Groundwater quality in Hungary a general view

Transcription

1 Freshwater Contamination (Proceedings of Rabat Symposium S4, April-May 1997). IAHS Publ. no. 243, Groundwater quality in Hungary a general view EVA DESEO Institute for Environmental Management (KGI), Budapest, Hungary JOZSEF DEAK Water Resources Research Centre (VITUKI), Budapest, Hungary Abstract More than 95 % of the drinking water demand of Hungary is met from groundwater. The first nationwide study of groundwater quality was made using chemical data from 5000 water supply wells. The wells were arranged into eight groups according to geological properties and depth of the aquifers. Groundwater quality was characterized by twelve main chemical components, and analysis undertaken by statistical treatment and mapping of areal distributions. It was established that the majority of groundwater quality problems in Hungary is caused by naturally derived components. These chemical components (iron, manganese, ammonia, organic material, methane, arsenic and low hardness) have been derived by water-rock interactions involving subsurface flow for more than years according to environmental isotope studies. INTRODUCTION Groundwater quality is very important in Hungary because more than 95% of drinking water demands (3 X 10 6 m 3 day" 1 ) are met from this source. This huge amount of water is pumped from more than 5000 wells by local water works. Temporary control of the quality of raw water is prescribed for the companies operating these wells. A national database was established by collecting chemical data measured in the last 20 years. The first results of the nationwide investigation of groundwater quality in Hungary based on these data are presented in this study. DATA The Water Resources Research Centre has collected more than three million chemical data from more than 5000 water supply wells belonging to 33 regional water works companies. The frequency of sampling of raw water and the chemical components determined differ between the companies. The main 12 chemical components measured by most of the water works were used for characterising groundwater quality. These components are: nitrate, ammonium, iron, manganese, ph, chemical oxygen demand, total hardness, sodium, chloride, sulphate, electrical conductivity and bicarbonate. The frequency of measurements is variable from one to 12 samples per year depending on the vulnerability of the aquifer. Groundwater quality of each well was characterized as an average value for the year METHOD Groundwater quality in Hungary was characterized by statistical treatment and by

2 228 Eva Deseô & JozsefDeâk mapping the areal distribution of the 12 main chemical components. Because geological properties and depth of the aquifers determine groundwater quality, water supply wells were arranged into eight groups: K groundwater stored in the fractures of karstified limestone and dolomite, (a) 20 -y- N03 (mg/l) NH4 (mg/l) M D1 D2 D3 Fe (mg/l) m O ^1 \A fe S D1 D2 K Mn (mg/l) PH 1/ WA m %, m m m <m J.<\ m. WÂ. m K B S D1 D2 D3 04 D5 COD (mg/l) m- VA UO t>* UD ground v/ater type Fig. 1 Average values of the main chemical components characterizing the groundwater quality of different types of aquifers.

3 Groundwater quality in Hungary a general view 229 B bank-filtered water of the rivers, mainly of the River Danube, S the first, shallowest groundwater (at a depth of <20 m) in porous aquifers, D groundwater of the porous sediments of the basins. This type of groundwater (b) 300 total hardness (CaO) Na (mg/l) -1 L.2 H WM M R9 f W/» V& S D1 CI (mg/l) S04 (mg/l) "ipf sir BMWBBH S 01 D2 D3 D4 DS J2ZL J1 D2 D3 el. conductivity (us/cm) HC03 (mg/l) soo - _ oo I!.;' Fig. 2 continued.

4 230 Eva Deseô & JozsefDeâk is divided into five subgroups by depths: Dl m, D m, D m, D m, D m. These groups contain water supply wells of different number. RESULTS Statistical treatment The first step in statistical treatment was to calculate the average concentration of the 12 main chemical components for each type of groundwater (Fig. 1(a) and (b)). The average values satisfy the standards for drinking water quality excepting the concentration of iron, manganese and ammonium (in some type of groundwater). Because the standard deviations are rather high, the ratio of wells having chemical components exceeding the drinking water standard limit has great importance in evaluating regional groundwater quality. Table 1 shows the percentage Fig. 2 Areal distribution of iron concentration in the main aquifer D3 ( m).

