## 1 Introduction  
Controlling urban stormwater, particularly in areas with impermeable covers such as road pavements, parking lots, airports and roofs, is an important issue if local flooding or larger floods after rainfall is to be prevented. Many methods have been proposed to mitigate this issue [1; 2; 3; 4]. The amount of runoff from urbanized areas may be reduced by increasing the retention of runoff water close to its source by centralized and non-centralized sustainable urban drainage systems [2; 3]. One of the solutions recommended as best management practice is the use of systems that increase stormwater infiltration. The advantages of this solution are the attenuation of peak runoff, reduction of direct removal of runoff water to the reservoir, and groundwater supply [1; 5]. In the last decade, aside from infiltration systems (filter strips, soakaways, infiltration trenches, infiltration basins) and retention devices (e.g., retention ponds, lagoons, constructed wetlands), new solutions that have been proposed are focused on reducing the volume of surface runoff; these include alternative road structures, e.g., porous paving and asphalt surfaces [6]. However, [PERSON] et al. [7] emphasized the need to maintain the balance between the volume of retained water and groundwater quality protection, particularly taking into account the restricted efficiency of the subsurface surface layer of soil and construction materials to remove contaminants. Based on literature data from the last decade [8; 9; 10], urban stormwater transports large amounts of contaminants, including suspensions, biogenes, oil-derived substances, heavy metals, dissolved salts, polycyclic aromatic hydrocarbons, phenols and pesticides. In many countries, stormwater is considered as one of the chief contaminant carriers to the soil-water environment [8; 10; 11; 12]. Its treatment during infiltration through a soil layer to groundwater may be insufficient to fulfil the requirements raised by the Water Framework Directive [13]. Oil separators and settling tanks are commonly used in engineering practice as elements of preliminary treatment before introduction of runoff water to the receiver. The main functions of such devices include elimination of organic contaminants (coalescence and lamella separators) and suspensions (settlers/grit chambers) transported by stormwater from impermeable areas [1]. Due to the difficulties in the implementation of these devices, their application is recommended by producers and research institutes mostly in cases of specific hazard posed on the natural environment, e.g., on petrol stations, parking lots, road junctions, overpasses and bridges. Therefore, it is important to find and implement more effective solutions, which would ensure not only a lower quantity, but also maintain the acceptable quality, of stormwater runoff. They include permeable treatment zones, where the processes taking place within them may be classified as engineered natural attenuation. Many studies related to engineered natural attenuation of runoff water in infiltration systems exist, mainly from the USA [6; 14; 15; 16], Austria [4; 17], Germany [18] and Australia [19; 20]. Moreover, guidelines have been elaborated to control the degree of contamination removal in sustainable systems of stormwater runoff, particularly total suspended solids, Zn, Cu, organic contaminants, total phosphorus and total nitrogen e.g., in Netherlands, Germany and Australia [2]. These investigations may be subdivided into the following groups: (1) application of treatment chambers for small-surface catchments [14; 15; 16; 18; 21], (2) application of point permeable treatment zones for larger catchments [6; 7; 19; 20; 22; 23], and (3) application of linear permeable treatment zones [6; 24].