## 2 Materials and Methods
### Sampling and Sample Preparation for Analysis  
Water and sediment samples from each study site were taken monthly over a 13-month period from January 2019 to February 2020.  
Water samples were filtered through a 0.45 \(\upmu\)m PTFE syringe filter (VWR), acidified in situ (pH\(\sim\)2) with nitric acid (>68%, PrimarPlus trace analysis grade, Fisher Scientific, Waltham, MA, USA) and stored in 15 mL PP centrifuge tubes for further analysis.  
For the determination of the activity concentrations of \({}^{238}\)U and \({}^{234}\)U, two 1.5 L, acid-prewashed PE bottles were filled with filtered water, acidified to pH-2 with nitric acid and transported to the University of Vienna.  
Surface sediment samples were taken by hand using nitrile gloves by scooping sediment directly into 50 mL PP centrifuge tubes. Subsequently, samples were transported to the laboratory at the Zoology department of the University of Johannesburg under cooled conditions and protected from light. Samples were frozen at -21 \({}^{\circ}\)C for 48 h and subsequently freeze-dried (Christ ALPHA 2-4 LD plus, Osterode am Harz, Germany). The freeze-dried sediment samples were sent to the University of Vienna, Institute of Inorganic Chemistry. There, the samples were homogenized with a mortar and pestle, sieved through an analytical nylon sieve with a pore size of 1 mm, and filled into 15 mL centrifuge tubes for further processing.  
On an analytical balance, approximately 0.5 g of each freeze-dried, homogenized sediment sample was weighed into a glass tube. Samples were leached using 4.5 mL MilliQ water, 4.5 mL _aqua regia_, which was always freshly prepared in a molar ratio of 3:1 of HCl: HNO\({}_{3}\) (HCl: 34-37%, Trace Metal Grade, Fisher Scientific, HNO\({}_{3}\): >68%, PrimarPlus-trace analysis grade, Fisher Scientific, Waltham, MA, USA), and 1 mL of H\({}_{2}\)O\({}_{2}\) (>30%, for trace analysis, Sigma-Aldrich, St. Louis, MI, USA). Covered with air coolers, sediment samples were leached for 2 h at 130 \({}^{\circ}\)C in a heating block. The leached samples were transferred into 20 mL volumetric flasks and filled up to volume with MilliQ water. The content was subsequently filtered through 0.45 \(\upmu\)m PTFE syringe filter (VWR, Radnor, PA, USA) and stored in 15 mL centrifuge tubes until analyses. Samples of the certified reference material \"marine sediment PACS-2\" obtained from the National Research Council Canada (NRCC) were digested and treated in the same manner as described for the sediment samples.  
\begin{table}
\begin{tabular}{c c c} \hline
**Site** & **River** & **Coordinates** \\ \hline
1 & Tweelopiespruit & 26\({}^{\circ}\)06\({}^{\circ}\)27.5899\({}^{\circ}\)S \\  & & 27\({}^{\circ}\)43\({}^{\prime}\)19.8500\({}^{\circ}\)E \\
2 & Bloubankspruit & 26\({}^{\circ}\)02\({}^{\circ}\)48.9100\({}^{\circ}\)S \\  & & 27\({}^{\circ}\)43\({}^{\prime}\)09.5966\({}^{\circ}\)E \\
3 & Bloubankspruit (+Tweelopiespruit) & 26\({}^{\circ}\)00\({}^{\circ}\)31.2599\({}^{\circ}\)S \\  & & 27\({}^{\circ}\)44\({}^{\prime}\)28.9199\({}^{\circ}\)E \\
4 & Bloubankspruit (+Tweelopiespruit) & 25\({}^{\circ}\)58\({}^{\circ}\)28.9900\({}^{\circ}\)S \\  & & 27\({}^{\circ}\)50\({}^{\circ}\)34.8800\({}^{\circ}\)E \\
5 & Crocodile River & 25\({}^{\circ}\)58\({}^{\circ}\)40.2200\({}^{\circ}\)S \\  & & 27\({}^{\circ}\)50\({}^{\circ}\)37.4499\({}^{\circ}\)E \\
6 & Crocodile River (+Bloubankspruit + & 25\({}^{\circ}\)58\({}^{\circ}\)31.9400\({}^{\circ}\)S \\  & Tweelopiespruit) & 27\({}^{\circ}\)50\({}^{\circ}\)34.9900\({}^{\circ}\)E \\
7 & Lake Heritage Dam & 26\({}^{\circ}\)06\({}^{\circ}\)27.5899\({}^{\circ}\)S \\  & & 27\({}^{\circ}\)43\({}^{\prime}\)19.8500\({}^{\circ}\)E \\ \hline \end{tabular}
\end{table}
Table 1: Coordinates of sampling sites and information on the respective rivers at each site.  
Additionally, about 1 g of the freeze-dried, homogenized sediment was weighed into crucibles and ashed in a muffle furnace for 2 h at 550 \({}^{\circ}\)C to estimate the organic matter (OM) as percent ash free dry weight (AFDW) by loss on ignition [24].