<p>The presence of small artificial water reservoirs is typical of the landscape of small agricultural catchments. The old mill dammings are used today mostly for recreation and fish farming. Moreover, they are a very important part of the hydrological system of small rivers, stabilizing and shaping the discharge and physico-chemical characteristics of water. Coupled with the landscape function of the ponds goes their functioning in conditions of strong human pressure in the urban space. A canalized rivers in the urban parts of its courses picks up rubbishes washed down from the surface and coming from a storm sewer system and sometimes from leachate from a sanitary sewage system.</p><p>An example of hydrological systems operating in such environments are Dzierżązna and Sokołówka catchments (Fig. 1). The first one is located north of Zgierz in the impact zone of the city of Łódź. Despite agro-forestry character, a significant impact on water quality is the chalets area and the A2 motorway crossing the catchment. The Sokołówka catchment is flowing through the urban area of the Łódź city. Both of the river flows are highly variable, and the water is heavily contaminated and has a high load of suspension (Tab. 1)</p><p>In both of the catchments the role of artificial reservoirs created to improve the quality of water is the important issue. In the Dzierżązna catchment the several retention and infiltration ponds are working for regulating the outflow from the A2 motorway. In the Sokołówka valley the biofilter has been recently built (Fig. 2). It was constructed in order to purify urban storm- and waste water (sequential biofiltration system of Sokołówka – SSBS). The main purpose of the constructed SSBS was to remove sediments, suspended solids, particulate pollutants, petroleum hydrocarbons, heavy metals, nutrients and bacterial contamination from storm-water runoff through sedimentation and filtration mechanisms.</p><p>The paper discusses the construction of both systems and technical solutions. To evaluate the effectiveness of their operation analyzed changes in the basic characteristics of physicochemical water parameters (water temperature, conductivity (SEC), pH, the concentration of dissolved oxygen, turbidity (NTU)) measured before and after both of cleansing systems in the year and multiyear (Fig. 3, Fig. 4).</p><p>Attention has been also paid to the distribution of the mineral sediment deposited on the bottom of the biofilter SSBS system (Fig. 5). It was found that lowering the temperature in the winter and the appearance of a compact ice cap adversely affect the ability of biofilm to absorb pollutants causing (relative to the hot half year) an increase of conductivity, a higher pH and reducing of oxygenation of water.</p><p>Identified mean changes of the selected physicochemical parameters of water discharged from the area of the A2 motorway to the Dzierżązna river system and as a result of the SSBS system operation to Sokołówka river (Fig. 6).</p><p>All the measured parameters are reduced after passing through both of systems. As a result of mechanical sedimentation the greatest reduction is suspended material transported by water – in the SSBS on average about 16% less than in untreated waters (in the Dzierżązna catchment this element was not measured). The pH of the water in both systems changes to the same extent – decreases by 10% in the Dzierżązna, and 7% in SSBS.</p>
Evaluation method of allowable concentration of pollutants considering the state of bottom sediments
