Problems Of Lake Eutrophication Caused By The Operation Of Pumped-Storage Power Plant

 

MAREK GENEJA^*, TERESA JARZEBINSKA^*, WOJCIECH MAJEWSKI^**

 

^*Technical University of Gdansk, Faculty of Environmental Engineering, Poland

Narutowicza 11/12, 80-952 Gdansk, Poland

tel. +48 58 347 16 24, +48 58 347 29 30, fax +48 58 347 24 13,

e-mail: mgen@sunrise.pg.gda.pl, tjarz@sunrise.pg.gda.pl

 

^**Polish Academy of Sciences, Institute of Hydroengineering, Gdansk, Poland

Koscierska 7, 80-953 Gdansk, Poland

tel. +48 58 552 39 03, fax +48 58 552 42 11, e-mail: wmaj@ibwpan.gda.pl

 

 

ABSTRACT

Lake Zarnowiec is (since 1983) the lower reservoir of the pumped-storage power plant. The studies carried out in 1994 had to indicate how the operation of Zarnowiec power plant influences the ecological state of the lake. The chemical water quality and trophy assessment of the lake and its susceptibility to degradation was an important part of these studies. In the paper the main results of the investigation are presented.

 

Keywords: water resources, water quality, eutrophication of the lake, lower eservoir, pumped-storage power plant

 

LAKE ZARNOWIEC

Lake Zarnowiec is situated in the northern part of Gdansk province. It is one of the largest and most beautiful lakes in this region and it plays important role in creating the landscape. The lake has very good water quality and is used intensively for recreation and fishery. Therefore preservation of water quality and ecological values is of prime importance. The lake belongs to the catchment of Piasnica River. Piasnica Górna flows into the lake and flows out of the lake through the control weir as the Piasnica Dolna (Fig.1.). River catchment amounts to 310 km². Piasnica Dolna discharges into Baltic Sea.

 

Since 1983 the Lake Zarnowiec forms lower reservoir of the pumped-storage power plant. During the turbine cycle maximum power of pumped-storage power plant Zarnowiec is 716 MW with discharge to the lake 700 m³/s. During pump cycle maximum power taken from the power grid is 800 MW and the discharge to the upper reservoir amounts to 600 m³/s. The water surface elevations in the lake (operation of pumped-storage power plant) vary daily about 1.0 m. During the year water elevations range between 0.70 and 2.00 m. Minimum water elevation appears during morning hours (when upper reservoir is full) and maximum - late in the evening when upper reservoir is empty. In natural conditions average water elevation was 1.34 m above sea level and the lake had the surface area of 14 km² with a total volume of 118 Mio m². The length of the lake is 7.6 km and the maximum width 2.6 km (Fig.1.). The maximum depth is 19.4 m and the average depth is 8.4 m. Minimum observed water surface elevation was 0.92 m and maximum 1.78 m. The bottom of the lake consists of sand and gravel, and in some areas is covered with organic sediments.

 

Surface inflow to the lake comes from the direct catchment and 2 small tributaries (Piasnica Górna and Bychowska Struga). The average surface inflow to the lake is small and amounts to 1.7 m³/s and the groundwater inflow was estimated as 0.6 m³/s.

 

In the past outflow from the lake was a natural one. Now with significant daily variation of water surface elevation the weir which controls the outflow from the lake has been constructed. The minimum biological outflow was determined as 0.3 m³/s.

 

STUDIES OF LAKE ZARNOWIEC

In the seventies it was planed to construct pumped-storage power plant and nuclear power plant on the shores of the Lake Zarnowiec. The operation of both projects would result in considerable hydrodynamic, thermal and environmental changes in the lake, therefore comprehensive studies of the lake were undertaken. They included hydrology, hydrodynamics, thermal regime, hydrochemistry, hydrobiology, ichthyology, and meteorology of the lake region. These studies started in 1973 and were continued till 1990.

 

In May 1983 the pumped-storage power plant was put into operation, and in 1990 it has been decided to abandon the construction of nuclear power plant.

 

In 1993, after 10 years of the operation of pumped-storage power plant, it has been decided to carry out another comprehensive study which had to indicate how the operation of the power plant influenced the ecological state of the lake, and what changes of the lake are caused by the power plant and what is due to varying use of the land of the catchment.

 

This study included: changes in the shoreline, lake hydrodynamics and thermal regime, water quality assessment of the lake and its susceptibility to degradation, environmental and trophic conditions of the lake, hydrobiological conditions (phyto- and zooplankton, zoobenthos, primary productivity), and fishery state of the lake (resources and communities). Studies, in their predominant part, were based on the direct measurements in the lake as well as on the laboratory analysis of the samples taken from the lake. Studies were carried out by interdisciplinary team from Technical University of Gdansk (Faculty of Environmental Engineering), Institute of Hydroengineering of the Polish Academy of Sciences in Gdansk, and the Institute of Inland Fishery in Olsztyn.

 

CHEMICAL AND BIOLOGICAL INVESTIGATIONS

RANGE AND METHODOLOGY OF THE INVESTIGATION

The investigations carried out in 1994 on Lake Zarnowiec included, among others, chemical and biological tests of the lake water and the surface water courses in the catchment of this reservoir. These tests were aimed at the determination of the effect of the Zarnowiec power plant and the pollutants from the catchment area on the quality of the lake water. Fig.1. shows the layout of points of water sampling for the tests. The qualitative assessment of the pelagic zone water was carried out using water from the southern station (points 1-2 ) and the northern station ( points 3-4 ). The samples were taken from the surface water layer and from the bottom layer. The water quality of the eastern and western lake littoral zone has been estimated on the basis of test results of samples taken from regions with anticipated pollution ( Fig.1., points 6, 7, 8, 10, 11, 12 ). The quality of main surface water courses was determined using the results of the samples taken from the mid-stream of the Piasnica Górna (point 5), the Piasnica Dolna (point 9) and the Bychowska Struga (point 13). Also some random qualitative analyses of the wastewater discharged to the lake by the sewage treatment plant in Nadole were carried out.

