APPLICATION OF ARITHMETIC FORMULAE IN DETERMINING VOLUME OF SEA WATERS INFLOW INTO ELBLĄSKA BAY RIVER ELBLĄG LAKE DRUZNO HYDROLOGICAL SYSTEM

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1 A C T A G E O P H Y S I C A P O L O N I C A Vol. 52, No APPLICATION OF ARITHMETIC FORMULAE IN DETERMINING VOLUME OF SEA WATERS INFLOW INTO ELBLĄSKA BAY RIVER ELBLĄG LAKE DRUZNO HYDROLOGICAL SYSTEM Roman CIEŚLIŃSKI Department of Hydrology, Gdańsk University ul. Dmowskiego 16a, Gdańsk georc@univ.gda.pl Abstract This paper is part of broader studies aimed at finding several mathematical formulae allowing to determine the volume of inflow of salty or slightly salty waters into water reservoirs of the coastal zone of the Baltic sea with input data limited to only one chemical index. The main aim of the paper was an evaluation of the intensity of inflows of sea waters into a chosen coastal water body within the Polish zone of the southern Baltic. This evaluation was performed on the basis of calculations of the percentage share of sea water in the waters of the chosen coastal reservoir using the formula suggested by J. Bolałek, determined on the basis of the mixing index η introduced by Ovchynnikov. Key words: lake Druzno, complete inflow, incomplete inflow, mixing index. 1. INTRODUCTION The phenomena of intrusions of sea waters into coastal reservoirs causing changes in the chemical composition of lake water are poorly explored, due to complex chemical and biochemical processes continuously taking place in both water bodies. Hence, it is essential to precisely identify physical and chemical structure of water itself and the properties of solutions of electrolytes occurring in it, as well as mechanisms of ex-

2 522 R. CIEŚLIŃSKI change of mass of land and sea water and places in which this mixing occurs. This is still very important as there is too little attention in literature to the two-way exchange of waters between coastal reservoirs and the sea, which very often results from a lack of data on the discharge of water from the given lake into the sea and in the opposite direction. Thus, we now have to search for other ways which would allow us to pinpoint an intrusion situation and to determine the scope and intensity of the influence of a particular intrusion of sea waters on the waters of seaside reservoirs. What seems to be most appropriate is the use of arithmetic formulae based on a chosen physicalchemical index or working out changes in water quality on the basis of several characteristic chemical indices. The quality of water should be understood as the set of physical and chemical properties distinguishing a given water from other (Wolford and Bales, 1996). 2. THE AIM AND SCOPE OF THE PAPER This paper is part of broader studies aimed at finding several mathematical formulae allowing to determine the volume of the inflow of salty or slightly salty waters into each water reservoir of the coastal zone of the Baltic sea with the input data limited to just one chemical index. The main aim of the paper was an evaluation of the intensity of inflows of sea waters into a chosen coastal water body within the Polish zone of the southern Baltic. This evaluation was performed on the basis of calculations of the percentage share of sea waters in the waters of the chosen coastal reservoir using the formula suggested by Bolałek (1993), which was determined on the basis of the mixing index η by Ovchynnikov (1963) and Macioszczyk (1987). An additional aim of the paper was to determine periods when an inflow of sea waters occurred and work out a division of inflows with respect to intensity and spatial range, as well as to specify frequency of the inflow occurrence. In addition, an attempt was made to relate the occurring inflows with hydrometeorological conditions. 3. RESEARCH AREA An example of an area where the phenomena of two-way water exchange are clearly visible is the catchment of the river Elbląg, especially the system formed by lake Druzno, the river Elbląg and Elbląska Bay. There occur both fast changes of water quality and changes in concentrations of individual substances which can remain in an insoluble state. The research area (Fig. 1) covered the body of lake Druzno (coastal reservoir), south-western part of the Vistula Lagoon (sea water body), Elbląska Bay, watercourses draining into the lake and the river Elbląg through which the lake contacts the sea.

