## 2 Materials and Methods  
### Study Area  
Approximately one-third of the Danube basin area (29%) is situated within the borders of Romania. Consequently, the river plays an integral role in the lives and well-being of the people who inhabit this region. The Lower Danube Basin represents the third portion of the Danube catchment area, where the river is compelled to traverse the mountains once more, forming a remarkable gorge. The Lower Danube begins at the Iron Gates, not far after the Danube enters Romania (west of Bazias), forming a natural border with Serbia and then with Bulgaria, and ends at Coastal Izmail [63]. Not far from this point, the Danube River divides into two branches (Chilia and Tulcea), marking the beginning of the Danube Delta [64, 65].  
From a geomorphological perspective, [PERSON] (2001) delineates the Danube River into nine reaches [66], with the Romanian reaches being represented by the following numbers: Reach 6 (from the Sava River at Belgrade up to the Iron Gate Dam, between 1202 km and 943 km), Reach 7 (from the Iron Gate Dam up to the confluence with the Jantra River, between 943 km and 537 km), Reach 8 (from the Jantra River up to Reni, between 537 km and 132 km), and Reach 9 (from Reni up to the Black Sea, between 132 km and 12 km). The aforementioned reaches exhibit a range of distinctive features. Reach 6 encompasses the Southern Carpathians, characterized by mountainous terrain. In contrast, Reach 7 encompasses the Romanian Plain, comprising loess, Aeolian sediments, and a steep, 150 m high Danube embankment on the right side. Reach 8 features a low relief with alluvial islands, while Reach 9 encompasses the Delta, which is home to a unique ecosystem.  
From a geological perspective, the tributaries from both orogens of the Romanian territory, the Carpathian orogen and the North Dobrogea orogen [67], which exhibit significant differences in terms of geological age and characteristics, contribute their waters to the Danube River, together with the tributaries from the Moesic Platform. In the last sector, the Danube Delta corresponds geologically with the Pre-Dobrogea Depression [67]. The Carpathian Orogen is part of the extensive Alpine belt, which encompasses the Pyrenees, the Betic Cordillera with the Balearic Islands, the Rif, the Tell-Atlas, the Apennines, the Alps, the Carpathians, the Balkans, the Dinaric Mountains, and the mountainous regions of Anatolia, Iran, Crimea, the Caucasus, and the Turkmen-Khorasan Mountains, continuing further eastwards [68]. Several mineral deposits have been exploited over time within the Banatitic Magmatic and Metallogenetic Belt (BMMB), as described in [69; 70]. The belt extends for 1500 km from the Apuseni Mountains (Western Romanian Carpathians) in Romania, towards the Southern Carpathians (in Romania and Serbia) and further to the southeast (in Bulgaria), hosting the only significant porphyry-copper deposits of Europe [70], along with very diverse deposits of Cu, Au, Mo, Zn, Pb, and Fe, which are of magmatic-hydrothermal origin.  
Mining these deposits has contributed to the anthropogenic impact of trace elements on the environment in the Lower Danube, along with other contamination from industry in Romania. This has been added to the contamination already transported by the Danube waters from upstream cities and capitals. Another source of contamination with trace elements (Cd, for example) comes from fertilizers used in agriculture.