## 3 Materials and Methods
### Sampling and Methods for Decomposition of Mineral Soil Components  
The samples collected in linen bags in an air-dry state were transported to the Institute of Soil Science, Plant Cultivation and Fertilisation in Pulawy, where preliminary operations were performed, consisting of manual grinding of air-dry samples in a porcelain mortar and sieving them through a sieve with a mesh diameter of 1 mm (which is equivalent to separation into so-called earthy and skeletal parts). The samples were then crushed in \begin{table}
\begin{tabular}{l l l l} \hline
**mg/kg** & **Standard San Joaquin Soil** & **AAS\({}_{\mathbf{1}}\)** & **ICP-MS\({}_{\mathbf{1}}\)** \\ \hline Cr & 60–115 (79) & 67 & 78.8 \\ \hline Cu & 26–40 (32) & 27 & 29.3 \\ \hline Fe & - & 19.18 & 29.528 \\ \hline Mn & - & 282 & 292.9 \\ \hline Ni & 65–90 (78) & 84 & 79.9 \\ \hline Pb & 12–18 (13) & 13 & 14.4 \\ \hline Zn & 87–120 (100) & 87 & 97.6 \\ \hline \end{tabular}
\end{table} 
Table 2: Standard material analyses [40]. Figure 3: Sampling sites. a 12-min cycle in an electric agate mortar to a grain diameter of \(<\)0.1 mm. The soil dust obtained as a result of grinding was stored in paper bags in an air-dry state. The material thus prepared was used for further studies. The concentrations of heavy metals in the soils of the study area were determined using two methods of transferring components from soil to solution:  
* with the IUNG method of hot digestion of soil material with a mixture of concentrated HNO\({}_{3}\) and HClO\({}_{4}\) after incineration of organic matter [49];
* with a two-step decomposition method, i.e., oxidative acid digestion followed by non-oxidative acid digestion (HNO\({}_{3}\) and HF, respectively) in a microwave oven [50]. Complete decomposition of soil minerals is a relatively difficult operation, so cold or hot leaching of soil components with mineral acids and mixtures is often used. The chemical components brought into solution by strong acids during hot leaching are assumed to be close to their total content in the soil [49]. In soils with a similar grain size distribution, the relation between the amount of chemical components in the digestion solution and their total content is relatively constant. To determine the so-called total content of individual elements in the soil, the method of hot digestion of soil material with a mixture of concentrated HNO\({}_{3}\) and HClO\({}_{4}\) was adopted. Acid digestion of organic soils is preceded by the ashing of the organic matter. Soil ashing releases components bound in the organic matter. Digestion of the incineration residue with a mixture of HNO\({}_{3}\) and HClO\({}_{4}\) causes decomposition of most of the soil minerals and the passage of the constituents into solution. In the following part of the article, this decomposition is referred to as the IUNG method and was carried out at the Department of Soil Science and Soil Erosion of the IUNG in Pulawy. The second method of decomposition was a two-stage decomposition, i.e., digestion with oxidizing acid and then with non-oxidizing acid (HNO\({}_{3}\) and HF, respectively), hot, in a microwave, which was carried out in the laboratory of the Department of Hydrogeology and Water Protection of the AGH University of Science and Technology (AGH) in Krakow. Methods for the Determination of Heavy Metals in Soil and Assessment of the Precision of These Determinations  
The concentrations of selected heavy metals (Zn, Pb, Cu, Fe, Mn, Ni, Cr) were determined in solutions obtained by both digestion methods. The solutions obtained from the decomposition by the IUNG method were determined twice: by the AAS method (AA500 flame, PG Instruments, Leicestershire, UK) in the IUNG laboratory and by the ICP MS method (ELAN 6100 Perkin Elmer, Waltham, USA) in the AGH laboratory. Solutions obtained from two-stage decomposition were analysed only on ICP MS in the AGH laboratory.