3 SEA WATER INFLOW INTO COASTAL RESERVOIRS 523 Fig. 1. Area of investigations.

4 524 R. CIEŚLIŃSKI 4. CHARACTERISTICS OF THE MAIN HYDROGRAPHIC OBJECTS Vistula Lagoon The Vistula Lagoon, sometimes called Fresh Lagoon, used to be one of the main recipients of the river Vistula. After the arms of the delta were cut off from the main channel of the Vistula, the role of the sea factor in the lagoon increased substantially (Majewski, 1960). It spreads in the form of a longitudinally stretched rectangular of 90.7 km of length and mean width of 9.2 km from Żuławy Wiślane up to the Sambia peninsula and mouth of the Pregoła river. Its area is 838 km 2. The only connection of the lagoon with Gdańsk Bay is by Pilawska Strait. The Vistula Lagoon is a very shallow reservoir. Mean depths are only m. Only within the channel of a water route running in the middle of the lagoon do the depths reach values larger than 3.0 m and maximum 5.1 m (Sołowiew, 1975). Elbląska Bay In the southern part of the Vistula Lagoon there may be separated the area of Elbląska Bay which constitutes the southernmost part of the Vistula Lagoon (Szarejko, 1955). This water region has a shape of a sack of 7.5 km length, about 1.0 km width (between 0.6 and 1.4 km) and an axis situated along the SSW NNE line (Błaszkowski et al., 1998). Its area is small, only 7.0 km 2. The whole water region is very shallow. Only on the very borderline between the bay and open waters of the Vistula Lagoon are there depths of 2.0 m. River Elbląg The river Elbląg begins by flowing out of the north-western end of lake Druzno and drains into the Elbląska Bay. Nowadays, the river Elbląg in its full length has a form of an embanked canal. In the area of the town of Elbląg its channel is reinforced by a concrete casing, deepened and broadened in order to perform the function of a harbour wet dock. Its width on the water surface is between 35.0 m and 64.0 m, and depth from 1.7 m at lake Druzno to 4.3 m in its lower section, and the length is 14.5 km (Bocheńska, 1995). Lake Druzno Lake Druzno is situated in northern Poland, 14 km from the Vistula Lagoon. Despite quite a large distance (about 11 km in a line) it remains in a hydraulic relationship with the waters of the Elbląska Bay and Vistula Lagoon not only because of the connection through the river Elbląg but mainly by a slight difference of water levels in the lake and bay on the average only about 0.1 m. The area of the water surface in the lake with a mean water stage is about 1450 ha, its mean depth is only 1.2 m and maximum depth 3.0 m. Due to fluctuations in water stages reaching over 1.5 m, the

5 SEA WATER INFLOW INTO COASTAL RESERVOIRS 525 area of the water surface may vary between about 1260 ha with low stages and about 1900 ha with high stages, and even to over 2900 ha with extremely high stages (Cyberski and Mikulski, 1976). The capacity of the lake is 22.4 million m 3, area of the catchment is 1084 km 2, and water exchange index ) is 9.8 (Mikulski, 1964; 1970). Lake Druzno may be divided into a swamp zone and proper water area. The water area, in turn, may be divided into two zones (Buliński, 1998): water surface with vast patches of floating vegetation and rushes zone (Fig. 2). Fig. 2. Lake Druzno. Tributaries of lake Druzno 12 watercourses drain into lake Druzno; they flow radially from Wysoczyzna Elbląska, Nizina Warmińska, Pojezierze Iławskie and Żuławy Wiślane, as well as Elbląski Canal and a dozen of canals draining waters from polders directly into the lake through pumping stations (Fig. 3). The river inflow into lake Druzno is estimated at 6.9 m 3 /s which is million m 3 yearly (Mikulski, 1970). One watercourse flows out of lake Druzno the river Elbląg and there are several sluices allowing to discharge water from the lake onto polders when irrigating. The runoff through the river Elbląg into the Vistula Lagoon is estimated at 7.0 m 3 /s (Silicz, 1975). ) Ratio of river inflow to the capacity of the lake a multiple of annual exchange.

