## 3 Results and Discussion  
### Heavy Metal Concentrations in Runoff  
Heavy metals found primarily in the airborne particulate matter are particularly hazardous substances. Deposited on the ground together with particulate matter, HM are then washed off the catchment and transported, with the stormwater, into the wastewater receiving body. High concentrations of Zn, Pb and Ni in the stormwater result from the widespread use of these elements in the automotive and fuel industries. The presence of copper and chromium in the atmosphere is mainly caused by coal combustion and industrial activity [27; 28; 29; 30; 31]. Zinc had the highest concentration of all HM analysed (Table 2). In stormwater samples collected from the Jesionowa SWTP catchment, Zn concentrations ranged 319-5731 ug\(\cdot\)dm\({}^{-3}\), and were higher than the values observed for the IX Wiekow Kielc SWTP (0-3873 ug\(\cdot\)dm\({}^{-3}\)) and for Mitosa SWTP (45-3160 ug\(\cdot\)dm\({}^{-3}\)). These values are similar to the range specified for Bialystok, Poland (200-6000 ug\(\cdot\)dm\({}^{-3}\)) [27], and for Belgrade (284-6200 ug\(\cdot\)dm\({}^{-3}\)) [32]. The high concentrations of Zn in the studied catchments result from significant percentage (9.4%-14.3%) of the area of roof covers in land development [33], especially in the industrial part of the SWTP Jesionowa. The highest concentration of Pb (1405 ug\(\cdot\)dm\({}^{-3}\)) was observed in the IX Wiekow Kielc SWTP catchment, and it fits within the limits for runoff from motorways and major roads (3-2410 ug\(\cdot\)dm\({}^{-3}\)) [34]. The high Pb values can be caused by a large volume of vehicular traffic in the city centre--exhaust gas, among others, is a source of Pb [28]. Conversely, the lowest Pb concentrations (1-343 ug\(\cdot\)dm\({}^{-3}\)) were observed in the Mitosa SWTP catchment, which may result from the prevalent single-family housing and local character of roads. However, Pb concentrations are higher than limits for Paris [35; 36] and Genoa [37], although they fit within the range for runoff from suburban and residential roads (10-440 ug\(\cdot\)dm\({}^{-3}\)) [34]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Years**} & \multicolumn{2}{c}{**Value**} & \multirow{2}{*}{**Cd**} & \multirow{2}{*}{**Cu**} & \multirow{2}{*}{**Cr**} & \multirow{2}{*}{**Ni**} & \multirow{2}{*}{**Pb**} & \multirow{2}{*}{**Zn**} \\  & & & & & & & \\ \hline \multicolumn{8}{c}{IX Wiekow Kielc SWTP} \\ \hline \multirow{3}{*}{2009–2018} & Range & 0–162 & 0–1068 & 0–350 & 0–168 & 0–1405 & 0–3873 \\  & Median & 20 & 128 & 70 & 45 & 411 & 430 \\  & Mean & 17 & 113 & 46 & 31 & 342 & 298 \\ \hline \multicolumn{8}{c}{Witosa SWTP} \\ \hline \multirow{3}{*}{2015–2018} & Range & 1–766 & 3–959 & 3–319 & 2–341 & 1–343 & 45–3160 \\  & Median & 37 & 358 & 70 & 38 & 55 & 663 \\  & Mean & 22 & 262 & 67 & 23 & 14 & 550 \\ \hline \multicolumn{8}{c}{Jesionowa SWTP} \\ \hline \multirow{3}{*}{2016–2018} & Range & 4–649 & 5–660 & 14–2236 & 13–304 & 13–1115 & 319–5731 \\  & Median & 16 & 128 & 177 & 74 & 138 & 1148 \\ \cline{1-1}  & Mean & 7 & 99 & 117 & 80 & 101 & 842 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary statistics describing heavy metals concentrations in the analysed catchments. Maximum Ni concentration found in the IX Wiekow Kielc SWTP catchment (168 \(\upmu\)g\(\cdot\)dm\({}^{-3}\)) was lower than the values for the other two facilities (304 and 341 \(\upmu\)g\(\cdot\)dm\({}^{-3}\)). The values are located within the range specified in literature for runoff from car parks and industrial areas (2-493 \(\upmu\)g\(\cdot\)dm\({}^{-3}\)) [38]. On the other hand, the highest median of Ni concentrations for the Jesionowa SWTP (80 \(\upmu\)g\(\cdot\)dm\({}^{-3}\)) exceeded the values observed in China (20.3 \(\upmu\)g\(\cdot\)dm\({}^{-3}\)) [39].