# Risk Assessment of Soil Contamination with Heavy Metals from Municipal Sewage Sludge
## 2 Materials and Methods  
### Materials  
Sewage sludge (S1, S2, S3) was collected from wastewater treatment plants located in the Swietokrzyskie Voivodeship (Table 3). According to the proximity principle, locations for natural sewage sludge disposal sites (P1, P2, P3) were selected as close as possible to the wastewater treatment plants (Figure 1). For S2 and S3, the same location of sludge utilisation was chosen due to the location of these sewage treatment plants. The characteristics of the soils in the accepted locations are presented in Tables 4 and 5. The sorption properties of the soils are arranged according to their type. A poor sorption complex is characterised by sandy soils, while organic soils have a particularly high sorption capacity. [PERSON] considers that the organic substance in an acidic environment is a more active sorbent for heavy metals than most mineral compounds [25]. Soils have an acidic pH in the range of 4.1 to 4.5. A low pH affects the increased mobility of cadmium and copper. An acidic reaction, a low humus content, and a poor sorption capacity influence increased lead migration. The sorption of zinc depends on the reaction and the granulometric composition. It forms a permanent bond with the soil organic substance, which results in its accumulation. The content of organic substances and the low pH level of the soil boost zinc migration in the soil. The zinc content is high in the analysed soil; however, these are typical values characteristic of Polish soils. Nickel is strongly linked to the organic substance, and its solubility increases with acidity. Due to the susceptibility of nickel to the organic substance, its high mobility is maintained in many soils, even under alkaline conditions [26].  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Heavy Metal**} & \multicolumn{2}{c}{**Location of Potential Disposal Sites of Seweage Sludge**} \\ \cline{2-3}  & **P1** & **P2 = P3 * \\ \hline Cu & 22.4 & 3.0 \\ Cr & 0.37 & 3.3 \\ Cd & 7.4 & 0.09 \\ Ni & 4.6 & 2.4 \\ Pb & 4.6 & 10.8 \\ Zn & 39.6 & 19.3 \\ \hline \hline \end{tabular}  
* P2 = P3—the same location of sludge utilisation.  
\end{table}
Table 4: Content of the element from the heavy metal group in agricultural soils of Swietokrzyskie Voivodeship, mg-kg\({}^{-1}\) d.m. [28].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Location and Type of WWTP**} & **Equivalent Number of** & **Sewage Sludge** & \multicolumn{2}{c}{**Location of Potential Disposal Sites of Seweage**} \\  & **Residents** & **Treatment** & **Sludge** & **SWWTP to Potential Disposal Site of** \\ \hline Daleszvce & & & & \\ mech.-biol. SBR & 5000 & \
\begin{tabular}{c} Oxygen \\ stabilisation \\ \end{tabular} & P1—Wola Kopcowa & 13.5 \\ Skarzysko- & & & & \\ Kamienna & 59,500 &
\begin{tabular}{c} Fermentation \\ hybrid \\ \end{tabular} & P2—Wachock & 13.8 \\ Starachowice & 95,000 & Fermentation & P3—Wachock & 7.6 \\ \hline \hline \end{tabular}  
* P2 = P3—the same location of sludge utilisation.  
\end{table}
Table 3: Characteristics of wastewater treatment plants [27].