6 Kowalewka 526 R. CIEŚLIŃSKI Burzanka LAKE DRUZNO Wska Balewka Dzierzgoń Brzeźnica Marwicka Elbl ą ski Canal M ł ynówka places of discharge of waters from polders direction of flow of rivers km Fig. 3. Lake Druzno and its inflows. 5. TERMS RELATED TO GEOGRAPHICAL LOCATION OF DRUZNO RESERVOIR Druzno is a reservoir which defies any common definition. The reason for the difficulty is mainly the hydrological and hydrodynamic specificity of the system formed by the reservoir together with neighbouring water bodies as well as qualitative temporal and spatial changes occurring in the waters of this system. Several acceptable definitions and classifications of lake Druzno are presented below. Classifying Druzno as a lake is not limnologically justified (Mikulski, 1955), as the whole area of the lake may be divided into a swamp zone and proper water area of the reservoir. The mean and maximum depths are small, and relative depth is This is a value far below the ones quoted by Stangenberg (1936) of the smallest relative depths of Polish lakes. Thus, Druzno should not even be called a pond-like lake and should rather be named overflowed land or pond-lake. Along the southern Baltic coast there are numerous coastal lakes, among which lake Druzno is also quoted (Łomniewski, 1974; Mikulski and Bojanowicz, 1967; Mikulski et al., 1969), despite quite a substantial distance from the sea. This lake is supplied by river waters flowing from Żuławy and surrounding morainic plateaux. However, it is also under influence of waters of marine origin and of fluctuations of sea level (Pietrucień, 1983).

7 SEA WATER INFLOW INTO COASTAL RESERVOIRS 527 Some authors classify lake Druzno as belonging to deltaic lakes (Buliński, 1998; Lange, 1993; Choiński, 1991), with relic reservoirs remaining after a period of natural formation of a delta. These are usually reservoirs with an advanced process of transformation into land terrain, to which a huge production of plant organic mass decidedly contributes. Some authors, as Majewski (1972), Tadajewski et al. (1979), Jasińska (1991), claim that part of the coastal lakes may be regarded as estuaries. According to Pritchard (1952) as quoted by Majewski (1972): Estuary is a half-closed near-shore part of waters with free contact with the open sea and containing measurable quantities of sea salt. In such an estuary, sea water is diluted by fresh water from overland flow (Cameron and Pritchard, 1963). However, according to Jasińska (1991), this definition does not cover all estuaries, i.e., such where the outflow is smaller than evaporation. This author suggests applying the name estuary (river-mouth area) to the part of a river, as well as coastal lakes and lagoons, through which the river flows and which is influenced by the sea, together with the near-mouth area of the sea where the influence of the river is clearly noticeable. Still another definition of an estuary was suggested by American oceanographers. According to them, an estuary is a reservoir which has an indirect connection with the sea simultaneously being isolated from it to a certain degree. Here occur processes of periodical mixing of waters from overland flow and sea waters which significantly influence the discharge dynamics and physical and chemical properties of water (Majewski, 1994). Lake Druzno, through which waters from the whole catchment of the river Elbląg are discharged, and which intercepts some quantities of sea salt through the Vistula Lagoon and the river Elbląg, according to the definition suggested by Pritchard (1952), may be classified as an estuary reservoir. This reservoir is a fragment of a transition zone where constantly occur processes of mutual influence of land and sea waters (Łomniewski, 1958). The penetration of sea water into the near-mouth area of land waters brings about periodical increases of the number of ions characteristic of sea water (Cyberski and Mikulski, 1976; Łomniewski et al., 1972; Nowacki, 1974). The penetration to estuaries of this type generally occurs with favourable hydrological and meteorological conditions (Zaborowska-Młodzińska, 1963). 6. METHODS OF RESEARCH In the years systematic investigation of intrusions of slightly salty waters into the Elbląska Bay river Elbląg lake Druzno hydrographic system were undertaken in order to determine their intensity and range. In order to perform the above mentioned investigation there were chosen 21 sites of taking samples (Fig. 4). Five sites were chosen directly on lake Druzno, four along

8 Kowalewka 528 R. CIEŚLIŃSKI Elbląska Bay measurement sites ą Burzanka Elszka Wąska Balewka Marwicka Elbl ą ski Canal Dzierzgoń Brze ź nica km Fig. 4. Location of measuring sites. the river Elbląg, two on Elbląska Bay and one on the main tributaries of lake Druzno. One measurement point was located also on the Vistula Lagoon. Water samples from the surface layer were taken by means of a hydrographic cylinder of 3 dm 3 capacity. The taking and storing of samples were performed in compliance with recommendations given by Polish Standards. Samples of water were taken in several places along a given measurement profile. Taking samples of water only from the surface layer was dictated by earlier investigations of the quality of surface and near-bottom water of lake Druzno. In those investigations there were observed no significant differences in concentrations of physical-chemical indices for individual measurement columns, which is justified by the quasiestuary hydrographic system of the object of research and a small mean and maximum depth of Lake Druzno.

