## 1 Introduction  
With a total length of 2780 km, the Danube River crosses 10 countries and 4 capitals and eventually runs into the Black Sea through the Danube Delta, the largest European wetland [1]. Due to its biodiversity, The Danube Delta, together with the Razim-Sinoe lagoon, is stated as an UNESCO World Heritage Biosphere Reserve [2]. The Danube basin on Romanian territory is the largest, compared with other countries of the Danube River Basin [1]. Surrounded by six coastal countries--Bulgaria, Georgia, Romania, Russia, Turkey, and Ukraine--the Black Sea is one of the largest inland water basins. Being almost entirely isolated from the world's oceans, the Black Sea is the largest natural anoxic water basin in the world [3]. The Black Sea is also a reservoir for the contaminants from multiple sources, among which Danube, the Driester, the Dnieper, and the Don are the most significant [3]. Due to the location and climatic and historical conditions, the Lower Danube and Black Sea basins constitute an unique ecosystem [4]. In Eastern Europe, the rapid development of small industry tourism activities and urban area, together with a decrease of the intensive farming, led to the frequent change in a pollutant profile. High industrial pollution inputs from tributaries and poor control of the discharges significantly affected the Lower Danube's water quality [5]. Due to fish farming, reed harvesting and, more recently, increasing of the recreational and tourist industry, disturbances in the aquatic ecosystems of the Danube Delta have occurred [1]. Surface waters provide important services, including drinking water, irrigation, hydropower, tourism, fishing, and navigation. Crossing a territory inhabited by about 83 million people, the Danube River is permanently highly exposed to anthropogenic contamination [5]. As the contamination of water bodies pose significant public health risks, water framework policies aim to improve the management of water resources. Danube River Protection Convention (1994) represents the overall legal instrument for water management and transboundary co-operation in the Danube River Basin [6]. According to the requirements of the European Water Framework Directive (WFD), a process of selecting and monitoring of the relevant pollutants in the river basin started in 2001 [7]. Coordinated by the International Commission for the Protection of the Danube River (ICPDR), Joint Danube Surveys (JDSs) have been conducted on the Danube river basin every six years from 2001-2020 (JDS 1-4), providing reliable and comprehensive data on water quality and pollution level [8]. Furthermore, several surveys along shorter sections of the Danube River monitoring certain contaminants classes relieved dynamic and regional particularities of the pollution [9; 10; 11; 12; 13; 14]. Thus, different monitoring studies identified several classes of contaminants such as: pharmaceuticals [10; 13; 15; 16], personal care products [17; 18], endocrine disruptors [19; 20; 21; 22; 23], pesticides [2; 22], polycyclic aromatic hydrocarbons [11; 22], polychlorinated biphenyls [22], heavy metals [22; 24; 25], and microbiological contaminants [8]. However, past surveys, including JDSs 1-3, regarded a limited number of compounds and classes. Currently, due to the development of the advanced analytical technologies based on high-resolution mass spectrometry together with large databases such as NORMAN network, wide scope non-target screenings have been conducted, providing a broader perspective on the Danube pollution phenomenon [26; 27]. The present review aimed to systematize recent monitoring studies in the Lower Danube and North West Black Sea area in order to identify the current research status and layout future research challenges; such surveys provide policy-relevant data. Comparative discussions on the pollution of other rivers elsewhere are presented.