Origin of Bank Filtered Groundwater Resources Covering the Drinking Water Demand of Budapest, Hungary
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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 cal- culated by hydraulic modeling rarely match each other. The Danube water transit time calcu- lated 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 pollut- ants 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 wa- ter 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 settle- ments. The mixing of these two types of waters has been investigated by stable oxygen iso- tope 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 hydrau- lic 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 pollut- ants. 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 ON + 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 oo 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 tN 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. ~ N 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 N 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 infil- tration, so the shallowest groundwater of the island is a mixture of Danube water and the local infiltration.