9 SEA WATER INFLOW INTO COASTAL RESERVOIRS 529 Chemical analyses of samples of water were performed in the laboratory of the Department of Hydrology of Gdańsk University. They covered determinations of concentrations of chlorides. Determinations of chlorides were performed by means of MPM-3000 photometer made by WTW. Analyses were performed using mercury thiocyanate and iron nitrate. Additionally, results obtained by the Institute of Meteorology and Water Management in Warszawa were used in the research. They consist of the direction and speed of wind data in stations in Świbno, Kmiecin and Elbląg for , as well as results of water stages in the Vistula Lagoon, river Elbląg and lake Druzno also for In order to calculate the degree of mixing of lagoon and land waters there was used mixing index η whose formula and description is given below. The choice of this particular formula was dictated by the possibility of substituting expensive and laborious measurements of two-way exchange of waters (discharges) in reservoirs of the coastal zone. It also should be stated that water flow measurements are rare for Polish deltaic and coastal lakes. If they do exist, they concern a short measurement period. In the case of mixing index η there only has to be performed a measurement of concentration of chlorides for three water masses (sea, fresh and mixed water), and then mathematical calculation of the share of sea waters in waters of the hydrographic objects of interest. Additionally this method is one of the few methods in the world using only one chemical index. It should also be stated that this formula offers the possibility of confirming or excluding the occurrence of inflow of sea waters into a chosen object under study, informs us how far the inflow reached, and determines its intensity. 7. COEFFICIENT OF OVCHYNNIKOV AND FORMULA BY BOLAŁEK The degree of mixing of lagoon and land waters was determined by calculating the mixing coefficient η (Ovchynnikov, 1963; Macioszczyk, 1987): VM η =, (1) V where V M is the volume of lagoon water (in m 3 ), V S is the volume of fresh water (from watercourses) (in m 3 ), and: S S VM c b = V a c where a is the chloride ion concentration in waters of the Vistula Lagoon (for the purposes of calculations, the mean value of concentration was used, calculated from results exceeding 1000 mg Cl/dm 3, which was 1286 mg Cl/dm 3 ); b is the chloride ion concentration in fresh waters, in watercourses flowing into lake Druzno (the mean value of the concentration was used calculated for all watercourses draining into lake, (2)

10 530 R. CIEŚLIŃSKI Druzno, which was 18.5 mg Cl/dm 3 ), and c is the chloride ion concentration in water mixed in given stations of Elbląska Bay, river Elbląg and lake Druzno. The percentage share of lagoon water in waters of lake Druzno and the river Elbląg was calculated on the basis of the formula suggested by Bolałek (1993): VM 1 % WM = 100% = 100%. V + V 1+ 1η M S In the formula by Bolałek Ovchynnikov, chlorides were chosen as chemical indices, which was dictated mainly by the physical and chemical properties of this ion. Chlorides do not participate in the oxidation and reduction processes and they almost do not take part in absorption exchange processes, and they belong to the most active water migrants (Burchard et al., 1990). Additionally, the concentration of chlorides is the highest in sea waters, so they can be a good indicator of the sea origin of waters in fresh-water reservoirs (Łomniewski, 1969). (3) 8. TYPES OF INFLOW An analysis of the results showed that in the case of several measurement series, the values of chlorides were clearly higher than mean values recorded in the whole research period. These are the cases of recorded intrusions of slightly salty waters into the waters of lake Druzno and the river Elbląg (Table 1). On 24 September 1997, 26 August 1998, 25 February and 6 October 1999, intrusions were recorded in all measurement sites. These were complete intrusions covering all the hydrograhic objects. On 9 July and 30 October 1997 and 22 January 1998, the intrusions were observed only in some of the measurement sites. These were incomplete intrusions. In order to evaluate the intensity of intrusions, calculations were performed of the percentage share of lagoon water in the waters of Elbląska Bay, river Elbląg and lake Druzno using formula (3) obtained on the basis of formulae (1) and (2). It was assumed that a content of lagoon water exceeding 10% means an occurrence of intrusions of slightly salty waters. It was also assumed that the intrusion is weak if the percentage share of slightly salty waters is between 10.1% and 25.0%, it is medium between 25.1 and 50.0%, strong between 50.1 and 75.0%, and very strong above 75.0%. The analysis covered only these periods when values of results exceeding 10% occurred at least in the full length of the river Elbląg. 9. COMPLETE INTRUSIONS 24 September 1997 On the day of taking water samples there was recorded a change of wind direction from western into northern, thus the conditions were favourable for an occurrence of

