Origin of Bank Filtered Groundwater Resources Covering the Drinking Water Demand of Budapest, Hungary

ORIGIN OF BANK FILTERED GROUNDWATER RESOURCES COVERING THE DRINKING WATER DEMAND OF BUDAPEST, HUNGARY

I. FORIZS Laboratory for Geochemical Research of the Hungarian Academy of Sciences

J. DEAK Water Resources Research Centre Pic.

Budapest, Hungary

Abstract - The ratio of Danube water/infiltrated precipitation has been determined using stable oxygen isotope data on four parts of the protection area of the bank filtered water works supplying drinking water for Budapest, Hungary. These ratios comparing to those calculated by hydraulic modeling rarely match each other. The Danube water transit time calculated for few wells by isotopic data are usually shorter than those determined by hydraulic modeling. The relation between the 8 O values and the nitrate, chloride and sulfate pollutants shows that the source of the pollutants is on the island area (sewage water, agricultural activity and salt used for de-icing asphalt roads).

1. INTRODUCTION

The drinking water demand of more than two million inhabitants of Budapest is mainly covered by bank filtered water of the River Danube. In 1990 the average drinking water consumption of Budapest was 976,000 m3/d [1], and it was 780,566 m3/d in 1995 [2]. The bank filtered wells are located on the both sides of the Danube north and south of Budapest, in Budapest, and on the bank shores of the Szentendre Island and Csepel Island (Fig. 1).

The ratio of the Danube water and the infiltrated precipitation in the supplied water is a very important question related to the drinking water quality. The infiltrated precipitation is potentially polluted by agricultural activity and communal waste water of unsewered settlements. The mixing of these two types of waters has been investigated by stable oxygen isotope ratio.

1 ft The 6 O values of the water samples have been measured, evaluated and compared with the chemical composition of the water. The hydraulic modeling of the water flow system in the uppermost aquifer on the Szentendre Island has been made in a frame of an independent programme at the Technical University of Budapest [3]. The ratio of Danube water/infiltrated precipitation, and Danube water transit time have been determined for some wells by hydraulic model and the stable oxygen isotope data as well. Data obtained by the two methods have been compared. In the first year (1995) of our project we studied the most northern and middle part of the Szentendre Island and the area of the Dunakeszi Water Works on the left bank of the Da- nube (Fig. 1,2,3 and 6) determining the origin of the shallowest groundwater and the pollutants. In the second year (1996) we studied the Csepel Island area (Fig. 1 and 10) for the same purpose.

133 Fig. 2-3 [ water work units Fig. 4-5

Fig. 10-13

0 5 10 15 km i i i i

Fig. 1. Sketch map showing the bank filtered water work units supplying Budapest.

134 horizontal well • observation well settlement

1 water work units 0 1 2 km • • •

Fig. 2. Sketch map of the northern part of the Szentendre Island showing the names and locations of the wells sampled. 818O

+ horizontal well • observation well settlement

I water work units 0 2 km

Fig. 3a. The stable oxygen isotope compositions of the wells sampled on the northern part of the Szentendre Island. NO,

Kl8o*?zl Water W

+ horizontal well • observation well settlement

water work units 0 1 2 1 • 2 km •

Fig. 3b. The nitrate content of the wells sampled in the northern part of the Szentendre Island. Cl

horizontal well observation well settlement

I water work units 1 2 km

Fig. 3c. The Cl content of the wells sampled in the northern part of the Szentendre Island. so,

+ horizontal well • observation well settlement

1 water work units 0 1 2 km • •

Fig. 3d. The sulphate content of the wells sampled in the northern part of the Szentendre Island. 1N

filter well observation well settlement I water work units high way

10 km

Fig. 4. Sketch map of the middle part of the Szentendre Island with the names and locations of wells sampled.

140 518O

filter well observation well settlement I water work units high way

10 km

Fig. 5a. The stable oxygen isotope composition of the wells sampled in the middle part of the Szentendre Island.

141 NO. ~

filter well observation well

settlement I water work units high way

10 km

Fig. 5b. Nitrate content of the wells sampled in the middle part of the Szentendre Island.

142 Tritium

filter well observation well

settlement I water work units high way

10 km

Fig. 5c. Tritium content of wells sampled in the middle part of the Szentendre Island.

143 filter well N observation well t

settlement I water work units 0 100 m

Fig. 6. Sketch map of the area of the Dunakeszi Water Works showing the name and location of the sampled wells.

144 filter well t N observation well

settlement I water work units 0 100 m

Fig. 7. The stable oxygen isotope composition of the sampled wells of the Dunakeszi Water Works.

145 35

30-

A 25- AA& A

Ai o o 20+ A* 15 *

10 H 1 1 h -12.00 -11.40 -10.80 -10.20 -9.60 -9.00 -12.00 -11.40 -10.80 -10.20 -9.60 -9.00

SM3W

too- °o °o o o

o 80 o o o o o o o* 60 ° o o oo oo 40

20 -\ 1 1- -12.00 -11.40 -10.80 -10.20 -9.60 -9.00 [O/J

Fig. 8. Chloride, nitrate and sulphate vs. stable oxygen isotope composition of water samples collected on the Szentendre Island.

