## 3 Persistent Organic Pollutants  
Persistent organic pollutants (POPs) are non-polar organic compounds with high stability and high bioaccumulation properties, toxic at threshold level [78]. The potentially most-hazardous POPs include: industrial by-products dioxins and dibenzofurans, organochlorinated pesticides (OCPs) (dichlorodiphenyltrichloroethane and its four isomers (DDTs), hexachlorocyclohexanes (HCHs), aldrin, dieldrin, endrin, chlordane, hep  
Figure 5: Frequency of the detection of certain CECs reported in the monitoring campaigns on Lower Danube and North-Western Black Sea basins, published between 2010 and 2021.  
tachlor, toxaphene, mirex, hexachlorobenzene) and chemicals resulting from industrial processes such as polychlorobiphenyls (PCBs), brominated flame retardants (BFRs), and polycyclic aromatic hydrocarbons (PAHs). POPs fall under incidence of the Stockholm Convention [79], Water Framework Directive 2000/60/EU [74], Commission Regulation 850/2004/CE [78], and Directive 2008/105/EC [80]. Environmental quality standards (EQSs) concerning the presence in surface water of the priority pollutants are set. Of the hundreds of known PAHs, 16 have been designated High Priority Pollutants by the United States Environmental Protection Agency (US EPA) [81] and are consistently monitored in the water bodies and reported as \(\Sigma\)16 PAHs (US EPA PAHs). Similar, a set of six indicator PCBs (indicator \(\Sigma\)6 PCBs) was recommended by the EU for assessing the pollution by PCBs [82]. Due to high hydrophobic nature and low solubility in water, POPs can be adsorbed on sediment particles or water-suspended particles, leading to the accumulation and concentration in different compartments of the aquatic ecosystems (water, sediment, biota) [2]. International standardized methods are available for the quantification of POPs in surface water or sediments [43]. Solid phase microextraction and liquid-liquid or solid-liquid extractions are the methods employed for the sample processing phase. Due to the complexity of the POPs' nature, multi-methods allowing detection and quantification of a large number of contaminants, such as gas chromatography-mass spectrometry (GC-MS) or gas chromatography coupled with an electron capture detector (GC-ECD), are generally used in the monitoring programs. During 2010-2021, 16 studies were identified on POPs for the selected area (Table 4). Among POPs, PAHs were investigated in 10 studies, PCBs in 5, and OPCs in 11. Most studies were focused on the analysis of sediment or top soil (12 publications) and surface water (four studies). Although POPs analysis in aquatic organisms' tissues is highly relevant for the assessment of the water ecosystem pollution [2], only two studies considered fish as matrix. Previously investigation of highly persistent PCBs and DDTs in fish from Danube River and the Black Sea were performed by [PERSON], (2006) [2] and [PERSON], (2007) [83]. Most of the reviewed studies describe complex monitoring programs over several years [84; 85; 86]. The Danube Delta was investigated in 2 publications, the Black Sea in 4, the Dniester basin in 1, and the Danube basin in 11. Data systematized in Table 4 show values for total concentrations of \(\Sigma\)16 PAHs in sediment ranging from 70 to 6983 \(\mu\)g/kg, for \(\Sigma\)PCBs from 0.3-74 \(\mu\)g/kg and for \(\Sigma\)DDTs from 0.7 to 61.7 \(\mu\)g/kg (Figure 6). Compared with other data on river sediments in Europe, including the upper Danube, the concentrations of \(\Sigma\)16 PAHs (US EPA PAHs) in the sediment samples from the Lower Danube basin were higher than those reported for Danube in Hungary (8.3-1202 \(\mu\)g/kg) [87], Tiber River in Italy (157.8-271.6 \(\mu\)g/kg) [88], or from Durance River in France (57-1527 \(\mu\)g/kg) [89], but lower than those reported in the Ammer River in Germany (112-22,900 \(\mu\)g/kg) [90] and Ria de Arousa in Spain (45-7901 \(\mu\)g/kg dry wt) [91]. Data of PCB in sediments and fishes collected in the Danube Delta in 2001 have been reported [2].