11 SEA WATER INFLOW INTO COASTAL RESERVOIRS 531

12 532 R. CIEŚLIŃSKI intrusion. The wind speed was 3 4 m/s. Water stages on the day of taking samples were higher in the Vistula Lagoon than in lake Druzno by 22 cm. The hydrological situation was thus also favourable for an occurrence of intrusion. It was observed that the intensity of this intrusion, expressed in a percentage share of lagoon water in waters of the estuary (Fig. 5), was very strong in the full length of Elbląska Bay and river Elbląg, strong in the northern and central water region of lake Druzno, and weak in the southern. Vistula Lagoon Elbląska Bay river Elbl ą g Intrusion: very strong strong slight lake Druzno Fig. 5. Percentage share of lagoon water in Elbląska Bay river Elbląg lake Druzno hydrological system waters on 24 September August 1998 On the day of taking samples as well as the preceding day, climatological stations observed wind from northern and western sectors. During field measurement, for several morning hours, violent storm winds were recorded. The wind speed was 4 9 m/s. Water stages on the previous and the proper day were higher in the mouth section of the river Elbląg than the stages observed on lake Druzno by 19 cm (proper day) and 32 cm (preceding day). The above hydrometeorological conditions were favourable for an occurrence of lagoon waters intrusion as far as lake Druzno. The percentage share of lagoon water in the waters of the system indicated that a very strong intrusion was observed up to the measurement site in the city of Elbląg. On the river Elbląg above the city of Elbląg and in the northern and central water region of lake Druzno a strong intrusion was recorded, while in the southern region of lake Druzno a medium intrusion was observed.

13 SEA WATER INFLOW INTO COASTAL RESERVOIRS February 1999 Winds and water stages on the previous and proper day were favourable for an occurrence of intrusion, as winds were recorded from the northern and western sector. The wind speed on individual stations oscillated between 5 and 9 m/s, while water stages allowed water to flow from the Vistula Lagoon into lake Druzno, as on the Vistula Lagoon the observed water stages ranged between 571 cm (previous day) and 582 cm (proper day), and on lake Druzno the water stages on both dates were 540 cm. The percentage share of lagoon water in waters of the system indicated that in Elbląska Bay and part of the river Elbląg from the site below the city of Elbląg to the mouth, a medium intrusion was observed, while on the remaining section a weak intrusion occured. In the waters of lake Druzno in its full length also a weak intrusion was recorded. 6 October 1999 On the day of taking samples there was recorded a change of wind from southern to northern in stations Elbląg and Świbno and from southern to western in station Kmiecin. On this date the measured wind speed was between 1 and 3 m/s. Water stages on the preceding and proper day of investigation were higher on the Vistula Lagoon than on lake Druzno by 12 cm (preceding day) and 35 cm (proper day). The above data can confirm an occurrence of intrusion situation in this period. The percentage share of lagoon water in waters of the system indicated that in Elbląska Bay and part of the river Elbląg from the site below the city of Elbląg to the mouth, a strong intrusion was observed, and in the remaining section a medium intrusion (site in Elbląg) and weak (site above Elbląg). In waters of lake Druzno in its full length a weak intrusion was observed. 10. INCOMPLETE INTRUSIONS 9 July 1997 Both on the day of taking samples and the preceding one there were recorded winds from northern sector of 5 to 8 m/s speed. On the day of investigation also higher water stages were observed on the Vistula Lagoon (543 cm) than on lake Druzno (530 cm). During the previous day an opposite hydrological situation was observed, where higher water stages were recorded on lake Druzno (535 cm) than on the Vistula Lagoon (525 cm). This situation indicates that wind directions in the whole period were favourable for an occurrence of an intrusion. However, the diversity of hydrological situation on the day preceding the investigation and the proper day may have limited the range of the intrusion.