146 in A profile in B profile on Fia 3b-d on Fig. 3b—d -o- -o- 3 -o- -A- 2 -o- 3

-11.7 -11.4 -11.1 -10.8 -10.5 -11.6 -11.2 -10.8 -10.4 -10.0

SMOW

18 Fig. 9 . NO3, Cl, SO4 vs. 5 O in profiles A and B indicated on Fig. 2.

147 Szigetujfalu | ^ b^lSzigetszentmirton LEGEND • observation well • horizontal well settlement high way water work units

3 km i 1

Fig. 10 Sketch map of the Csepel Island showing the locations of the wells studied

148 2. TECHNIQUES APPLIED

2.1. Isotope analysis

Stable oxygen isotope measurements have been made on Finnigan MAT delta S mass spectrometer. The results are expressed in the conventional delta (5) notation in per mille (%o) relative to the VSMOW (Vienna Standard Mean Ocean Water) international standard in the following way.

•^sample " ^standard 618O = * 1000 [%o],

•"^standard where R;arnpie and R^dard indicates the 0/ O ratios of the sample and standard respectively.

Samples were prepared according to the conventional CO2-H2O equilibration method first described in [7].

Tritium measurements were made in the TriCarb Lab of the Water Resources Research Centre, Budapest, Hungary (analyst Miklos Siiveges) by the conventional scintillation method.

2.2. Chemical analyses

The NH4, NO3, NO2, Cl and SO4 content of the water samples were measured by the methods described in the Hungarian National Norm for drinking water quality determination. All the data have been got from the Water Works of Capital Corp. (Fovarosi Vizmuvek Rt., Budapest, contact person Adam Konnir).

2.3. Calculation of Danube water component

The basis for determining the origin of the drinking water supplied from bank filtered 1 8 wells is the fact that the 6 O value of Danube water is significantly different from the locally infiltrated precipitation [8]. The 8 O value of the Danube water varies seasonally between - 10 and -12 %o. Its annual mean value for the period of 1991-96 is -11.24%o at Vienna [4], - 11.0%o at Bratislava [5], and -11.0%o at Medve (halfway between Budapest and Bratislava) [6]. As there is no considerable inflow into the Danube between Budapest and Bratislava, we 1 R can use the value of -11.0%o as the average 8 O value of River Danube near Budapest. The multi-annual mean of infiltrating precipitation in Hungary is -9.5%o [9]. Using this difference we can calculate the mixing ratio of Danube water/infiltrated 1 n precipitation by 8 O data measured in production wells according to the following equation:

O '-'well ~ x " '-'Danube V1 ~XJ " ^infiltrated precipitation

From this the ratio of Danube water in percent is:

O '-'well " O '-'infiltrated precipitation 10 XDanUbe= * ° (%)• ^Danube " ® ^infiltrated precipitation

149 3. RESULTS AND INTERPRETATIONS

3.1. Northern and middle part of the Szentendre Island

3.1.1. Stable oxygen isotope data

Stable oxygen isotope measurements have been made on 80 samples from the above mentioned two areas. The name and locality of the wells sampled are indicated on Figs. 2 and 4. Results of stable oxygen isotope measurements are summarized in Tables I and II. Two profiles (indicated on Figures 2-3a-d) more-or-less perpendicular to the flow line of the River Danube have been studied.

All the wells are located on the river island. The two production wells (Table I) are very close to the river bank. Their 518O can be used for transit time calculations (see later), because the Danube water component in these production wells are almost 100% according to the hydraulic modeling [3].

1 R The 5 O values of the observation wells on the northern part of the Szentendre Island range between -9.6 and -10.9%o. These values are more negative than that of the local infiltration, so the shallowest groundwater of the island is a mixture of Danube water and the local infiltration. Going from the river bank to the center of the island, the Danube water component is less and less. The 618O values of the observation wells on the middle part of the Szentendre Island vary between -10.1%o and -11.0%o (Table II, Fig. 5a). All of these data are more negative than that of the mean annual precipitation indicating that the shallowest groundwater has a considerable Danube water component. While in the northern part of the Szentendre Island the most positive values can be found in the midline of the island, in the middle part of the island no such a distribution can be found. Two observation wells (F10 & Fl 1, Tables I, II, and Figs. 1 8 2, 3a, 4, 5a) have been sampled twice, first in June (8 0 = -10.76, -10.90%o), second time in 18 September (-10.47, -10.63). Second time the 8 O values were more positive by 0.28%o in both wells. 3.1.2. Tritium

The tritium content of the water samples collected in the middle part of the Szentendre Island has been determined in the Water Resources Research Center (VITUKI). Results can be found in Table II, and are plotted on Fig. 5c. The tritium content of the water samples varies in a rather narrow range of 17-30 TU (tritium unit), with two exceptions (S-63/a and F-50) where the tritium content is around 42 TU. These two wells are close to each other and may indicate a) a rather old water (the average T content of the precipitation at Nick (Hungary) in 1994 was 11.8 TU, in 1995 was 16.0 TU, and that of the Danube in 1994 was 24.9 TU [10]; higher sulfate and chloride contents support this idea) or b) a very young Danube water component (in August 1995 the mean T content of the Danube was 92 TU [10]).

