## 2 Materials and Methods
### Sample Prelevation and Element Detection  
Both sets of soil samples, from 2019 and 1988, were gathered from depths between 25 cm and 40 cm, based on the possibility of penetrating the soil. Soil sampling was conducted according to ISO 18400-104:2018--Soil quality--Sampling--Part 104: Strategies, using an auger soil sampler [(24)]. The sterile samples from 1988 were gathered according to the usual pedological methodology and were analyzed at the Pedology laboratory at the Forestry Research and Management Institute in Bucharest (the present Silviculture Research and Development National Institute). The micro elements (Fe and Zn) and potentially toxic elements (Pb, Cd, V, and Ra) were determined through atomic absorption spectrometry, the same method used for the analyses from 2019, but applied by a different laboratory, using equipments sourced from Varian Inc., Victoria, Australia. Because of unmaterialized sampling points in the open field without forest vegetation in 1988 and without knowing the sampling points' coordinates, but knowing just the area of the sampling points at that time in accordance with the aforestation project (the corners of the waste heap, the middle of the heap's large sides, and the central part of the plateau--yellow circles in Figure 1) the analysis from 2019 was not a perfect replication of the analysis from 1988 in terms of the sampling points but only with regard to the sampling area.  
The soil samples gathered in 2019 were analyzed in the laboratory at the \"Regele Mihai I\" Life Sciences University, Timisoara. For these analyses, a macro and micro elements analysis was conducted according to SR EN 14082/2003 [(25)]. As such, 3 g of samples were burned for calcination at 650 \({}^{\circ}\)C for 8 h (until a light grey ash was obtained) in a Nabertherm B150 oven, sourced from Nabertherm GmbH, Lilienthal, Germany. The resulting ash, dissolved in 20% HCI, was transferred with a final volume of 20 mL. The elements (Ca, K, P, Mg, Cu, Fe, Zn, Cr, Ni, Mn, Cr, Pb, and Cd) were determined through atomic absorption spectroscopy, by using the Varian 220 FAA equipment, sourced from Varian Inc., Victoria, Australia. The detection limits (DML) for the analyzed elements were 0.02 ppm for K and Mg, 0.06 ppm for Cu, Mn, Fe, Zn, and 0.03 ppm for Ca. In order to make the analysis estimate as accurate as possible, each sample collected from the sample points was divided in the laboratory into three sub-samples, which were separately analyzed. The initial results of the analysis represented the total content for each element and were expressed in ppm, reported to the weight of the fresh sample. In the analyses, the averagedetermination of the three sub-samples was included, except for the content variation of the element figure representation, where the average of all 21 sub-samples was used.  
In order to equate and compare with the results from 1988, the 2019 sample results were expressed in calculations as a percentage of the total. The transformation from parts per million to percentages was performed for each individual sample. This was not expressed (neither in the case of the samples from 1988 nor in the case of those from 2019) in the weight of each individual element and their sum does not summarize up to 100%. Each element was expressed as a percentage of the total mass of the sample. Thus, the sum of the percentages of the determined elements varied in the 8 samples from 2019 between 12% and 49%, the rest up to 100% being undetermined inert material. In the samples from 1988, the sum of the percentages of the determined elements varied between 35% and 56%. All chemical substances and solvents used in this study were of an analytic degree. The atomic absorption spectrophotometric method was used in analyzing both sets of samples (from 1988 and 2019).