## GROUNDWATER QUALITY ON THE LEFT SIDE OF THE DANUBE RIVER AT THE WATERWORKS KALINKOVO, SAMORIN AND GABCIKOVO

#### Maria VAVROVA

Western Slovak Waterworks and Sewage, Drienova 5, 82629 Bratislava, SLOVAKIA

CONCLUSIONS The history of ground water quality monitoring realised at the waterworks Kalinkovo, Samorin and Gabcikovo confirms that there are no significant changes in the ground water quality after putting the Gabcikovo hydropower structures into operation. According to the Slovak state norm STN 757111 ground water quality at all large mentioned waterworks is of drinking water quality and no additional treatment is necessary. The development of some qualitative parameters and water consumption is expressed in graphical form.

#### INTRODUCTION

Mainly social demand for the supply of healthy drinking water to the inhabitants coupled with intensive development of urban aglomeration, industry and agriculture in the territory of the former Western Slovak Country, required the orientation of a ground water survey for the construction of large-capacity well systems. Lack of drinking water in Bratislava, the capital of Slovakia, as a consequence of pollution of ground water at the water source for Bratislava at Podunajske Biskupice, accelerated the construction of the first large-capacity waterworks situated west of the village of Kalinkovo (constructed in the period 1971 to 1972). In following years waterworks were constructed at Samorin and later at Gabcikovo (Fig. 1). A permanent discharge of 850 l/s was recommended for exploitation at Kalinkovo. At Samorin the proposed discharge was at first 600 l/s, than later 900 l/s, and after the extension of the well system, to 1200 l/s. At locality "A" at Gabcikovo, it was recommended to exploit 1040 l/s of ground water.

The utilisation of the individual waterworks capacity depended on the requirements for municipal water supply which gradually increased and joined the system of these large-capacity waterworks. The quantity of water demanded for municipal water supply to Bratislava was decisive for a ground water waterworks at Kalinkovo, and partially also for waterworks at Samorin. Starting the waterworks operation on the right side of the Danube at Rusovce and the general decrease of water consumption by inhabitants and industry since 1991, decreased the requirements for municipal water supply from the waterworks, mainly from Kalinkovo. These circumstances were manifested in a decisive degree in the amount of water exploited from the water source at Kalinkovo (Fig. 2). Also decreases of water consumption have been recorded at other water supply systems in Slovakia (Fig. 3).

At present 200 l/s is distributed from the Kalinkovo waterworks. Ground water is exploited only because of ground water quality monitoring in the framework of the Gabcikovo project monitoring activities. This requires costly economical and operation measures.

A systematic operational measurement and watching of ground water quality from individual wells, according to valid standards, have been carried out since the beginning of the hydrogeological survey and construction of waterworks. Parallel with the construction of water supply wells, an observation system of deep multi-level observation wells was established on which quantitative and qualitative ground water observations have been realised. This system of wells is also used for monitoring the development of the impact of Gabcikovo structures on the ground water level and ground water quality.

Ground water exploited by the waterworks Kalinkovo, Samorin and Gabcikovo is of excellent drinking water quality and does not need any treatment. Chlorinating is used as preventive disinfectant of water in the municipal water supply system.

#### WATERWORKS KALINKOVO

The waterworks at Kalinkovo were set in operation in 1972. Ten wells with filter parts at a depth interval between 40 to 80 m were drilled for water exploitation. Well signed S-1 with a filter from 21.1 to 55.1 m was excluded approximately after two years of operation because of an increased iron and manganese content.

Ground water recharge from the Danube river water existed at all water level stages in the Danube. Impoundment of water in the Cunovo reservoir caused an essential increase of groundwater levels, in the region close to the waterworks and also in its wide surroundings, and this, without respect to the exploited amount of water from the wells. Water exploitation from wells, was occasionally interrupted in accordance on water needs in the water supply system. According to amount of water needed the number of operating wells has been changed. Because the possibility of accumulating pumped water at the beginning of the use of these wells did not exist, the repeated interruption of water exploitation from individual wells occured during the day. These operational procedures were negatively manifested in the water quality. There was found to be an increased share of amorphous material and sandy grains in the water, following by an occurrence of a higher number of psychrophilous bacteria (bioseston). An additional survey as well as an evaluation of operational procedures at the wells proved that hydraulic breaks caused by switching pumps on and off were responsible for that type of changes in the water quality. After accepting new measures in operational procedures at the wells, the exchange of pumps, and also partial measure changes in the municipal water supply system, the water quality was without defects. Based on the knowledge from the first years of operation the following principles for operation of water wells at Kalinkovo and later on also for the operation of the water wells at Samorin and Gabcikovo were formulated:

1. It is suggested to hold wells in continual operation at the constant recommended discharge.
2. Change of discharge is slow.
3. Due to interruption after sudden pumping, it is suitable to resume pumping after 12 hours.
4. It is necessary that each well chosen for the monitoring of ground water quality is permanently in continual operation at the recommended discharge.
5. If the required amount of water is smaller than the well's capacity, it is necessary to determine a regime of utilisation of the well system, taking into consideration the purposes of the monitoring and requirements of water supply.