3.1.3 Water chemistry

Table III and IV show the concentrations of some components, which are important con- taminants. The areal distribution can be seen at the Figs. 3b-d and 5a-b. It is very interesting to

150 Table I Stable oxygen isotope composition of the water samples collected on the area indicated on Fig. 2 (sampling in May-June 1995). Name of well 816O name of well 518O [%°]sMOW [%°]sMOW Production wells Observation wells (continued)

T/II.2. cs. -11.45 F-l -10.87 K/7. cs. -11.12 F-2 -10.87 F-3 -10.53 F-4 -10.52 F-5 -9.58 Observation wells F-6 -10.12 F-7 -10.91 171.100 -11.47 F-8 -10.69 T/II.31 -10.83 F-9 -10.79 T/II.69 -11.93 F-10 -10.76 T/II.5 -11.49 F-ll -10.90 K-16 -11.51 F-54 -10.74 K-64 -10.71 F-68 -10.85

Table II Stable oxygen isotope composition and tritium content of the water samples collected on the middle part of the Szentendre Island (Figs. 4, 5a, 5c, samples were collected in August-September, 1995) Name of 5'6O tritium name of 8i8O tritium well [%°]SMOW [TU] well [%o]sMOW [TU] Production well Observation wells (continued) S-15 -11.12 23.2 F-21 -10.31 25.5 Observation wells F-2 lft -10.37 21.4 S-21 -10.06 21.0 F-22 -10.93 20.5 S-54 -11.53 20.0 F-23 -10.31 20.5 S-63/a -10.47 42.0 F-24 -10.02 18.5 S-80 -10.64 F-25 -10.84 19.5 R-12 -10.50 16.8 F-26 -10.79 17.3 R-16 -10.83 23.4 F-26/a -10.86 19.5 R-18 -10.51 18.1 F-26/b -10.83 21.4 F-10 -10.47 20.8 F-26/c -10.16 21.0 F-ll -10.63 21.2 F-28 -10.26 22.3 F-12 -10.14 21.6 F-29 -10.61 20.8 F-13 -10.06 27.0 F-30 -10.35 25.2 F-14 -10.52 29.0 F-32 -10.42 21.0 F-17 -10.79 29.8 F-33 -10.17 20.8 F-18 -10.28 21.5 F-34 -10.83 21.2 F-19 -10.41 20.2 F-35 -10.99 26.2 F-20 -10.40 22.1 F-36 -10.81 22.7 F-20/a -10.34 20.2 F-3 7 -10.84 20.8 F-20/b -10.27 20.3 F-39 -11.00 27.3 F-20/c -10.23 20.2 F-50 -10.72 41.5

