2.1.5. Element Concentrations

Analysis of element concentrations was conducted with a Niton XL3t portable energy-dispersive X-ray fluorescence spectrometer (p-ED-XRF) by Thermo Scientific (Waltham, Massachusetts, USA). Internal parameter calibration was set to 'mining mode' and all measurements were recorded in %. Two certified reference materials (CRMs) were used for quality control: GBW07312 (stream sediment; National Research Center for Certified Reference Materials in China) and NCS DC 73387 (soil; China National Analysis Center for Iron and Steel). The CRMs were measured repeatedly after every eight samples. Since this study does not examine any absolute elemental concentrations but, rather, the overall variability throughout the profile, the CRMs were used only to ensure validity of the measured data. Recovery values were therefore not examined quantitatively.

Further processing of the p-ED-XRF data was necessary to produce reliable measurement results. Measurements with a value smaller than or equal to three times the 1-*σ* error were discarded. Following Schmidt et al. [43], the remaining results were analysed using a compositional data approach to manage the consistent sum-constrained model [44] via R [45]. Elements were only processed if the number of values below the limit of detection did not exceed 10% of the respective measurements. Subsequently, the data were transformed using centred log-ratio (clr; [44]):

$$z = clr(\mathbf{x}) = [ln(\mathbf{x}\_1/\mathbf{g}(\mathbf{x})), ln(\mathbf{x}\_2/\mathbf{g}(\mathbf{x})), \dots, ln(\mathbf{x}\_D/\mathbf{g}(\mathbf{x}))],\tag{1}$$

where *g*(*x*) = [*x*<sup>1</sup> × *x*<sup>2</sup> × ... × *xD*] × 1/*D* is the geometric mean of all parts of the observation *x* = [*x*1, *x*2, ..., *xD*]. This transformation was conducted using the compositions package for R [46]. Prior to the conducted cluster analysis, the data were additionally standardised using a robust z-transformation.