 

Lake Zarnowiec due to the operation of pumped-storage power plant has practically no thermal stratification (Majewski; 1996). Therefore chemical assessment of the pelagic and littoral zone waters was made taking advantage of the investigation results of following major indices related to lake water quality. These are: concentration of dissolved oxygen in the bottom layer (in summer), content of organic substances (BZT₅, ChZT_Cr) in the surface layer (in summer), content of phosphates in the surface layer (in spring), mean concentration of total phosphorus (in spring and summer) in the surface layer, concentration of mineral nitrogen in the surface layer (in summer), mean concentration of total nitrogen (in spring and summer) in the surface layer. The adapted scope and frequency of the analytical work were compatible with the investigation methodology and classification of lake waters applied in Poland (Kudelska; 1994).The investigations also involved the following supplementary parameters: pH, colour, total alkalinity, calcium, magnesium, sodium, potassium, chlorides, sulfates and of some heavy metals. The above mentioned parameters were subject to testing in the surface water layer in spring and summer. The range of the qualitative tests of the main surface water-courses situated in the catchment area of the lake referred to: pH, electrolytic conductivity, dissolved oxygen, sodium, potassium, total iron, manganese, chlorides, sulfates, ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, total nitrogen, phosphates, total phosphorus, ChZT_Cr, ChZT_Mn, BZT₅, dissolved substances and suspended solid matter.

 

The susceptibility of Lake Zarnowiec to degradation has been determined on the basis of the morphometric, hydrographic, and catchment area characteristics, including among others, mean depth of the lake, ratio of the lake volume to the length of the shoreline of the lake, characterictics of stratification, quotient of active bottom area and volume of the upper surface, percentage of annual water exchange, the Schindler coefficient and method of management of the direct catchment area (Kudelska, 1994).

 

The changes of the environmental conditions and lake trophy (eutrophication) have been characterized using the comparison between the results from the periods: 1974-76, 1981-84 and 1994 (Majewski, 1997). These analyses concerned additionally the water temperatures, mineral salt content, electrolytic conductivity, visibility of Secchi disc, and chlorophyll a. Also the dynamics of phyto- and zooplankton, zoobenthos and primary productivity have been examined.

 

RESULTS AND DISCUSSION

The mean values for water of the pelagic zone are presented in Table 1, which includes also the range of variability and the mean values of the main indices related to the quality of water in the eastern and western littoral zones. Taking into consideration the lake water classification used in Poland (Kudelska, 1994) and the criteria to determine the quality class, the waters of the Lake Zarnowiec in pelagic zone, in general, have been qualified to the first class of water quality. The water of shore zone reveals significantly worse quality ( it is connected with water level variations caused by the operation of the power plant which intensify the circulation of the nutrient elements). But the results of numerical analysis have shown that the operation of the power plant caused the decrease of the seasonal changes of physicochemical parameters and elimination of the differences between litoral and pelagial zones.

 

The biological (accumulation of phytoplankton and chlorophyll a in pelagial of the lake) and chemical processes occurring in the Lake Zarnowiec in 1994 (particularly in the eastern part), were characteristic for eutrophicated waters. Total nitrogen content was rather low, characteristic for mesotrophy. Levels of phosphorus were similar in particular years and were typical for eutrophic lakes, and occurence of particular groups during the study period (1973-1994) suggests the stabilization of the phytoplankton changes caused by the operation of the power plant. Trophy of pelagic zones can be defined as transitory between meso- and eutrophy.

 

CONCLUSIONS

Operation of the pumped-storage power plant have caused changes of ecological state and water quality of the lake. But, in general, it is to conclude, that pumped-storage power plant results in the stabilization of lake trophy during the year.

 

Lake Zarnowiec belongs to water reservoirs susceptible to degradation. Therefore the water quality of this reservoir can be subject to gradual deterioration. Due to potential eutrophication of Lake Zarnowiec, some measures should be taken to improve, first of all, the water quality of the Rivers Piasnica Górna and Bychowska Struga.

 

REFERENCES

Jarzebinska T., Geneja M., 1998. Assessment of water quality of lake Zarnowiec using GIS. In: Proceedings of the International Symposium on Water Management and Hydraulic Engineering, Dubrovnic, Croatia, September 14-19

 

Jarzebinska T., Majewski W., 1996. Problems of small river catchment management based on the example of Lake Zarnowiec. HydroGIS 96: Application of Geographic Information Systems in Hydrology and Water Resources. (Proceedings of the Vienna Conference, April 1996). Volume of Poster Papers

 

Kajak Z., 1983. Ecological characteristic of lakes in North-Eastern Poland versus their trophic gradient. Dependence of chosen indices of structure and functioning of ecosystems on trophic status and mictic type 42 lakes. Ecol. Pol. 31(2)

 

Kudelska D. et al., 1994. Instructions of Polish Environmental Protection Survey

 

Majewski W., 1997. Influence of pumped-storage powerplant Zarnowiec on natural lake (lower reservoir). In: Proceedings of the 27^th Congress of the International Association for Hydraulic Research, San Francisco, California, August 10-15.

 

Majewski W. (Editor in chief), 1996. State of Lake Zarnowiec after 10 years of the exploitation of pumped-storage power plant (in Polish). Monograph of the Committee of Water Resources of the Polish Academy of Sciences. No 11