## 4 Metals
A number of six complex studies considered water, sediments, and biota. As environmental pollution with heavy metals is well regulated, sampling, sample preparation, and analysis are generally carried out according to specific standards, e.g., ISO 5667-3:2018 [131] for water sampling, sample preservation, transport, and storage, ISO 5667-13:2011 [132] for sediments sampling, ISO 15587-1/2:2002 [133; 134] for water sample preparation, ISO 17294-2:2016 [135] concerning the ICP-MS method, ISO 11047:1998 [136] concerning flame and electrothermal AA for analysis. The most frequently reported metals, in the following decreasing order, were: Pb, Cd, Ni, Cr, As, Cu, Zn, Hg, Mn, Fe, Co, Al. The reported values show large variation for all matrices (Figure 7). Regarding spatial distribution of metal concentrations in water, it can be observed that, in general, the highest concentrations were reported for tributaries, followed by those in the Danube Delta biosphere Reserve, the lowest concentrations being reported in the Danube River. Thus, for Cd, a concentration range of 0.002-1.33 \(\upmu\)g/L was reported in the Danube River, values of 50 \(\upmu\)g/L were measured in the tributary Morava River, Serbia [109], and of 14.90 \(\upmu\)g/L in the tributary Yantra River, Bulgaria [111] as a maximum of 11.05 \(\upmu\)g/L in the Biosphere Reservation of the Danube Delta [123]. Similarly, Cu reported concentrations varying within a range 2.65-10.1 \(\upmu\)g/L, except the maximum value of 112.3 \(\upmu\)g/L in the Danube, reported in the Danube in sampling sites corresponding to a large industrial city (Galatj town) [117]. In the tributary Tisa River, a concentration of 70 \(\upmu\)g/L was measured [109], as a value of 6.7 \(\upmu\)g/L was reported in the Danube Delta [126]. Although lower concentrations of metals in seawater have generally been reported for Cu, a maximum value of 30.66 \(\upmu\)g/L was measured by [PERSON] et al. [128]. Cr concentrations ranged from 0.21 \(\upmu\)g/L to 9 \(\upmu\)/L in the Danube River. A maximum concentration of 64 \(\upmu\)g/L was reported in Olt River [107], as comparable maximum concentrations of 81.24 \(\upmu\)g/L [122] and 78.25 \(\upmu\)g/L [124] were measured in the Biosphere Reservation of the Danube Delta. A similar trend was observed for Hg, with concentration range of 0.001-0.117 \(\upmu\)g/L in the Danube and a maximum value of 68.15 \(\upmu\)g/L reported in the Yantra River, Bulgaria [117]. Low concentrations of Pb are generally reported in the Danube river in a range of 0.003-3.81 \(\upmu\)g/L with the exception of a maximum value of 21.4 \(\upmu\)g/L reported near the Galati town by [PERSON] et al. [117]. However, considerably  
Figure 7: Concentration range of the seven most-reported metals in surface water (**left**) and sediments (**right**) compared to the maximum acceptable concentration (MAC-EQS) according to the Decisions 2013/39/EU and Order 161/2006 (Romanian regulation). higher concentrations of 48.04 ug/L [122] and 34 ug/L [124] of Pb were reported in the Danube Delta Biosphere reserve. For As, concentration range of 0.5-16.96 ug/L was reported in the Danube River, a maximum of 10.1 ug/L in the tributary Yantra River [111], and a maximum of 16.25 ug/L in the Danube Delta [124]. Regarding the concentrations of metals in sediments, in general, the lowest concentration values were reported for the Danube Delta followed by those in the Danube River. Significantly higher values were reported for tributary, especially where sampling was carried out from reservoirs or between of shipping locks. Thus, a concentration range of 1.96-126.52 mg/kg Cu in sediments was measured in the Danube River [85] and 40 mg/kg in the Danube Delta [126], whereas, a maximum concentration of 263 mg/kg was reported in the Begej canal [84]. A similar trend was observed for Pb, Hg, Cd, As, Zn, and Cr.