is ameeting point of different flow systems, high variation in chemical content is understandable.



0

5000

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15000

20000

25000

30000

35000

40000

0 20 40 60 80 100

Na+K meq%

Cor

r. 14

C a

ge [y

ear]

Csepel Islanddeep groundwater

Danube-TiszaInterfluve

Figure 7. Radiocarbon ages versus Na C K meq% of deep groundwater samples (see Table I). Data for the Danube–Tisza interfluve arefrom Berecz et al. (2001)

y = 0.0991x - 12.273R2 = 0.918

-12.5

-12.0

-11.5

-11.0

-10.5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Tritium [TU]

δ18O

[‰] V

SM

OW

Figure 8. The υ18O versus tritium content of monitoring well F23 during the period 1999 to 2002. Uncertainty is indicated

Regarding the shallowest aquifer, which is used for drinking water and obtained near the riverbank,ascending old groundwater has been identified in one monitoring well, i.e. F23 north of Halasztelek (Figure 2).The υ18O values and tritium content of F23 are plotted in Figure 8. The average υ18O value is �11Ð5‰, whichis more negative than that of the Danube, indicating some contribution from the deep groundwater. If onecompares the υ18O values with the tritium content (Figure 8) one notes a good linear correlation (R2 D 0Ð918).Lower υ18O values correlate with lower tritium content. The average tritium content of precipitation atBudapest for the period 1998–2002 is 10 TU, and for the Danube it is 20 TU.

The isotope data of well F23 (Figure 8) cannot be explained with a two-component mixing model of Danubewater and infiltrated precipitation; thus, there must be a third component. A linear mixing of bank filtration(υ18O D �10Ð9‰, 20 TU) and precipitation (υ18O D �9Ð4‰, 10 TU) must lead to tritium values between10 and 20 TU in well F23. This is not the case, the observed values range between 6 and 13 TU. Thus, itis probable that a deep groundwater component (υ18O D �12 to �13Ð5‰, 0 TU) contributes. Although for



the three-component mixing model the real values of their isotopic composition are not known, we can useaverage values for calculating the mixing ratios. The following equations are valid:

υ18OF23 D xυ18Odeep groundwater C yυ18ODanube C zυ18Oprecipitation �1�

TF23 D xTdeep groundwater C yTDanube C zTprecipitation �2�

x C y C z D 1 �3�

The average values are υ18OF23 D �11Ð41‰, υ18Odeep groundwater D �13‰, υ18ODanube D �10Ð9‰,υ18Oprecipitation D �9Ð4‰, TF23 D 8Ð7 TU, Tdeep groundwater D 0 TU, TDanube D 20 TU, Tprecipitation D 10 TU.Solving the three equations with the above average values we get 11% Danube water, 24% deep groundwater,and 65% precipitation contents for the average F23 water. If one makes calculations with measured υ18O andtritium content in the F23 water at a specific date, then another ratio will result for the three components.For example, when υ18OF23 D �11Ð8‰, TF23 D 5Ð8 TU then the proportions are 19% Danube water, 61%deep groundwater, and 20% precipitation content. One can notice that the calculated ratios depend highly onthe input data, but because of the character of the flow system (sudden changes in the river water level and,consequently, in the flow directions), these big differences in the calculated ratios can be real.

SUMMARY

By applying environmental isotope measurements (υD, υ18O, tritium, υ13C, radiocarbon) to the shallow anddeep aquifers of the alluvial Csepel Island (south of Budapest) and to the River Danube, four different sourcesof water were identified. These are characterized as follows.

Danube water: seasonally varying υ18O value with an unweighted multi-annual mean of �10Ð9‰ for theyears 1998–2002. The seasonal variation is almost the opposite of that of precipitation; most negative υ18Ovalues occur in late spring and summertime. Average tritium content is 20 TU.

Infiltrated precipitation: the υ18O value is between �9Ð0 and �9Ð5‰. The average tritium content of theprecipitation is 10 TU, and that of infiltrated precipitation must be nearly identical.

Lake water: the υ18O value is >�9Ð0‰. On a υD–υ18O diagram the lake water plots to the right of theLMWL.

Ascending deep groundwater: the υ18O value varies between �12Ð0 and �13Ð5‰. The radiocarbon ageis between 16 000 and 30 000 years BP, which probably means this infiltrated during and around the LGM(Wurm III) period of the latest glaciation.

The infiltration of river water from the Danube and Rackeve–Danube and water from Kavicsos Lake arewell seen in the cross-section along the M0 highway (Figure 9). Around Kavicsos Lake the infiltrated lakewater dominates. Wells close to the river branches are characterized by intensive riverine water infiltration.Between Kavicsos Lake and a 1 km bank zone of the River Danube (big branch), infiltrated precipitation isthe main source of water in the shallowest aquifer.

The proportions of the four water sources described above in the shallowest aquifer are going to be calculatedusing a three-dimensional non-permanent hydrodynamic model created using the VISUAL MODFLOWsoftware (Waterloo Hydrogeologic Software). Isotopic data and hydraulic data will be incorporated into themodel. Dispersion modelling using υ18O data will be made as described by Małoszewski et al. (1990). Theresults of this are planned to be published in separate papers in the future.

ACKNOWLEDGEMENTS

We are grateful for the support provided by the Fovarosi Vızmuvek Rt. (Water Works of the Capital CityInc.) to carry out the isotope hydrogeochemical study of Csepel Island. SziKo Ltd (Szigethalom) and Mr Bela



M08 Ráckeve-Danube

M07

M06

M09

M05

M04

M01Danube-11.5

-10.5

-9.5

-8.5

-7.5

-6.5

-5.5

-4.5

0 1000 2000 3000 4000 5000 6000 7000

Distance from the river bank along the M0 high way [m]

δ18O

[‰] V

SM

OW

Figure 9. The υ18O cross-section along the M0 highway (see Figure 2). The υ18O values are average values of samples taken once everyquarter year in the period 1999–2001. The υ18O value for the Danube indicated in this figure is the average value for the period 1999–2001

Beleznay (Szigetszentmiklos) provided access to wells and chemical data. I.F. is grateful to the HungarianScientific Research Fund (OTKA) for supporting the isotope hydrological study of the River Danube by projectno. T 035032. We are very grateful to Tyler B. Coplen (USA) for revising the manuscript and improvingits English. Finally, we received with thanks the valuable suggestions of two anonymous referees, whichimproved this publication.

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Documents

Origin of shallow groundwater of Csepel Island (south of Budapest, Hungary, River Danube): isotopic and chemical approach