Changes of Water Quality Indices with Depth at Drinking Water Intake from Dobczyce Reservoir

The paper presents a comparative and statistical analysis of the changes of water quality indices values with depth on the water intake from Dobczyce reservoir. Moreover, seasonal variability of the tested indices was analysed and water quality with its usable values were assessed. Dobczyce reservoir is a 12 km long and 1.0 km wide submountain dam reservoir. The reservoir area at the maximum impoundment level (maximum water level) is 915 ha, its capacity 112.1 mln m(3) and mean depth 10.2 m. It is situated 20 south of Krakow, in the malopolskie voivodship, myslenicki county. Raw water from the reservoir is collected on tower intake which makes possible water intake from three depth levels. Real water extraction on the intake is about 1.0 m(3).s(-1), guaranteed 2.5 m(3).s(-1) and permissible 5.5 m(3).s(-1). The investigations conducted in 2008 and 2009 covered selected physicochemical water indices: the temperature, pH, turbidity, electrolytic conductivity, total suspended solids, dissolved oxygen, oxygen saturation, silica, phosphates, total phosphorus, as well as bioindicator - chlorophyll "a". At weekly intervals, water samples for laboratory analyses were collected from three depth levels of the reservoir directly at the tower water intake for Krakow. Physicochemical and biological analyses, using referential methods were conducted in the Laboratory of the Municipal Waterworks and Sewer Enterprise in Krakow. For each analysed index mean values for 2008 and 2009 and for the 2008-2009 period of analyses were computed, divided into the winter (October-March) and summer (April-September) half year and for the calendar year (January-December). Statistical significance of the differences between the values of individual indices registered on various depth levels on the reservoir were estimated by nonparametric U Manna-Whitney'a significance test, on alpha = 0.05. For all investigated indices box-whiskers plots were made which suggestively presented the median, extreme values and 10-90% percentiles. The assessment of Dobczyce reservoir water for drinking water supply for people was conducted following the methodology described in the Regulation of the Minister of the Environment [Dz. U. 2002 Nr 204, poz. 1728]. Selected quality parameters of the tested water were also confronted with the requirements stated in the Regulation of the Minister of Health dated 29 March 2007 on drinking water quality for people [Dz. U. 2007 nr 61, poz. 417]. Trophic state of Dobczyce reservoir was assessed on the basis of total phosphorus and chlorophyll "a" concentrations, using the criteria stated by OECD. On the basis of data analysis it was established that mean annual values of water temperature, pH, oxygen saturation and concentrations of dissolved oxygen and chlorophyll "a" were decreasing with depth. On the other hand, in the vertical profile, turbidity, electrolytic conductivity, as well concentrations of total suspended solids, phosphates and total phosphorus were increasing. Higher water temperatures and pH values were noted in the summer half-year, whereas dissolved oxygen, oxygen saturation and phosphates in the winter half-year. On average for the investigated period, water collected in Dobczyce reservoir on the depth of 3.55 and 9.65 m was classified, respectively to A2 and A3 water category, therefore it required appropriate typical or high performance typical treatment. On the other hand, due to low oxygen saturation in the summer half-year, water on the level of 15.87 m did not meet the standards. Irrespective of the depth, pH and salinity fulfilled the requirements for water intended for direct consumption by people, whereas water turbidity was unsatisfactory all the time. Mean annual concentrations of total phosphorus qualify water collected in the reservoir as mezotrophic, while average and maximum concentrations of chlorophyll "a" allow to regard the water as respectively mezotrophic or eutrophic. Due to relatively big differences in values of the analysed indices, better quality raw water intended for running water outside the growing period supply should be skillfully collected from various depths generally in the from the shallowest level, in the vegetation season from the middle level and from the lowest level on case higher concentrations of chlorophyll "a" and plankton occur on the first two depths of the reservoir: