The deterioration of water quality in the opencast lignite mining Leknica region (lake district) was evaluated in 2007 by studying the underground water sampled in the area of a lake from 2 piezometers (located at the water inlet and outlet) and the lake water taken from 2 levels (5 m and 11 m). The water samples were analyzed for pH, neutralization potential, Fe(II), Al, Ca, sulfates, org. C and dissolved matter. Indices of water satn. with Fe, Al and Ca compds. were calcd. by using Phreeqc program. On the inlet to the take, the underground water contained sulfates, Ca and dissolved mater 80-82, 11-11.2, and 117-125 mg/L, resp., whereas at the outlet the resp. concn. were 762-780, 99-117, and 1037-1040 mg/L.