14 534 R. CIEŚLIŃSKI The percentage share of lagoon water in waters of the estuary indicated that in the whole Elbląska Bay a very strong intrusion was observed, in the full length of the river Elbląg a strong intrusion, in the northern water region of lake Druzno a medium intrusion, and in the central one a weak intrusion. In the southern region of the lake there were no symptoms of an intrusion at all (Fig. 6). Vistula Lagoon Elbląska Bay river Elbl ą g Intrusion: very strong strong medium slight lake Druzno no intrusion Fig. 6. Percentage share of lagoon water in waters of Elbląg Bay river Elbląg lake Druzno hydrological system on 9 July October 1997 The meteorological and hydrological situation both on the previous and proper day were unfavourable for an occurrence of intrusion. However, an analysis of the hydrometeorological situation in the period of a few preceding days showed that there were recorded high-speed winds from the northern sector (Table 2), and water stages allowing a flow from the Vistula Lagoon to lake Druzno (Table 3). The percentage share of lagoon water in waters of the system indicated that in the northern part of Elbląska Bay a medium intrusion was recorded, while in the southern a weak intrusion. In the full length of the river Elbląg and in the northern water region of lake Druzno also a weak intrusion was recorded. In the remaining regions of lake Druzno no intrusion was recorded.

15 SEA WATER INFLOW INTO COASTAL RESERVOIRS 535 Table 2 Direction and speed of wind from 24 to 28 October 1997 measured in meteorological stations: Elbląg and Świbno Day Elbląg Świbno direction speed direction speed 24 NW 7 m/s N 10 m/s 25 N 6 WNW 7 26 N 4 NW 7 27 NNW 2 NW 5 28 W 4 W 4 Table 3 Water stages from 24 to 28 October 1997 in gauging stations: Batorowo, Elbląg and Żukowo and differences W between water stage on lake Druzno and Vistula Lagoon Day Batorowo (Vistula Lagoon) Elbląg (river Elbląg) Żukowo (lake Druzno) W cm 553 cm 540 cm 11 cm January 1998 On 21 January 1998 measurement stations Elbląg and Świbno observed winds from the northern sector. On 22 January 1998 such winds were measured only on station in Świbno (ENE). The wind speed was about 2 m/s. Water stages in the above days were higher on the Vistula Lagoon and river Elbląg than on lake Druzno. The difference in water stages between the Vistula Lagoon and lake Druzno was 56 cm on the previous day and 33 cm on the proper day. Thus, on the day of investigations there was recorded a lowering of water stages in comparison with the previous day. The above situation might indicate a stopping of intrusion. The percentage share of lagoon water in waters of the system indicated that in the whole Elbląska Bay there was a medium intrusion, while in the full length of the river Elbląg a weak intrusion was observed, and in the whole length of lake Druzno no symptoms of intrusion were observed.