151 Table III Some chemical components of the watei samples collected on the middle part of the Szentendre Island (Figs. 4, 5a-b) Name NH4 NO3 NO2 a SO4 of well [mg/1] [mg/1] [mg/1] [mg/1] [mg/1] 1 2 1 2 1 2 1 2 1 2 Production well S-15 0.02 0.01 7.3 7.1 0.00 0.00 19.9 | 17.7 40.8 44.7 Observation wells S-21 0.00 0.01 13.9 19.1 0.00 1 0.01 23.5 25.3 50.4 51.7 S-54 0.02 0.40 6.3 5.3 0.02 0.03 16.3 14.1 38.4 32.9 S-63/a - 0.00 - 29.0 - 0.01 - 36.5 - 124.7 S-80 0.00 0.00 14.2 10.7 0.00 0.01 24.5 25.3 52.8 54.1 R-12 0.00 0.02 1.6 1.4 0.00 0.01 17.3 15.2 48.0 32.9 R-16 0.00 0.02 6.6 6.9 0.00 0.00 20.9 17.2 48.0 37.6 R-18 0.14 0.03 5.3 8.9 0.00 0.00 23.5 25.3 67.2 61.2 F-10 0.01 0.01 10.6 12.4 0.00 0.00 22.4 22.2 96.0 91.7 F-ll 0.03 0.00 11.5 12.2 0.00 0.01 17.3 19.2 52.8 58.8 F-12 0.01 0.01 12.8 13.8 0.01 0.00 15.8 17.2 55.2 51.7 F-13 1.45 1.77 0.1 0.1 0.01 0.03 20.9 23.7 124.8 159.9 F-14 0.01 0.00 17.4 15.4 0.00 0.00 31.1 31.8 88.8 82.3 F-17 0.00 0.13 11.3 14.5 0.00 0.00 22.4 22.2 72.0 70.6 F-18 0.01 0.03 7.1 3.7 0.01 0.00 18.4 19.2 48.0 56.4 F-19 0.03 0.01 20.6 22.0 0.00 0.00 22.9 20.7 60.0 58.8 F-20 0.01 0.00 8.9 0.6 0.00 0.00 19.4 22.2 67.2 65.9 F-20/a 1.24 34.10 25.2 23.0 0.00 0.02 53.0 49.9 124.8 110.5 F-20/b 3.70 11.80 22.9 30.0 0.00 0.00 38.8 48.5 96.0 101.1 F-20/c 0.00 0.00 6.6 8.8 0.00 0.01 18.9 20.7 52.8 58.8 F-21 0.00 0.00 16.8 0.6 0.00 0.00 23.5 21.2 108.0 94.1 F-21/b 0.00 0.03 35.2 29.0 0.00 0.00 39.8 37.4 96.0 96.4 F-22 0.01 0.03 10.6 11.8 0.01 0.00 18.9 20.2 55.2 51.7 F-23 0.01 0.00 11.2 17.5 0.00 0.02 26.0 26.3 48.0 61.2 F-24 0.00 0.06 74.1 78.0 0.00 0.01 49.9 48.5 120.0 112.9 F-25 0.00 0.00 76.8 87.0 0.01 0.01 44.9 55.6 96.0 110.5 F-26 0.00 0.05 23.7 25.0 0.00 0.00 33.7 30.8 96.0 91.7 F-26/a 0.01 0.05 70.5 86.0 0.01 0.01 60.2 58.6 129.0 129.4 F-26/b 0.00 0.05 66.4 67.0 0.00 0.01 56.1 51.0 139.0 134.1 F-26/c 0.00 0.05 13.7 15.5 0.00 0.00 18.9 23.2 31.1 47.0 F-28 0.01 0.01 11.7 15.5 0.00 0.00 17.9 20.7 48.0 47.0 F-29 0.00 0.01 17.2 10.7 0.01 0.00 21.9 21.2 48.0 54.1 F-30 0.00 0.01 27.1 32.0 0.02 0.02 20.9 20.7 79.2 75.3 F-32 0.02 0.01 22.2 37.0 0.00 0.00 25.5 29.8 72.0 70.6 F-33 0.01 0.01 34.4 33.0 0.00 0.00 20.9 24.2 67.2 61.1 F-34 0.01 0.01 10.7 9.2 0.00 0.01 20.4 22.2 48.0 47.0 F-35 0.02 0.03 2.8 5.1 0.00 0.00 18.4 19.2 76.8 77.6 F-36 0.02 0.03 1.2 3.2 0.00 0.00 22.9 22.2 79.2 75.3 F-37 2.61 0.01 0.5 2.0 0.04 0.01 18.4 18.7 40.8 37.6 F-39 0.01 0.00 11.3 11.2 0.09 0.01 21.1 29.3 57.6 70.6 F-50 0.00 0.00 19.1 25.0 0.00 0.01 25.5 25.3 96.0 105.8

1: first sampling May-June, 1995 2: second sampling August-September, 1995

152 1 8 compare the 5 O values with the chloride, nitrate and sulfate content of the water samples collected on the Szentendre Island (Fig. 8). A slight trend can be observed. The more positive the 8 O value the higher is the Cl, NO3 and SO4 content. The more positive 6 O value means higher component from the infiltrating precipitation, and lower component from the River Danube. The relation between the 81 O value and the chemical components is more striking along the A and B profiles on the northern part of the Szentendre Island (Fig. 3b-d, and Fig. 9). This trend is a clear indication that the source of the Cl, NO3 and SO4 pollutants is on the island and not the Danube water. 3.2 Dunakeszi Water Works

This water works was established on the left bank of the River Danube and the area differs 1 O from the others, because it situates not on a river island. The 8 O values of all the filter wells are the to or more positive than the average 518O value of the Danube (Table V, Fig. 6-7) indicating that the infiltrated precipitation component is considerable.

The 8• 18O, values of the observation wells range between -9.1 and -11.1 %o (Table V, Fig. 6-7). The most negative ones are the closest spatially to the Danube. About 500-800 meters off the river, the Danube does not affect the stable oxygen isotope composition of the shallowest groundwater.

Table IV Some chemical components of the water samples collected in the northern part of the Szentendre Island (Fig. 2, 3b-d)

Name of NH4 NO3 NO, Cl SO4 well [mg/1] [mg/1] [mg/1] [mg/1] [mg/1] 1 2 1 2 1 2 1 |2 1 2 Northern part of the Szentendre Island

