## 1 Introduction  
In coastal areas, human settlement and its continuous growth have affected estuarine and coastal ecosystems over the years [1, 2]. The urban and industrial wastewater discharge into water bodies induces dispersion of contaminants, pathogens, and nutrients in natural aquatic ecosystems. As a result, the water quality degrades in these ecosystems, threatening both marine life and human societies that depend on these water bodies [3, 4, 5]. It is thus essential to ensure that society's development is managed sustainably for good environmental state preservation in these areas. Directive 91/271/CEE of the Council of the European Communities is dedicated exclusively to regulating urban wastewater treatment. In Annex I of Directive 91/271/CEE, the adequate treatment of urban wastewater is delineated, from collection to discharge, and it also lists urban wastewater treatment plants' (WWTPs) water quality requirementsthat have to be met before the sewage water flows into sensitive ecosystems. In Portugal, for instance, Law-Decree 152/97 transposes Decree 91/271/CEE of the Council. The bathing water quality around the country must be coherent with Directive 2006/7/CE of the European Parliament and the Council. Before 2000, pollution of water bodies and degradation of water quality were already a general concern in the European Union (EU). However, the water policies were not unified among the member states [6]. Hence, there was room for reinforcing the Community's requirements to preserve the good environmental status of its water bodies. The Water Framework Directive (WFD) was implemented in 2000 to regulate the balance between ecosystems conservation and human activities. The WFD offers guidelines to the member states for the protection of inland surface waters, transitional waters, coastal waters, and groundwater. The main aim of the WFD is to contribute to the reduction and further elimination of the emission of harmful substances into bodies of water to achieve concentrations near background values of the naturally occurring substances in those environments. In order to decrease the impact of wastewater on the estuarine environment, sea outfalls became the solution for directing the discharge point away from the coast. Furthermore, the ocean seems to be the best alternative for pre-treated effluent input due to the high self-purifying capacity inherent to its composition, the constant water flow, and the dimensions, thus reducing the level of treatment required in the WWTP [7]. In mainland Portugal, for instance, where the coastal hydrodynamic conditions favor the dilution and dispersion of the effluents [8], there are 21 operational sea outfalls; 19 of them discharge urban wastewater and 2 industrial effluents [9]. A reasonable number of studies have employed satellite imagery to monitor the impact of wastewater discharge in the coastal environment [4,10, 11, 12]. Overall, those studies proved the Sentinel-2 (S2) ocean color data to be adequate to monitor sea outfall turbid plume dispersion. For instance, [PERSON] et al. [11] monitored the coastal area of two cities along the coast of the Black Sea affected by wastewater from the sea outfall's pipe cracks. [PERSON] et al. [12] monitored the biophysical impact of direct sewage discharge in a shallow nearshore environment in California. Satellite technology developments and improvements have made the investigation of coastal processes possible without the necessity of scheduling aerial/boat surveys for monitoring purposes, as in the past. In this context, S2 twin-satellite imagery emerged as a fundamental and effective tool to monitor the coastal waters, as it offers large areas of coverage (swath width of 290 km on the ground track) within a high temporal (2-5 days) resolution, with spatial resolution up to 10 m. Data access has no inherent cost, presenting as a cheaper and more efficient tool to assist effluent discharge monitoring.