5 Groundwater quality in Hungary a general view 231 Table 1 Percentage of wells having chemical components exceeding the drinking water standards. Component N0 3 (mg 1 ') NH 4 (mg I-') Fe (mg I' 1 ) Mn (mg l" 1 ) COD (mg l' 1 ) Total hardness CaO (mg 1"') Conductivity (US cm ') CI (mg I- 1 ) Standard limit >40 >0.2 >2 >0.3 >0.2 >10 <50 >350 >1600 >350 K B S Dl D D D D of wells where the concentration of the main chemical components exceeds the standard limits. The average ammonium concentrations of the vulnerable karstic, bank-filtered and shallow groundwater are rather lower than in the protected deep wells. There are two different standard limits. For the vulnerable aquifers, where surface contamination might cause the presence of ammonium, the limit is 0.2 mg l" 1, and for the protected, deep groundwater it is 2 mg l' 1. The average value of ammonium concentration for deep wells (D5) is higher than 2 mg l" 1, and 52% of D5 wells exceed this limit. The increase in ammonium concentrations with depth (Fig. 1(a)) shows that the ammonium is naturally derived. The areal distribution for ammonium concentrations shows that the highest NH 4 values are characteristic of the discharge areas of the regional groundwater flow-systems. The average iron concentrations are also too high (>0.3 mg l" 1 ) in each type of groundwater, except the karstic water. The unacceptably high iron concentrations can be found not only in average values, but in terms of frequency too (in Dl, D2 and D3 types the iron content is not suitable in more than 50% of wells). Figure 2 shows the areal distribution of iron concentration in the D3 aquifer ( m) which can be considered as the main source for exploitation to provide water supplies. The map reveals that concentrations of iron are higher than 0.3 mg l" 1 in 80% of the investigated area. The average manganese content exceeds the limit of 0.2 mg l" 1 in the shallowest groundwater types (S, Dl, D2, D3) including also the bank-filtered resources (B). In these types of groundwater, more than 20% of wells have an unsuitable manganese content. Characterization of different types of groundwater The presence of tritium suggests karstic water (K), mainly in the shallow aquifers and springs, is one of the most vulnerable groundwater resources, although karstic water looks to be the best quality groundwater resource on the basis of average chemical composition. The average nitrate content (13 mg l" 1 ) is lower than the

6 232 Eva Deseô & JozsefDeâk 80 - là 60 - E, co O i i ' screen=15-33 m i ; 1 1!!! 1 :! ÎT 1^! J! I T+f j \ \/k ' mi a» i j i i i j 1? 1 j 1 1 i i i i i i i i i period (years) Fig. 3 Increasing N0 3 concentrations in a vulnerable karst aquifer (Sopron) standard for drinking water, but examining the distribution of N0 3 values (Table 1), it can be seen that 6.7% of karstic wells exceed the limit of 40 mg l" 1. This is a great problem because karstic water is the only drinking water resource in 20% of Hungary. In some small villages, where the nitrate level is too high, the drinking water demand has to be met by bottled water. The karstic wells, which were withdrawn from operation before 1992 because of their nitrate problems, were neglected in our study. These wells had low N0 3 content at the time of construction. The water quality deteriorated through some decades of operation as a result of overexploitation (see Fig. 3). Bank-filtered resources in Hungary, which can be dominantly found along the River Danube, meet more than 30% of the country's drinking water demands. Two million inhabitants of Budapest have obtained drinking water (for a century) from the islands to the north and south of the city. Temporary water quality problems are caused by the mixing of the pumped water with a higher proportion of background shallow groundwater during periods of lower water level in the river. Shallow groundwater (S) resources are exploited at a very low rate. This resource is the most vulnerable because of the very short transit time of the pollution from the surface to the aquifer. The shallow groundwater is polluted by nitrate under the unsewered villages (Deseô, 1995) although in the areas between villages low nitrate contents can be found. Settlements occupying only 5% of the whole area have an average N0 3 content of 188 mg l" 1. On the other hand, 80% of the period (years) Fig. 4 Increasing N0 3 concentrations in polluted shallow groundwater (Szombathely well no. 14).

7 Groundwater quality in Hungary a general view N03 (mg/l) Fig. 5 Relationship between nitrate concentration and depth in Gôdôllô. surrounding areas have shallow groundwater without any nitrate. As a result of carefully locating the wells tapping shallow groundwater, the average N0 3 content is only 17 mg l" 1, and less than 15% of wells contain nitrate higher than 40 mg l" 1. The deeper groundwater (D) is the most protected groundwater resource. According to Fig. 1(a), the average N0 3 content decreases with depth from Dl to D5. The relatively high average content of Dl (20-50 m) groundwater (5 mg l" 1 ) is the result of some wells (4.3%) having high nitrate concentration. These wells had also low nitrate content at the time of construction, but the polluted shallow groundwater gave additional recharge becausa of overexploitation as illustrated by the example of Szombathely (Fig. 4). The proportion of the additional recharge from the polluted shallow aquifer decreases with depth, as an example of data from Gôdôllô shows (Fig. 5). CONCLUSION One of the most important facts to be established by the study is that the greatest part of the contamination in groundwater is naturally derived. These chemical components (iron, manganese, ammonia, organic material, methane, arsenic and low hardness) have been dissolved from rock by subsurface flow which, according to environmental isotope studies (Deâk, 1995), has been taking place taking place for more than years. The concentration of these components in groundwater often exceeds the drinking water quality standards, mainly in the discharge areas of the regional groundwater flow systems. Monotonie changes of groundwater quality can be observed along flow paths from the recharge to the discharge areas. Transit time, and regional and local flow conditions of groundwater were analysed by environmental isotope data. In contrast to this very well protected groundwater, the vulnerable groundwater resources (bank-filtered, karstic and near-surface groundwater) sometimes contain

8 234 Eva Deseô & JozsefDeâk pollutants (mainly nitrate) originating from the surface. Statistical analysis of groundwater quality data shows that the pollution of nitrate in groundwater appears to be lower than has been supposed. REFERENCES Deseô, É. (1995) Groundwater quality and vulnerability in the Danube Tisza Region. VITUKI Report. Deâk, J. (1995) Limits of the groundwater exploitation on the alluvial fan of River Maros and the surrounding areas of the Great Hungarian Plain. VITUKI Report.