## 2 Materials and Methods
### Sample Processing  
All samples were homogenized in a porcelain mortar and sieved (<2 mm) before analyses. To analyze the total organic carbon (TOC) and the total nitrogen (N\({}_{\text{t}}\)), the samples were dried for 16 h at 105 \({}^{\circ}\)C and ground in a pebble mill. The concentrations of TOC and N\({}_{\text{t}}\) were determined by a C/N analyzer (vario EL cube, Elementar, Langenselbold, Germany) after combusting the samples at 950 \({}^{\circ}\)C [34].  
Grain size analysis was conducted by suspending 0.3-1 g of a respective sample in water added with 1 mL of a dispersing agent (4% Na\({}_{4}\)P\({}_{2}\)O\({}_{7}\)). Each suspension was sonicated for 10 min. Grain size distributions were subsequently determined by laser diffractometry (LA 960V2, Horiba). Acidity was determined by repeated shaking of the dried samples in 0.01 molar CaCl\({}_{2}\). After a reaction time of two hours, the pH value was measured by means of a glass electrode (WTW Weilheim, Germany) [35].  
Metals and metalloids were extracted as pseudo-total concentrations by heating (85-90 \({}^{\circ}\)C, 2 h) 1 g of a respective sample in aqua regia from 15 mL of HCl (37%) and 5 mL HNO\({}_{3}\) (65%) [36]. The extracts were subsequently filtered with cellulose filters (particle retention 1-2 \(\upmu\)m, Sartorius) and diluted with ultrapure water. Since a potential element release in soils can be estimated by their mobilizable element proportions, the easily exchangeable element concentrations were determined in ammonium-nitrate extractions. For this purpose, 10 g of each sample was shaken (1.5 h) in 25 mL of 1 M ammonium-nitrate [37]. Since this extraction is usually performed on dried samples and oxic conditions were found in the field for the vast majority of soils, it was performed on dried soil samples (CS). In contrast, the organoleptic characteristics of RS indicated highly anoxic conditions. To minimize the possible influences of drying on easily exchangeable element concentrations, these were determined on field-fresh (previously frozen) RS. The extracts were centrifuged at 3000 g, filtered, diluted to 50 mL with ultrapure water, and stabilized by the addition of 0.5 mL HNO\({}_{3}\) (65%). All extracts were stored in pre-cleaned polypropylene centrifuge tubes at 4 \({}^{\circ}\)C until analysis. The aqua regia extractable concentrations of aluminum (Al), chromium (Cr), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) were analyzed by ICP-OES (Optima 8300, Perkin Elmer) using matrix adaption and a recovery control by certified reference material (NIST 1646a, ERM-CC020) [38]. As well as the ammonium-nitrate extractable concentrations of all elements, the concentrations of aqua regia extractable arsenic (As), cadmium (Cd), and vanadium (V) were determined in diluted extracts by ICP-QQQ-MS (8800, Agilent) [39].  
The results of ICP-OES and ICP-QQQ-MS analyses were used to calculate the enrichment factor (EF), which relates the concentration of a metal(loid) to the concentration of a reference element of low variability and is suitable for assessing anthropogenic element enrichment [40]. Because its calculation normalizes sample-specific variations in grain sizes, it is useful for comparing different samples or groups of samples. Due to its comparatively low mobility under varying redox conditions, Al was used as a reference element. The EF was calculated according to [PERSON] and [PERSON] (2016) [41] by the equation:  
\[\text{EF}=(\text{M/Al})\text{s}/(\text{M/Al})\text{r}\]  
where (M/Al)s is the mean metal(loid)/Al ratio of a respective sample divided by (M/Al)r, which is the mean element-specific ratio to Al of European river sediments according to [PERSON] et al. (2005) [42], using the following values: Al = 5.4 (%); As = 9.5 (mg kg\({}^{-1}\)); Cd = 0.527; Cr = 31 (mg kg\({}^{-1}\)) Mn = 716 (mg kg\({}^{-1}\)); Ni = 28.6 (mg kg\({}^{-1}\)); Pb = 29.8 (mg kg\({}^{-1}\)); V = 33 (mg kg\({}^{-1}\)); Zn = 98 (mg kg\({}^{-1}\)).