Operating wells

T7IL2.cs 0.00 0.01 5.5 6.6 0.00 0.00 14.7 15.1 36.0 35.3 K.7.cs. 0.00 0.01 4.9 5.0 0.00 0.00 16.3 14.1 40.8 32.9 Observation wells T/I.100 0.00 0.01 1.6 2.8 0.00 0.00 15.8 14.7 33.6 42.3 T/II.31 0.01 0.03 9.9 13.9 0.00 0.01 20.9 20.7 43.2 44.7 T/II.69 0.00 0.01 5.2 6.2 0.00 0.00 13.8 10.7 33.6 37.6 T/II.5 0.00 0.01 4.3 6.7 0.00 0.00 15.3 12.6 33.6 38.0 K-16 0.03 0.01 2.7 4.3 0.02 0.01 18.4 15.2 33.6 35.3 K-64 0.02 0.14 11.6 16.5 0.02 0.02 19.4 23.2 62.4 70.6 F-l 0.31 0.27 0.5 0.3 0.00 0.00 24.5 27.8 110.4 117.6 F-2 0.00 0.00 12.3 12.9 0.00 0.00 20.9 19.7 57.6 56.4 F-3 0.00 0.01 40.0 41.0 0.00 0.19 25.5 29.8 91.2 72.9 F-4 0.01 0.03 23.8 19.3 0.00 0.01 23.5 26.3 57.6 49.5 F-5 0.01 0.03 24.7 25.0 0.00 0.01 26.0 25.8 55.2 70.5 F-6 0.00 0.03 19.6 23.0 0.00 0.01 21.9 21.7 62.4 75.3 F-7 0.00 0.00 13.5 20.0 0.00 0.00 21.4 20.9 48.0 54.1 F-8 0.00 0.00 37.2 36.0 0.00 0.03 34.7 19.1 93.6 91.6 F-9 0.02 0.08 32.2 29.0 0.01 0.03 27.5 28.3 76.8 61.1 F-54 0.00 0.02 16.8 21.0 0.00 0.00 21.4 23.2 45.6 47.0 1: first sampling May-June, 1995 2: second sampling August-September, 1995

153 Table V Stable oxygen isotope composition of the water samples collected on the area of the Du- nakeszi Water Works (Fig. 6) and of Danube water. Name of 5'°O name of 51SO well [%o]sMOW well [%°]sMOW Production wells Observation wells (cont'd) II/2 -11.12 FK-10 -10.43 II/8 -10.75 FK-35 -9.41 11/15 -10.96 FK-36 (-7.39) 11/23 -10.89 FK-37 -11.06 Danube -11.52 FK-38 -9.63 (28.06.95) FK-39 -9.75 Observation wells FK-41 -9.65 FK-1 (-7.98) FK-43 -9.74 FK-3 -9.68 FK-44 -9.05 FK-6 -9.81 FK-45 -9.24 FK-7 -9.85

3.3 Origin of groundwater and its relation to pollutants on the Csepel Island

3.3.1 Stable oxygen isotope and chemical data

The stable oxygen isotope composition and the concentration of some chemical components of water samples studied are in the Table VI.

1 ft The stable oxygen isotope data are plotted on Fig. 11. The 8 O values of the water of the production wells are around -11.0%o (Table VI, Fig. 11), which is identical with that of the average 818O value of the Danube. So we can draw a conclusion that the water supplied by the 1 ft production wells are 100% or very close to 100% of Danube water. The 8 O values of the observation wells varies in a rather wide range from -7.89%o to -11.85%o (Table VI, Fig. 11). Usually in the inner part of the island the 818O value is identical or very close to that of the infiltrated multiannual precipitation (-9.3%o) (see Tokol, north and west of Szigetszentmik- 16s and southwest of Szigenijfalu on Fig. 11).

There are two observation wells on the left bank side of the Rackeve Danube (the smaller branch), these are F40 and F42 (Fig. 10). The corresponding 8 1 RO values are -9.3 l%o 1 R and -8.90%o (Fig. 11), which are identical with the 6 O value of the infiltrating precipitation. At the same time the 8 O values of the observation wells on the right bank side (F36, F39, F10, F54, F58, Fig. 10 and 11) are more negative (from -10.43%o down to -10.87%o) showing the effect of the Rackeve Danube. Although we do not have 6I8O data for the Rackeve Da- nube, we can suppose that it is the same as that of the Danube (or a little bit more positive). Keeping in mind that the water level in the Rackeve Danube is higher by about 1-3 meters than the water level in the Danube, it is reasonable to suppose that the shallowest groundwater 1 R flows from east-northeast to west-southwest. This supposition is supported by the 8 O values as well. The groundwater on the left bank side is not affected, while that on the right bank side of the Rackeve Danube is affected by the river water. The water of the Rackeve Danube infil- trates into the gravel material, then mixes with the infiltrated precipitation and flows to the direction of southwest-west. Also we can suppose that the shallowest groundwater on the left bank side of the Rackeve Danube flows under the river and mixes with the infiltrating

154 Table VI Stable oxygen isotope and chemical composition of water samples collected on the Csepel Island (Fig.10) Production wells

I8 Name Date 6 O NH4 N03 Cl H.5 12.09.96 -11.00 0 5 23 H.14.cs 13.09.96 -11.01 0 8 20 H-15.cs 13.09.96 -10.93 0 10 24 H-19.cs 13.09.96 -11.01 0 8 22 R.12 30.08.96 -11.14 0 3 23 Observation wells

