Summary:

The paper contains preliminary results of investigations on transformations of organic and biogenic compounds in small water reservoir. The reservoir Górny Młyn near Końskie (Świętokrzyskie District) located on the Czysta river was chosen as an object of investigations. The bases of analysis were hydrological and chemical investigations covering measurements of water volume inflowing and flowing from the reservoir as well as determination of its chemical composition. The investigations which covered six measurements series were carried out between May and July in the year 2006. The following indicators of pollution were determined in laboratory by means of reference methods in taken samples of water: BOD₂, P-PO₄, N-NH₄, N-NO₂ and N-NO₃. Hydrometric measurements showed that the main source of alimentation of investigated reservoir is the reservoir Szawelnia located above, from which inflow to the Górny Młyn reservoir fluctuated between 0,003 do 0,120 m³⋅s^-1. Taking into account small stream „without name”, the total amount of water inflowing to the reservoit during investigations was on an average 0, 055 m³⋅s^-1, while the amount of flowing from the reservoir was on an average 0,051 m³⋅s^-1. The crucial influence on quality of water inflowing and flowing from the decanter had concentration of nitrite nitrogen, which several times exceeded permitted limit 1,000 mgN-NO₂⋅dm^-3, determined for in obligatory law for the lowest V purity class. The remaining determined indicators were located in limits for the highest I, or at least for II purity class. It was stated that reservoir held carrying in together with inflowing water charges of organic compounds and compounds of phosphorus and nitrogen. Water flowing from the reservoir contained on an average 1,200 kg⋅d^-1 (9,0%) of organic compounds less, of 0,182 kg⋅d^-1 (51,0%) P-PO₄ less, of 3,080 kg⋅d^-1 (99,0%) N-NH₄, less, 11,349 kg⋅d^-1 (68,0%) N-NO₂ less and 1,302 kg⋅d^-1 (81%)j N-NO₃ less. This retention function the reservoir performed relating to delivered charges of biogenic compounds is the serious threat for eutrophication of water which is stored. The threat is confirmed by calculated quantities of permitted and dangerous charge for phosphorus and nitrogen. During investigations the charge of phosphorus (3,370 mgP⋅m^-2⋅d^-1) held in the reservoir Górny Młyn was about three times higher than the permission one and about 1,5 times higher then dangerous one. On the other hand the charge of nitrogen held which is the sum of charges determined its mineral forms (291,315 mgN⋅m^-2⋅d^-1), was about 19 times higher than the permitted one and 10 times higher the the dangerous ones.