ABSTRACT: After stopping the production in the Nickel plant in Sereď in year 1993, a period of 30 years of continual pollution of groundwater ended. Groundwater pollution occured in 3 places: in the area of nickel production, in places of ash disposal and in place of disposal of lyer. Maximum ammonium ions pollution of groundwater, the area of cca. 200 x 300 m was created around the disposal place of lyer. Contaminated groundwater is slowly moved in the general direction of flow. At the moment this pollution is monitored by groundwater quality monitoring.