**1. Introduction**

The resources of surface waters in Poland are relatively low, not equally distributed and characterized by seasonal variability. The total annual runoff is about 57 km3 of water, which is equal to 1600 m3 /person—this is a third of the European average runoff per person (statistic data). In order to improve the availability of water, quite a number of storage water reservoirs have been built in different parts of the country. Generally, water storage behind large dams is applied whereby the water is used for power generation, municipal water supply and flood control. Large storage reservoirs with controlled discharge provide larger amounts of water for periods of drought or low flow in the streams. In Poland, there are about 100 reservoirs characterized by a capacity of over 1 million m3 each. The total storage capacity of the Polish reservoirs is about 4000 million m3 , which constitutes about 6% of the annual runoff (statistic data). Unfortunately, this storage capacity has not been sufficient up to now. Moreover, the strong anthropopressure still observed makes that the water quality is not satisfactory in many cases. So, the efforts are being undertaken to increase waterretention and eliminate or, at least, strongly decrease water pollution.

One of the larger water reservoirs in Poland is the Turawa reservoir, operated since 1948. It is used to drive a hydroelectric power plant, as a source of water for municipal water supply systems, and also for recreation.

The Turawa reservoir is situated between 50°42′27″ and 50°44′32″ north latitude and 18°04′51″ and 18°10′59″ east longitude in Mała Panew Valley, which is the part of Równina Opolska (Opole Plain) (**Figure 1**).

**Figure 1.** Localization of the Turawa reservoir in Poland (Terra/Modis NASA).

It is situated on the Mała Panew river 16 km away from where it flows into the Odra river. The two other smaller rivers supplying the reservoir with water are the Libawa and the Rosa. The participation of these rivers in the surface water inflow to the Turawa reservoir is 87, 9 and 4%, respectively.

Morphometrical parameters of the Turawa reservoir are as follows: capacity, 95.5 hm3 ; surface area, 20.9 km2 ; length, 7.4 km; maximum width, 3.45 km; average width, 2.26 km; length of the shore line, 29.5 km; maximum depth, 13.68 m and average depth, 4.70 m. The water turnover time in the Turawa reservoir in the years 1996–2005 was 59 days on average: the shortest 37 days in 2001 and the longest 87 days in 2003. The turnover time was calculated as the quotient of the sum of water evaporating, water exchanged with catchment and water that flows out of the reservoir divided by the annual average capacity of the reservoir. The water turnover time is the theoretical value, which allows for the approximate assessment of the process. On the basis of this parameter, the Turawa reservoir can be classified as a high-flow water body.

The Turawa reservoir plays an important role in Poland, but due to the strong agricultural and industrial anthropopressure its water quality in the studied period was rather poor. The quality of water in the Turawa reservoir is determined by the composition of the water of the rivers flowing into the reservoir, that is, the Mała Panew river, the Libawa river and the Rosa river and also by the processes proceeding in the reservoir itself. The Mała Panew is the most polluted of the above-mentioned rivers; it carries industrial, municipal and agricultural pollutants. The largest amounts of wastewater are discharged from the several cities in the neighbourhood of the reservoir [1, 2].

The main source of nitrogen and phosphorous compounds is the application of fertilizers in arable land in the catchment of the Mała Panew and the Libawa rivers.

Any pollution prevention and remedial programmes need the determination of the large number of physico-chemical parameters characterizing water quality and the dynamics of the changes of these parameters.
