Summary:

Organic sewage of the potato industry is being purified in soil filters – in the form of filter fields established on permeable soils. In seventies of the 20th century they were making an attempt at adopting the conception of agricultural using organic sewage for forest conditions. The forest sewage treatment plant for the Potato Industry Plant in Iława was started up in 1984. For distributing sewers a rain gun was put on with rotational sprinklers. Area of irrigated object covers 216 hectares of pine tree stands in the Iława Forest District (from 2001 the sewage treatment plant was limited to 175 ha). In 1984 a net of 30 piezometers was put on, from what 16 they located on the area of the sewage treatment plant, and 14 outside its borders. These piezometers served mainly for observation of the level of groundwater. A few times a year from some piezometers testers of water were being taken for quality examinations. The aim of the following paper was an assessment of changes of the groundwater quality near the forest sewage treatment plant in Iława, occurring under distribute sewers from the potato industry in 2006–2007 years. A physicochemical composition of testers of water was judged from 7 piezometers. Analyses of testers of water were carried out in two accredited laboratories. They included analyses among others: the pH, the electric conductivity and concentrations of Ca, Mg, Na, K, NO^3-, NH⁴⁺. These rates of the quality best are describing character of changes of the quality of groundwater which are occurring under the influence of distributing of sewers in the forest. Minimum values of the electric conductivity and NO^3-, Ca, Mg, Na and K concentrations were in groundwater on the edge of the sewage treatment plant and outside the sewage treatment plant on the road of the inflow of waters toward the sewage treatment plant. Maximum values of the electric conductivity and concentrations of mentioned ingredients were stated in groundwater in the central part of the sewage treatment plant, as well as the pose with sewage treatment plant, but in waters running off from the area of the sewage treatment plant toward the Drwęca river. It has been stated that a concentration of nitrate was particularly height, even over 100 mg·dm^-3.