Domestic wastewater is the most significant source of pollution in Hungarian settlements. In order to eliminate this pollution the construction of sewage networks is ongoing in the country. The investments have a significant impact on the local environment, especially on groundwater resources. In the present case study changes in groundwater quality are evaluated after the construction of the sewage network. In the summers of 2013 and 2017 water samples were collected from 40 ground water wells. The comparative investigations were performed using a contamination index. The following parameters were used to calculate the contamination index: pH, EC, NH4+, NO2-, NO3-, PO43-, CODps, Na+. The degree of contamination (Cd) of the groundwater was classified into 4 categories: Cd>3 very heavy contamination, 3>Cd>2 heavy contamination, 2>Cd> 1 moderate contamination, 1>Cd low contamination. The results show strong groundwater contamination. In 2013, in 67.5% of the examined wells, the water quality was very heavily or heavily contaminated. Only 15% of the water samples had a low degree of contamination. Three years after the sewage network was set up, significant changes were observed. In 2017 the proportion of highly or heavily contaminated wells dropped to 50%, thus the number of moderately contaminated wells more than doubled. The number of low-polluted wells also grew, but their proportion did not reach 20% in 2017. On the basis of the results, it can be concluded that the purification process has started, but it may takes several years. Further environmental measures are needed in order to eliminate these pollutants.