Wpływ zbiornika retencyjnego na fizykochemiczne właściwości rzeki himalajskiej (Tista, Indie) = Impact of a dam reservoir on the chemistry of a Himalayan river (the Teesta, India)

W artykule przedstawiono rolę nowo wybudowanego zbiornika retencyjnego w kształtowaniu właściwości fizykochemicznych i składu chemicznego rzeki Tisty (dopływ Brahmaputry) w Himalajach. Badania terenowe przeprowadzono w okresie pomonsunowym w latach 2013-2015. Analiza składu chemicznego wody w profilu podłużnym rzeki powyżej i poniżej zbiornika wykazała, że w zbiorniku następuje obniżenie stężenia większości podstawowych jonów (Cl– , K+, Na+, Mg2, NO3 i PO4 3–), odwrotny trend zaobserwowano tylko w przypadku jonów Ca2+, SO4 2– i NH4+. Istnienie zapory nie wpływa na stężenie jonów F– . Poniżej zbiornika odnotowano nieznaczne wzbogacenie wód w jony metali: Cu, Ni, Zn, Cr, C i Sr. Mniejsze stężenie jonów w rzece poniżej zapory wskazuje na procesy samooczyszczania wody retencjonowanej w zbiorniku. Zmiany właściwości fizykochemicznych oraz stężeń jonów następujące w zbiorniku są niwelowane przez czynniki środowiskowe jeszcze na obszarze Himalajów na odcinku do 15 km poniżej zapory.

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In the light of a growing demand for energy, the government of India has decided to tap into the potential of rivers by building numerous hydroelectric plants, with a view to doubling the amount of energy produced by water. One of the regions in which such plants are being built is the basin of the River Teesta (as a right-bank tributary of the Brahmaputra). The main aim of this article has been to present chemical-composition characteristics and physico-chemical parameters of water in the Teesta in its Himalayan course, with account being taken of the role the newly-built reservoir plays in shaping the chemistry of water in the river. Fieldwork entailed measurement of physico-chemical parameters of the water, as well as sampling for the purposes of chemical analysis. This was done in the post-monsoon period in the years 2013-2015. Sampling and measuring points were located along the Teesta in the Darjeeling Himalaya, over a length of approximately 43 km, between the borders of Sikkim and West Bengal and the southern margin of the Himalayas. Above the newly-built reservoir, measurements were made at points P1 and P2; while within it, measurements were made and samplings taken in the centre (at P3 by the shore). The two points located below the dam were P4 and P5. The results of the analysis of main physico-chemical properties and chemical compositio data for the water in the Teesta allowed conclusions as detailed in the following points to be drawn. The decrease in water temperature caused by the reservoir is slight, at approximately 1.0°C, with the indication being that very weak thermal stratification has developed, due to the rapid exchange of water in the reservoir. As for Total Dissolved Solids, marked stability of values across a narrow range is to be observed. The values for ANC in turn indicate that there is no risk of acidification at any point along the section examined. The newly-built reservoir is responsible for a decrease in concentrations of most of the main ions (i.e. Cl– , K+, Na+, Mg2, NO3 and PO4 3–). The reverse trend was only to be observed in respect of Ca2+, SO4 2– and NH4 +. The dam does not influence F– concentrations. The reservoir causes minor enrichment of water in ions of most metals like Cu, Ni, Zn, Cr, C and Sr. The more limited enrichment of water in the Teesta below the dam indicates self-purification processes taken place in the Teesta Reservoir where metals are concerned. The changes in physico-chemical properties and concentrations of ions caused by the reservoir are usually normalised by environmental factors before the Teesta exits the Himalayas (i.e. within 15 km of the reservoir). The results of the study are relevant, in light of the construction of several further reservoirs in the Teesta catchment in the near future, which can lead to significant transformations of the natural environment, including hydrochemical changes that determine water quality.