## 2 Materials and Methods
### Analytical Determinations by Means of Portable X-ray Fluorescence (pXRF)  
In this research, an Olympus Vanta C Series with a 4 W Ag anode X-ray tube (VCA) and a silicon drift detector was used. The VCA has an excitation source ranging from 8 to 50 keV and is capable of detecting elements from magnesium (Mg) to uranium (U). As is the case with many pXRF instruments, there are various factory methods available, each with different calibrations. The \"soil\" method is more suited for trace concentrations, whereas the \"geochem\" method is more accurate at higher concentrations. Both methods were used to determine as many elements as possible with the best data quality [27] without moving the analyser between two measurements [5]. In this study, pXRF data > 1 wt.% were selected from the geochem method, whereas values < 1 wt.% were determined using the soil method. Both methods used three beams set to 20 s each, with a total of 1 min for analysis, replicated three times. Although the payback for longer analyses would be lower LODs, a reasonable measurement time was selected to maintain the analysis fast enough to characterise a large number of samples. The accuracy was evaluated by means of certified reference materials (CRMs; \"PACS-3\" and \"MESS-4\", Marine Sediment, NRCC, Whitehorse, YT, Canada), which were analysed in the same batch of samples using both methods. Confirmatory Analyses: Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and Direct Mercury Analyser (DMA-80)  
As a confirmatory analysis for the determination of metals (Cu, Fe, Mn, Pb, Zn) and metalloids (As, Sb) except for Hg, 250 mg of \"lab-prepared\" samples were acid-digested in PTFE vessels through a total acid dissolution in a closed microwave system (Multiwave PRO, [PERSON]) using two heating steps. In the first mineralisation step, inverse aqua regia (3:1, HNO\({}_{3}\) 67-69% and hydrochloric acid HCl 34-37%; hydrofluoric acid (HF, 47-51%, VWR Normatom), suprapure H\({}_{2}\)O\({}_{2}\) (30%, Merck, only in the case of soil samples), and U.S. EPA method 3052 [28] were used. In the second step, boric acid (suprapure H\({}_{3}\)BO\({}_{3}\) 6%, Merck) was added to buffer the excess HF. Blank samples and CRMs were also digested for each microwave batch to evaluate the quality of the analysis. After mineralisation, the solutions were diluted up to a volume of 25 mL by adding Milli-Q water and stored at 4 \({}^{\circ}\)C until analysis. Total digestion with HF was chosen for its capability to dissolve refractory minerals such as silicate minerals. Major and trace elements were determined by means of ICP-MS (NexION 350x equipped with an ESI SC autosampler, Perkin Elmer). In order to avoid and minimise cell-formed polyatomic ion interference, the analysis was performed using Kinetic Energy Discrimination (KED) mode. The instrument was calibrated via an analysis of standard solutions ranging between 0.5 and 500 ug/L prepared by dilution from multistandard solutions for ICP analyses (Periodic Table MIX1 and MIX2, TraceCERT Sigma-Aldrich). Several aliquots of CRMs were analysed to check for accuracy, and allowable recoveries ranging between 70-119% (PACS-3) and 83-107% (MESS-4) were obtained. Except for Cu in PACS-3 CRM, ICP-MS produced 106 of 112 results (94.6%) within the 80-120% recovery acceptance range for all analytes and replicates in the two CRMs. Moreover, potential matrix effects were evaluated by means of laboratory-fortified samples prepared by spiking a standard solution different from that employed for instrument calibration (Multielement quality control standard for ICP, VWR Chemicals) into actual samples. The repeatability of the ICP-MS analysis expressed as RSD% was <3%. Total Hg in powders was determined using a Direct Mercury Analyzer (DMA-80, Milestone, Sorisole, Italy) atomic absorption spectrophotometer, according to EPA method 7473 [29]. The LOD was calculated as 3 times the standard deviation of 10 blanks and divided by the slope of the calibration curve was 0.004 mg/kg.