16 536 R. CIEŚLIŃSKI 11. CONCLUSIONS As a result of the performed research on the influence of the sea on the quality state of waters of lake Druzno it is possible to draw several final conclusions. 1. The performed measurements showed that for 35 periods on which samples of water were taken, 7 document intrusions of waters of the Vistula Lagoon into the estuary. It was observed that intrusions covered either the whole estuary (the so-called complete intrusions) or only a part of it, during which slightly salty waters reached only the northern water region of lake Druzno (the so-called incomplete intrusions). 2. The choice of formulae by Ovchynnikov and Bolałek proved to be appropriate. These formulae confirmed or excluded the possibility of an occurrence of intrusion of sea waters into chosen hydrographic objects. Additionally they informed how far the intrusion reached and determined how intensive the intrusion was. This method was verified by two other methods based, like the formula by Bolałek Ovchynnikov, on a chemical analysis. In the first case this was a method based on stechiometric systems using hydrochemical classification by Alekin (1970), while the other method consisted in calculations applying mathematical proportion using a chosen chemical sea index (Cieśliński, 2003; Cieśliński and Drwal, 2003). 3. The recorded intrusions occurred only under favourable hydrometeorological conditions. It was the hydrometeorological situation that decided if there occurred a complete or incomplete intrusion. Moreover, it was observed that a substantial change of even one of the two factors, e.g., wind direction, resulted in a fast disappearance of hydrochemical effects of an intrusion of lagoon waters into the estuary. Thus, it is important for the interpretation of the results for individual intrusions of sea waters to take into account additionally the history of each analysed period. 4. This paper is only part of broader studies aimed at finding several mathematical formulae allowing to determine the volume of the inflow of salty or slightly salty waters into each water reservoir of the coastal zone of the Baltic sea with the input data limited to results for just one chemical index. The author intends to calculate the inflow of salty waters into a coastal reservoir on the basis of mathematical proportions using one physical-chemical index: of chlorides or proper conductivity. 5. Further research, aimed at determining the influence of sea waters on hydrographic objects of the coastal zone in time and space, should be supplemented by verification procedure on the basis of measurements of hydraulic parameters in chosen measurement profiles. Additionally, there should be determined functional relationships between the difference of water levels, mean wind and intensity and scope of the intrusion. The above conclusions indicate that the Elbląska Bay river Elbląg lake Druzno hydrological system forms a compact hydrodynamic system which may be classified as quasi-estuary, because only periodical and not cyclic penetrations of slightly salty waters occur in this system. This periodicity results from unique meteorological and

17 SEA WATER INFLOW INTO COASTAL RESERVOIRS 537 hydrological situations typical of the southern Baltic and from hydrographic features of the very system. References Alekin, O.A., 1970, Hydrochemistry, Gidrometeor. Izd., Leningrad (in Russian). Błaszkowski, J., R. Bogdanowicz and R. Cieśliński, 1998, Geographical environment of Elbląska Bay and its surroundings, Dept. Hydrology, University of Gdańsk, Gdańsk (unpublished, in Polish). Bocheńska, T., 1995, Water Resources and Management in Catchments of the Elbląg Province, Regionalny Zarząd Gospodarki Wodnej w Gdańsku, Elbląg (unpublished, in Polish). Bolałek, J., 1993, Matter Cycling Between the Sediments and Overlying Water in Pucka Bay, Wyd. Uniwersytetu Gdańskiego, Gdańsk (in Polish). Buliński, M., 1998, The Lake Druzno Reserve Nature and problems of protection of deltaic lake. In: J. Herbich and M. Herbichowa (eds.), The Vegetable Cover of Pomeranian the Differentiation, Dynamics, Threat and Protection, Przewodnik Sesji Terenowych 51. Zjazdu PTB, Sept. 1998, (in Polish). Burchard, J., U. Hereźniak-Ciotowa and W. Kaca, 1990, Methods of Investigation and Assessment of Quality of Surface and Underground Waters, Wyd. Uniwersytetu Łódzkiego, Łódź (in Polish). Cameron, W.M., and D.W. Pritchard, 1963, Estuaries. The Sea, Academic Press Inc., New York London, vol. 2. Choiński, A., 1991, Catalogue of Polish Lakes. Part 2: Masurian Lakeland, Wyd. Uniwersytetu A. Mickiewicza, Poznań (in Polish). Cieśliński, R., 2003, The influence of present intrusion of sea waters on content of chlorides, sodium, potassium and magnesium in waters of lakes of South Baltic coastline. In: R. Gołębiewski (ed.), Ewolucja Pojezierzy i Pobrzeży Południowobałtyckich, Fundacja Rozwoju Uniwersytetu Gdańskiego, Gdańsk, (in Polish). Cieśliński, R., and J. Drwal, 2003, The influence of present intrusion of sea waters on seacoast in South Baltic. In: Theoretical and Practical Problems in Present Limnology, University of Belarus, Minsk, (in Russian). Cyberski, J., and Z. Mikulski, 1976, Hydrological relations. In: B. Augustowski (ed.), Żuławy Wiślane, Gdańskie Towarzystwo Naukowe, Gdańsk, (in Polish). Jasińska, E., 1991, Dynamics of Waters in Estuaries of Polish Rivers, Prace IBW PAN 24, Gdańsk (in Polish).

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