Hidrológiai Közlöny 1971 (51. évfolyam)

1. szám - Valló Sándor: Összefüggés a felszíni vizek minősége és a hidrológiai viszonyok között

Special Conference Number, Szebellédy, L. Hidrológiai Közlöny 1971. 1. sz. 15 irrigation, introduetion into the soil, or fish-ponds is already beneficial in this respect, although applied mostlv for interests other than the protection of lakes. Most effective in eutrophication control are preventive measures, with the primary objective of preventing wastes, including biologicallv treated effluents from entering the lakes. Potential measures for improving the quality of eutrophicated lakes include the aeration of deeper low-oxygen layers, their removal through outlets, the dredging of bottom sludge and the removal of somé of the vegetation. At the same time caution must be exercised in destroving the plants within the laké itself (the deeav of algae is an additional source of organic pollution), in the complete removal of sea-weed vegetation by plánt devouring fish (the weeds consume the same nutrients as the algae) and in the uncontemplated exchange of water. The latter is not beneficial, unless the replacement water is of better quality and poorer in plánt nutrients and is available at a rate ensuring a residence time shorter than the generation period of algae. In rivers the danger of eutrophication is less pronounced, since on the one hand, nutrients tend less to accumulate on the bottom, and on the other, water is continually mixed by turbulence, while the bed is actuallv flushed by highwater waves. The interests of nature conservation should alsó be mentioned in this context. In order to protect the bird sanctuary of Kisbalaton, situated at the south-western end of Laké Balaton in Hungary, the control of pollution in the Zala river discharging into the laké is of primary importance. Eutrophication is not yet serious in Laké Balaton itself. Isolated occurrences of plancton invasion, expansion of sea-weed areas and algal blooming are signs of warning, stressing the importance of wastes treatment. The interests of watersupply must alsó be observed here, the water of Laké Balaton being used increasingly—after proper treatment—for supplying drinking water. Laké Velence is the largest alkaline and mostly eutrophicated laké in Central Europe. The western part has been declared a conservation area because of the wide variety of birds nesting there. Protection and "rejuvenation" of the old laké, mainly for recreation and water sports, has turnéd one of the most exciting engineering problems in the sound solution of which biologists, limnologists and experts of various branches of water management cooperate. Conservation and sanitary authorities exercise strict control of pollution. c) Thermal stratification in reservoirs of small and médium depth (10 to 20 m) is of an adverse effect on water quality, as far as water supply and ereatment is concerned. This stratification is subtect to seasonal variations. Under moderate climates the temperature of water is rather uniform, due to wind action and precipitation, which ensure adequate mixing of water. In early summer the upper layer becomes warmer, specific gravity is reduced and a period of stagnation follows. In the upper layer (epilimnion) plánt microorganisms prolificate under the influence of sunshine. Sooner, or later the nutrients in this upper layer of a few m thickness. temperature of which may attain 20 to 22 °C, are exhausted, since the nutrient P- and N salts are confined, owing to stagnation, to the lower layers (hipolimnion) of rapidly decreasing temperature. As a consequence the phytoplancton decay and sink together with other settling substances entering the reservoir to the bottom, where the originally small reserves of oxygen are soon exhausted, creating even anaerobic conditions in the bottom sludge. Investigations performed in reservoirs in Hungary revealed the bottom layers to be completely devoid of oxygen, to contain 20 to 30 mg free C0

2 per litre, to have a pH lower than 7 and several mg Fe and Mn compounds per litre in summer stagnation periods. In winter difficulties may be due to the ice cover. The uptake of dissolved oxygen is retarded, C0

2 is increased and the soil-dissolving potential becomes higher under such conditions. The adverse consequences of thermal stratification are difficult to control. In theory, the watersurface could be protected from sunshine (such attempts have been made) to impede the prolification of phytoplancton. An ideál solution would consist of transferring dissolved oxygen from the oxygen-rich epilimnion to the hipolimnion without clisrupting the metalimnion situated between the two. This would alsó prevent the plánt nutrients settled to the bottom from rising and causing further prolification of algae. Present practice is thus imperfect, but no other practicable solution being available, the water is recirculated artificially. The development of stagnation areas most susceptible to algal blooming is thus eliminated and flow conditions in the entire reservoir are favourably influeneed. The x-eservoir system of the London Municipal Waterworks on the Thames and Lee rivers provides a totál storage space of 165 millión cu.m and acting as a settling and biological treatment space yields 6 miihon cu.m/day even in very dry years. Several methods of mixing have been developed and introduced on a routine basis. The experiences gained in the VVraysbury reservoir are deseribed by P. Cooley. The reservoir of 35 miihon cu.m capacity at 21 in depth is operated in three ways: 1. Under normál conditions water sufficient to cover consumption is pumped from the river into the reservoir. At rising temperatures water is injected in jets 1.5 m above the bottom through 6 pipes of 1070 mm diameter, at a velocity of 3 m/sec. Of the pipes two are horizontal, while pairs include angles of 22.5 degs. and 45 degs., respectively, with the horizontal. 2. In the period with small temperature differences, low-velocity inflow occurs through two pipes of 2.9 by 2.9 m cross-section, in order to avoid turbulence. 3. When owing to low stages no water is pumped from the river pumps are operated to create verticai circulation in the reservoir.