Thermodynamic equilibrium modelling of groundwater from Lower Jurassic aquifi er explored in Gostynin GT-1 geothermal borehole. By using komputer application PHREEQCI connected to wateq4f.dat database conducted numerical spetiation modelling for sample of water picked up during documentary work on Gostynin GT-1 borehole. From models raised, that in fi z-chem conditions occure in instalation in Gostynin during it works thermal sollution destablizated from thermodinamical poin of view. Tendency to precipitate or dessolve particular minerals illustrated on the drawing, appear that oxygen is noteworty, it content in water can impact thermodynamical stabilization of solution.