This article introduces results of observations concerning changes in groundwaters located under the area of an exploiting plant-soil treatment plant. The research period included months from December 2008 to December 2009. In this period authors analyzed water samples from piezometers located around and directly on the irrigated area. Water samples were collected one a month. Each sample was analyzed for the following pollution indexes: BOD₅, COD_Cr, total nitrogen, total phosphorus, total suspension. Groundwaters were collected from the existing piezometric net (P1-P3) that was installed before main exploitation and from the complementary net (P4-P6) completed in 2008. Research showed that composition of groundwater located directly under the irrigated area declined in quality. The highest pollution indexes were observed in piezometer No. 4 for almost all indices. Research and direct observations showed improper exploitation of the treatment plant caused by overloading the irrigated area with high pollutant loads, resulting in decreasing treatment capability. For the purpose of regeneration of soil profile, further operation of the treatment plant should be verified in regard to decreasing single doses of sewage.