Y.I 06.09.96 -10.35 0 24 64 F.10 27.08.96 -10.32 0 24 90 F.21 16.09.96 -9.43 0 640 283 F.23 12.09.96 -11.85 0 1 176 F.27 16.09.96 -9.03 0 135 139 F.28 13.09.96 -9.61 0 22 211 F.32 10.09.96 -9.10 1 180 314 F.35 09.09.96 -7.89 0 210 121 F.36 09.09.96 -10.87 1 2 21 F.39 27.08.96 -10.60 2 105 87 F.40 27.08.96 -9.31 0 310 73 F.42 27.08.96 -8.90 0 210 137 F.44 06.09.96 -10.26 0 8 49 F.45 05.09.96 -9.22 0 18 40 F.46 30.08.96 -9.39 0 18 33 F.54 06.09.96 -10.72 10 1 32 F.55 30.08.96 -9.85 0 4 38 F.56 30.08.96 -10.76 0 39 53 F.57 06.09.96 -10.81 0 28 43 F.58 06.09.96 -10.59 7 1 34 F.62 04.09.96 -9.37 0 32 36 F.65 03.09.96 -8.48 0 60 60 F.69 03.09.96 -9.34 0 115 36 F.76 29.08.96 -9.49 0 1 28 F.77 05.09.96 -9.96 1 2 147 F.77A 05.09.96 -9.80 1 1 103 F.78 05.09.96 -10.74 1 350 179 H.56 11.09.96 -10.86 0 1 32 H.60 13.09.96 -10.91 0 1 19 H.I 09 11.09.96 -10.44 0 2 38 SZ.6 09.09.96 -10.27 0 220 111 R.12.18 30.08.96 -11.15 0 1 22

155 518O

|Szigelu]falu| v & |SzigetszentmArton| LEGEND • observation well + horizontal well 3 settlement highway —• water work units

3 km

Fig. 11 The stable oxygen isotope composition of the wells sampled on the Csepel Island.

156 NO,

Szigetujfalu] v b^lSzigetszentmirton LEGEND • observation well + horizontal well settlement high way •• water work units

3 km

Fig. 12 The nitrate content of the wells sampled on the Csepel Island

157 Cl

Szigetujfalu| v fc/Tszigetszentmarton LEGEND • observation well + horizontal well settlement high way water work units

3 km

Fig. 13 The chloride content of the wells sampled on the Csepel Island

158 Csepel Island Csepel Island

o 600

500-

400 o d O z 300 o o o o 200 o o 100 o o o 0 o—&8cc&-a —1 1 -12.0 -11.0 -10.0 -9.0 -8.0 -7.0 -12.0 -11.0 -10.0 -9.0 -8.0 -7.0

8 O [%o]sMOW 5 O [%O]SMOW

Csepel Island

X 6

4 -

-12.0 -11.0 -10.0 -9.0 -8.0 -7.0

5 0[%o]sMOW

Fig. 14 Chloride, nitrate and ammonium content vs. stable oxygen isotope composition of water samples collected on the Csepel Island.

159 Rackeve Danube water. The water table levels [3] support these suppositions, because the water level on the left bank side of the Rackeve Danube nearby and between the two studied observation wells (F40, F42; Fig. 10) is higher than on the right bank side. This flowing system discharges into the Danube.

The Danube (big branch) has only a little effect on the shallowest groundwater of the Csepel Island, especially where there are chains of production wells (Fig. 11). The production wells exploit a lot of water and this way serve as a barrier and only a little amount of Danube water gets behind the production wells. This can happen in two cases: 1. highest Danube water level or 2. when a group of production wells are out of work because of maintenance. So the 1 fi 6 O value of the observation wells "behind" the production wells are rather close to that of the infiltrating precipitation or between the average Danube water and the infiltrated precipitation (F46= -9.39;H109= -10.44; H56= -10.86%o). 3.3.2. Pollutants: nitrate, chloride and ammonium

The distributions of the above pollutants are rather irregular (see Fig. 12 and 13). The main source of nitrate in many cases is the communal sewage water, see e.g. Rackeve, Tokol, Szigetszentmiklos and Csepel settlements. The agricultural activity is also a source of nitrate. The main source of chloride pollution is the salt used in winter time for de-icing the asphalt roads.

Comparing the data of these pollutants with 6 O values (Fig. 14) we can observe a 1 8 slight correlation. We can find higher nitrate and chloride concentrations when the 6 O value is greater than -11.0%o (with one exception, F23), it means when the water has component of infiltrated precipitation. Between the ammonium content and 6 O value there is no correlation (Fig. 14). Usually higher concentration of ammonium can be found where the chemical condition is rather reducing.

4. COMPARING THE HYDRAULIC AND ISOTOPIC MODELS ON THE SZENTENDRE ISLAND

4.1. Transit time calculated by hydraulic modeling and stable oxygen isotope data

The two dimensional hydraulic modeling of the flowing system in the shallowest aquifer of the Szentendre Island has been made by Gy. Molnar at the Technical University of Bu- dapest, Hungary [3]. For the hydraulic modeling the following data were used: Danube water level, water level in the observation wells, bottom and upper surface morphology of the aquifer, hydraulic conductivity, exploitation rate, rate of precipitation and infiltration. These data were collected during the years of 1992, 1993 and 1995. Using the software of this hydraulic modeling the ratio of Danube water/infiltrated precipitation and the transit time of Danube water from the river to the wells have been calculated for some wells (see Table VIII and IX).

