Our paper evaluates the effectiveness of pollutant removal from stormwater in a semi-natural treatment plant and develops stochastic models of their transformations in a soil bed. The examined wastewater treatment facilities consisted of reduction chambers and a retention-infiltration reservoir. The following indicators of pollution were subjected to analysis: BOD₅, COD, total suspended solids, and chlorides. It was shown that COD, total suspended solids, and chlorides are the indicators that have the greatest impact on stormwater quality. However, the possibility of removal of the analyzed pollutants from stormwater is affected by the sedimentation process occurring in the reduction chambers and in the retention-infiltration reservoir, as well as by the process of infiltration through the soil bed. The developed models of pollutant transformation during wastewater infiltration through the bed were presented as the regression equations, where the selected pollution indicators in the filtrate were the dependent variable, while the analyzed pollution indicators in water from the retention-infiltration reservoir, coefficients of constant reaction kinetics determined for each variable, and the time of infiltration of wastewater through the soil bed were adopted as independent variables.