The quality of water in the Busko Blato reservoir

The article presents the results of chemical and microbiological measurements of the water quality in the Busko Blato reservoir, the largest in Europe, in the period 1989 to 1998. The aim was to observe possible changes in water quality during the past decade and its influence on the downstream section of the Cetina River and its numerous tributaries and reservoirs, where there are two large regional water supply systems. This reservoir was built in order to exploit the high hydroelectric potential of small streamflows in the karst fields of south-western Bosnia. It has strongly influenced the hydrologic conditions of the downstream section of the Cetina River. Busko Blato is a relatively shallow reservoir with the mean depth of 5 m. It covers an area of 57 km2. The water exchange in the reservoir is slow, occurring from one to several years. When the water inflow exceeds the hydroelectric plant demands the excess water is delivered by a reversible channel and pre-pumped into the Busko Blato reservoir. The total and carbonate water hardness in the reservoir is lower than in the Cetina River and in typical karst streamflows, since the water in this reservoir comes mainly from surface sources including numerous springs with high capacity oscillations, torrents and rainfall. Measurements, carried out during the past years, showed a significant increase in nitrate content as well as in the coli bacteria and a less significant increase in total phosphorus content. The water quality was evaluated as mezotrophic according to the total phosphorus content based on the Carlson index of trophic level, a lower quality than in the Peruca reservoir, upstream on the Cetina River. The latter contains a smaller phosphorus content and was classed between oligotrophic and mezotrophic. According to these data, employing the Vollenweider equation, the total phosphorus content in this water should not exceed the mean of 20 mg m(-3) P. The results indicate that the water in the Busko Blato reservoir did not deteriorate the water quality on the downstream section of the Cetina River.