In the case of some production and observation wells, where the Danube water ratio was close to 100% (according to the hydraulic modeling), the transit time of Danube water was calculated (Table VIII) using stable oxygen isotope data measured in the wells and the 5 O values of the Danube water at Bratislava, Slovakia (Fig. 15). The calculation was made

160 in a very simple way: taking the 818O value of the well, the same value was looked for back- way on the 5 O curve of the Danube (Fig. 15) considering the 2-3 days needed for the Da- nube water to get from Bratislava to the Szentendre Island. Dispersion has not been taken into account. These transit times are in the range from half to 3 months (Table VIII). The uncer- tainty is ±5 days.

Comparing the transit times calculated by stable oxygen isotope data to the transit times determined by hydraulic modeling (Table VIII) we can conclude that in 4 out of 7 cases (T.II.5; T.II.69; T.II.2. cs.; S-15.cs.) the latter ones are two-three times longer, in 2 out of 7 cases (T.I. 100; K.7.cs.) the former ones are longer, and 1 out of 7 cases the two data are almost the same (S.54). Explanations for this discrepancy can be 1) imperfect input parameters used for hydraulic modeling 2) isotopic data were determined only once for the wells, while the hydraulic modeling was based on data collected during three years 3) the water level of the Danube at the time of sampling was unusually high (Table VII). This high water level could resulted in a faster water flow, and so shorter transit time.

Table VII Danube water levels at three points along the Szentendre Island Nagymaros Vac Budapest Mean water level in June 1995 (cm) 357 399 508 Mean water level of June averages from 1986-1995 206 224 339 (cm) Mean water level of year averages from 1986-1995 136 148 259 (cm)

For a more characteristic and more reliable isotopic modeling a time series data col- lecting would be necessary.

Table VHI. The stable oxygen isotope composition, tritium data and calculated Danube water ratio and transit time for some wells on the Szentendre Island Well number Isotopic Ratio of Danube water Transit time from the Tritium Date of comp. [%] Danube sampling 61SO Hydraulic Isotopic Hydraulic Isotopic [TU] [%o]sMOW model model model model T.II.5 -11.49 91 US 84 days - 40 days 08.06.95 T.II.69 -11.93 100 U9 37 days ~ 15 days 07.06.95 T.II.2. cs. -11.45 98 U$ 130 days ~ 40 days 12.06.95 T.I. 100 -11.47 99 25 days ~ 40 days 06.06.95 K.7. cs. -11.12 96 m 29 days > 60 days 12.06.95 S-15.cs. -11.12 34 days -15 days 23.2 31.08.95 S-54 -11.53 99 444 23 days ~ 20 days 20.0 31.08.95 K-64 -10.71 61 84 16 years S-63/a -10.47 12 67 64 years 42.0 S-80 -10.64 100 79 31 years F-25 -10.84 93 38 years 19.5

161 Danube at Bratislava (data from Michalko, Bratislava)

-10 - -10.5 - » jsampling 1 \ sampling 2 < o -11 - oo ~0O -11.5 - -12 : S^ -12.5 : ; i i 1 ; 1 ; 1 : L in in in m in in in in in m en en en en en en en en en en c\i CO in CO r^ cd o> 12 . o o o o p p o in CD oCD iCO in oin d CO CM Date

Fig. 15 Stable oxygen isotope composition of the River Danube at Bratislava, Slovakia in the year 1995. Data from [5].

Table IX Stable oxygen isotope composition and the calculated ratio of Danube water components in % for the wells in the inner part of the Szentendre Island. Well number Isotopic com- Ratio of Danube water % position 8 o' O [%o]SMow hydraulic model isotopic model F-l -10.87 99 91 F-2 -10.87 84 91 F-5 -9.58 31 5 F-6 -10.12 27 41 F-9 -10.79 8 86 F-10 Jun.6. -10.76 88 84 F-11 Jun.6. -10.90 62 93 R-16 -10.83 91 89 S-63/a -10.47 12 67 F-10 Sept.7. -10.47 88 65 F-11 Sept. 12. -10.63 62 75 F-12 -10.14 35 43 F-20 -10.40 87 60 F-20/a -10.34 90 56 F-20/b -10.27 88 51 F-21 -10.31 86 54 F-21 to -10.37 62 58 F-24 -10.02 76 35 F-25 -10.84 39 89 F-26 -10.79 46 86 F-26/a -10.86 42 91 F-29 -10.61 51 74 F-36 -10.81 74 87 F-37 -10.84 89 89

162 4.2. Ratio of Danube water calculated by hydraulic modeling and stable oxygen isotope data

