## 2 Materials and Methods
### Sampling  
Nine sampling locations were selected for the study area, encompassing over 1000 km along the Danube River, from Bazias (km 1075) to Sulina (near km 0--Figure 1). Seven sampling stations were situated on transverse profiles of the Danube River, with two supplementary transverse profiles located in the lower reaches of the Jiu and Olt tributaries. The selection of the locations was based on the criteria established in [71], which stipulates that the sampling location should be correlated with a Trans-National Monitoring Network station or a national monitoring station for water/sediments. Additionally, the location should be representative for the water body, have a different geological background, be easily accessible, and have information on potential sources of contamination. The final station coordinates were established in collaboration with the National Water Authority in Romania (ANAR). The collection of suspended and bottom sediments from the transverse profiles was conducted in accordance with the ISO sampling guidelines for sediments [72; 73]. Three verticals were established on each transverse profile, designated as \"Left\" (at 50 m from the Danube River's left bank), \"Centre\" (in the center of the river), and \"Right\" (at a distance of 50 m from the Danube River's right bank). At each vertical, 30 L of water was collected using a submersible pump from the following depths: 0.5 m below the water surface, the middle depth, and 0.5 m above the bottom of the river. A composite water sample with suspended sediment, comprising 90 L, was obtained by using this method and deposited in a plastic barrel. After 24 h, the supernatant was removed, and 8 L of water with settled suspended sediment was collected and refrigerated (4 \({}^{\circ}\)C) in a plastic bottle. Figure 1: Sample locations: (**a**) Bazias; (**b**) Pristol; (**c**) Zával; (**d**) Islaž; (**e**) Svishtov—Zimnicea; (**f**) Oltenita—Tutrakan; (**g**) Silistra—Chiciu; (**h**) Hársova; (**i**) Sulina; (**j**) distribution of the sampling sites in Romania. Background map credentials: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Kong Kong), Esri Korea, Esri (Thailand), NGCC © OpenStreetMap contributors, and the GIS User Community. To collect the bottom sediments, a Van Veen grab with a volume of 250 cm\({}^{3}\) was deployed from a manual-driven winch at each vertical (left, center, and right). Samples of the riverbed sediments were obtained from the uppermost 5 cm. A bottom sediment sample, a replicate, and a duplicate were collected and stored in 750 mL glass jars, also kept in low-temperature conditions. In the case of the Jiu and Olt tributaries, the bottom and suspended sediments were sampled from a single vertical profile situated at the center of the river. Active floodplain sediment (overbank) was collected close to the river transverse profiles, with appropriate consideration given to the extent of the flood hazard maps [74]. Floodplain sediment samples were collected from two depths using spades and shovels: the topsoil or top layer (0-10 cm) and the bottom layer (40-50 cm). This approach was undertaken to attempt to separate recent contamination from older contamination and potentially identify a pre-industrial, non-contaminated state [75]. A composite sample was created by combining the sediment collected from five sampling locations along a linear riverbank profile of 250-500 m in accordance with the methodology outlined in the Geochemical Atlas of Europe [76]. The sampling campaign was conducted in the autumn of 2020 (active floodplain sediments) and, for logistical reasons, in the late spring of 2021 (suspended and bottom sediments). Given the difficult sampling conditions, 54 unique samples were collected and transported under refrigerated conditions to the laboratory. The floodplain samples and the bottom sediments were subjected to a sieving process, during which each sample was divided into two distinct grain sizes: <0.063 mm and <2 mm.