## 2 Materials and Methods
### Sequential Extraction of Metals from Dried Sediment and Soil Samples  
A six-step sequential extraction procedure, based on the approach proposed by [58], was applied to fractionate the heavy metals from sediment and soil samples into six fractions: exchangeable (F1), bound to carbonates (F2), bound to Mn oxides (reducible) (F3), bound to organic matter (F4), bound to Fe oxides (reducible) (F5), and residual (F6). Extractions were performed using dried samples at the Institute for Regional Development (University of Castilla-La Mancha). First, approximately 1 g of the dried samples from cores BA-BN (\(n=14\)) were weighed and sealed in 50 mL polyethylene (PE) vials. The subsequent extraction used different extraction media (see following paragraph). After each successive step, samples were centrifuged at 7000 rpm for 15 min to separate the extract (supernatant) from the residual sediment. Extracts were then filtered through a  
Figure 1: (**A**) Location of the Segura river basin in Europe. (**B**) Location of Pétrola basin in Segura river basin. (**C**) Land use map of Pétrola basin and location of control points from the studied areas: Pétrola Lake (green squares), affected by wastewaters (black circles), dryland cultivation (blue rhombus), and irrigated croplands (red triangles). 0.45 \(\upmu\)m nylon filter (Millipore(r), Burlington, MA, USA), transferred into 15 mL PE vials, and acidified by adding 0.5 mL of ultrapure HNO\({}_{3}\) (69%, TraceSELECT(r) Fluka, Hong Kong, China). Then, the residual pellet was washed with 10 mL Millipore water and centrifuged at 7000 rpm for 5 min, the supernatant was discarded, and the pellet used for the next extraction. All chemicals used for the extraction (Sigma-Aldrich, St. Louis, MO, USA) were of analytical grade (p.a.) unless otherwise stated. The exchangeable fraction (F1) was extracted using 40 mL of 1 M KNO\({}_{3}\) (pH = 7) for 14 h under constant shaking (adapted from [76]). The fraction \"bound to carbonates\" (F2) was extracted by adding 35 mL of 1 M sodium acetate solution to the residue from step 1 and adjusting to pH 5.0 with glacial CH\({}_{3}\)COOH, followed by continuous agitating for 5 h on an orbital shaker at 300 rpm (adapted from [77]). The residue was treated with 35 mL of 0.1 M NH\({}_{2}\)OH\(\cdot\)HCl in 0.1 M HNO\({}_{3}\) (pH = 2) for 2 h [77] to obtain the fraction \"bound to Mn oxides\" (F3). The fraction \"bound to organic matter\" (F4) was obtained from the residue from the previous step by adding 35 mL of 0.1 M Na\({}_{4}\)P\({}_{2}\)O\({}_{7}\) solution (pH = 10) and leaving it for 36 h [77]. Then, the residue was used to extract the fraction \"bound to Fe oxides\" (F5) by adding 35 mL of 0.04 M NH\({}_{2}\)OH\(\cdot\)HCl in 25% (\(v/v\)) CH\({}_{3}\)COOH at 96 \({}^{\circ}\)C with occasional agitation for 5 h [59]. Finally, the residual fraction (F6) was extracted from the residue from F5 with 30 mL of ultrapure 4 M HNO\({}_{3}\) at 85 \({}^{\circ}\)C with occasional agitation for 12 h [78]. All the extracts were shipped to the lab of the Institute of Inorganic Chemistry at the University of Vienna for element analysis. In addition to this sequential extraction, a pseudo-total digestion was carried out in the laboratory at the University of Vienna. This digestion should yield a 70-90% recovery of total metal concentration in the sample, depending on the species and bonding of the metal with the silica matrix [79]. Approximately 0.5 g of the homogenized dry sample were digested. Acid leaching was conducted in glass tubes equipped with air-coilers in a heating block for 120 min at 130 \({}^{\circ}\)C after adding 9 mL of 34% HNO\({}_{3}\) (TraceSELECT(r) Fluka) and 1 mL H\({}_{2}\)O\({}_{2}\) 30% (TraceSELECT(r) Fluka). Digested samples were transferred into 20 mL flasks and brought to volume with Millipore water. Afterwards, samples were filtered through 0.2 \(\upmu\)M PTFE syringe filters (VWR).