In the inner part of the Szentendre Island, where the transit time of the Danube water calculated by the hydraulic model fell into the range of decades (e.g. K-64; S-63/a; S-80; F-25 in Table VIII) the ratio of Danube water/infiltrated precipitation have been calculated by both models (Table IX). Data are plotted on Fig. 16. Hence the transit times in these cases are more than ten years, the supposition has been made that the water in these observation wells are 1 8 well mixed and their 8 O values are an average values characteristic for the ratio of mixing. Comparing the ratios calculated by the two different models we can see (Fig. 16) that the two calculated ratios are rarely the same. There is a high discrepancy in many cases. Two wells (F-10 and F-ll) were sampled two times (in June and September 1995). The 8• 18O values of the first sampling was -10.76%o and -10.90%o, while of the second sampling they were -10.47%o and -10.63%o respectively (Table IX). The differences are signifi- cant. As a consequence the calculated Danube water ratios differ considerably as well. So our supposition regarding the well mixed water in the inner part of the Szentendre Island was false. The ratio of Danube water/infiltrated precipitation changes frequently even in those observation wells where the transit time is some decades.

Ratio of Danube water in %

Q. 2 o 0)

10 20 30 40 50 60 70 80 90 100 Hydraulic modelling

Fig. 16 Ratio of Danube water in % calculated by hydraulic and isotopic methods are plotted.

163 5. SUMMARY

On the Szentendre Island the groundwater in the observation wells contains a considerable amount of Danube water. The Danube "flows" under the island.

In the case of the Dunakeszi Water Works, left bank of the River Danube and not island area, the Danube component of the supplied drinking water is less than 100%. The shallow groundwater behind the production wells is affected by the Danube in a lane of 500-800 m off-shore. The Danube water component decreases gradually.

1 8 Comparing the 8 O values with the Cl, NO3 and SO4 content of the water samples 1 R collected on the Szentendre Island, we can see a trend: the more positive the 8 O value the higher is the Cl, NO3 and SO4 content. This is a strong indication that the source of the Cl, NO3 and SO4 pollutants is on the island and not the Danube water. Csepel Island area: from the stable oxygen isotope data we can conclude that the Rackeve Danube affected the right bank side groundwater only, but not the left bank side groundwater. This fact indicates that the groundwater in the shallowest aquifer flows from east-northeast to south-southwest. Most probably the Danube discharges this flowing system.

1 fi Comparing the nitrate and chloride contents with the 6 O values, we can conclude that the water has higher amount of these pollutants where it has an infiltrated precipitation component. The transit time of Danube water calculated by hydraulic modeling is longer in average than those calculated by the stable oxygen isotope data. For more reliable calculation based on isotopic data a time series sampling of both the Danube and the wells would be necessary (if possible along a flow path).

The ratio of Danube water/infiltrated precipitation calculated by hydraulic modeling based on data collected during three years (average ratio) rarely matches this ratio calculated by stable oxygen isotope composition measured ones in the wells. The reason is that the ratio of these two kinds of water changes frequently even in those wells where the transit time of the Danube water is some decades.

ACKNOWLEDGEMENTS

Financial support for this study was provided in part by the Hungarian National Scientific and Research Fund (OTKA T0114968) and by the International Atomic Energy Agency (contract number HUN/8126). The authors are thankful to Dr. Gyo'rgy Molndr (Technical University of Budapest) for instructive discussions on hydraulic modeling, and Adam Kontur (Fovdrosi Vizmuvek Rt. Budapest) for providing data on water chemistry.

REFERENCES

[1] OLLOS, G., WISNOVSZKY, I., BAKONYI, P., BUZINKAY, P., LIEBE, P., RAD- VANYI, R., RATH, I., SALI, E., SZOLNOKI, CS., Integrated Water Resources Man- agement in Urban and Surrounding Areas (Case study). Budapest Technical University, "Romai Kiadoi es Nyomdaipari" Publishing Co., Budapest (1993).

164 [2] Fovarosi Vizmuvek Reszvenytarsasag (Water Works of Capital Corp., a brochure in Hungarian, 1996). [3] MOLNAR, GY., Determining the hydrologic protection area of the water works on the Szentendre Island by means of hydraulic modelling. Report. Archive of the Fovarosi Vizmuvek Rt. (Water Works of Capital Corp.), Budapest (in Hungarian, 1996). [4] RANK, D., Bundesforschung- und Priifzentrum Arsenal, Vienna, personal communication. [5] MICHALKO, J., Data from the Archive of the Geological Survey of the Slovak Repub- lic, Bratislava (1996), personal communication. [6] DEAK, J. Water Resources Research Centre, Budapest, personal communication. [7] EPSTEIN, S., MAYEDA, T., Variation of 18O content of waters from natural sources. Geochimica Cosmochimica Acta 4 (1953) 89-103. [8] DEAK, J., DESEO, E., BOHLKE, J.K., REVESZ, K., "Isotope hydrology studies in the Szigetkoz region, Northwest Hungary", Isotopes in Water Resources Management (Symp. Proc. Vienna, 1995), Vol. 1, IAEA Vienna (1996) 419-432. [9] FORIZS, I., Origin of groundwaters and determination of recent shallow groundwater component by stable isotope measurements. Ph.D. thesis, Kossuth University, Debrecen (in Hungarian with English abstract, 1995). [10] SUVEGES, M., Water Resources Research Centre, Budapest, personal communication.

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