The results of the requirements for water supply from the waterworks operations, showed that well S-4 has been in operation since 1993 only because of ground water quality monitoring, thereby ensuring objective water quality information.

Water quality from the Kalinkovo waterworks is systematically watched since setting the water wells into operation (1972). The ground water quality exploited from this locality fully complies to the parameters required by the state norm STN 757111 for drinking water. The physical and chemical quality of water is constant without expressive changes. Course of changes in water quality from well S-4, which was selected as representative for the Kalinkovo waterworks (situated in the middle of the well system, closest to the Danube and now reservoir) is shown in a selected extension of parameters in Fig. 4. The start of the Gabcikovo project and filling the reservoir with water, did not have a negative influence on the quality of exploited ground water. The presence of chemicals indicating water pollution was not detected. According to microbiological and biological criteria of the STN 757111, ground water from the wells is permanently suitable as drinking water.

Apart from the water wells, a system of deep multi-level observation wells was established. The original observation wells were supplemented by new objects PZ-12 and PZ-13, which belong to a group of piezometric observation wells established in the way used by the PHARE program entitled: Danubian Lowland - Ground Water Model. The observation wells are situated between the reservoir and the waterworks' wells. The observation wells are 70 m deep, equipped with eight piezometric tubes each.

The changes in distribution of single parameters indicate oxidation-reduction processes, coupled with the transformation of the chemical composition of water up to the depth of 25 m. We can connect these changes to the quality of infiltrating surface water and infiltrating water from the shallow coastal part of the reservoir. Also this "shallower" water is of good drinking water quality. Standard microbiological and biological examination of water from individual deep piezometric tubes of observation wells PZ-12 and PZ-13 document a minimal occurrence of microbiological and biological representatives. We can connect the increase of these parameters in the autumn period of 1994 to changes in water levels, increase in water temperature and, it is not possible to exclude an influence of microbiotope on the inside walls of observation wells and piezometric tubes. Generally, at the Kalinkovo waterworks territory the water quality was without expressive changes during 1994. We can state, that in this territory the natural transformation of the chemical composition of infiltrated water from the reservoir occurs.

#### SAMORIN WATERWORKS

The waterworks well system consists of six wells with filters at depths from 45.0 to 90.0 m. The waterworks were set in operation in 1975. The amount of exploited water from this waterworks depends on demand. Therefore only the well HGS-2 is in continual operation, which after the start of the waterworks operation, was selected as representative for the observation of the ground water quality development. Setting of the Gabcikovo project into operation caused a decisive increase of ground water levels in exploitation and observation wells.

The ground water quality of the Samorin waterworks, according to systematic observation since 1975, corresponds to the requirements of drinking water in whole to the parameters of water quality according to the norm STN 757111. The physical and chemical quality of water are constant, without any expressive changes. A slight increase in dissolved solids was recorded in 1988 at the time of transition to increased exploitation at 900 l/s. From a standpoint of microbiological and biological criteria the ground water is permanently of good quality drinking water. During setting the Gabcikovo project into operation, as well as in the following period the water quality was constant and equal to the water quality from the previous period. The presence of organic or inorganic chemicals indicating pollution or the deterioration of water quality was not detected.

#### GABCIKOVO WATERWORKS

The Gabcikovo waterworks (locality "A") is situated south of the city, Gabcikovo. The waterworks consist of 13 wells at 85.0-90.0 m depth. The filter part of the wells is located at the 50.0 to 80.0 m interval. It was recommended that the wells should be exploited up to 1040 l/s. Gradually since 1986 the waterworks started to supply water. Ground water quality has been systematically observed since the start of water exploitation according to state norm STN 757111. The ground water quality fulfils the criteria for drinking water quality in full extent. From a standpoint of microbiological and biological criteria for drinking water, the water from the wells is permanently of a satisfactory quality. We have not recorded a deterioration of water quality in connection with the construction and operation of the Gabcikovo project.

The history of ground water quality monitoring at the Kalinkovo, Samorin and Gabcikovo waterworks and at other observation wells stated the fact, that every chemical and biological analyses should be checked and interpreted consequently in relation to the actual hydrological, hydrogeological, technical and operational conditions and situation. Apart from this, it is important to document the technical state of the monitoring object, the procedure of sampling and the transport of samples, the laboratory analytical methods used and all other circumstances. Deep theoretical knowledge is necessary to avoid misinterpretations of data and misuses of various computer programs including those of a statistical nature. It is necessary to pay attention to these realities in order to make the interpretation of long-dated water quality results more objective.

#### REFERENCES

  Vavrova, M., Takacova, J., Hudecova, M., 1995: Final report about results of monitoring of quantitative and qualitative groundwater regime in area of water sources Kalinkovo, Samorin and Gabcikovo during period 1.7.1994 - 31.12.1994. Final report (in Slovak). Western Slovak waterworks and sewage, Bratislava.