## 2 Methods  
### Study Site  
The Ria de Aveiro lagoon (Figure 1) is an example of a coastal ecosystem whose environmental quality was wrecked due to an increased volume of poorly treated or untreated effluent input, which required an important process of re-qualification. The coastal lagoon's main sources of pollution were domestic and industrial sewage discharge, as well as wastewater from fish and shellfish production [20]. Hence, to mitigate the wastewater discharge into the lagoon, the Sao Jacinto sea outfall was built in 1998. Nowadays, it drains the effluent from the Cacias, Ilhavo, and Sao Jacinto WWTPs and from 10 municipalities, which accounts for over 300,000 inhabitants [21; 22]. The Sao Jacinto sea outfall, which lies on a nearshore continental shelf of 1.4 to 22.5 m depth, is located 3 km north of the Barra inlet channel north jetty and in front of the Sao Jacinto Dunes Natural Reserve. According to [PERSON] and [PERSON] [23], the mean outflow of discharged wastewater was 0.8 m\({}^{3}\)/s. However, recent data on the Sao Jacinto sea outfall flow rate and operational characteristics are not publicly available. Moreover, the submarine outfall has an extension of 3.378 km, reaches a depth of 15 m, and has a diameter of 1.6 m [22]. The diffuser contains 20 operational holes, disposed alternatively with an angle of 30\({}^{\circ}\) and 150\({}^{\circ}\) about the horizontal axis, found in the last 98 m of the outfall [23]. In addition, Sao Jacinto beach, north of the Barra inlet channel, has held the Blue Flag award for the past 17 years and received the Golden Flag award in 2020 [24; 25; 26]. These recognitions are given to beaches that hold good environmental and urban state conditions, promote sustainable use of the space, and present high water quality during bathing season. Hence, monitoring wastewater dispersion becomes imperative. Regarding the marine outfall's discharge, the initial behavior of the wastewater jet is mainly determined by the diffuser hole's geometry and location on the pipe, the flow rate of the fluid, buoyancy force, and momentum. The buoyancy force will act when the density gradient between the adjacent ocean and sewage inflow dominates the dynamics of the jets, overlapping the discharge momentum due to its distance from the pipe's mouth. As the wastewater jets are less dense than seawater, they reach the surface due to the buoyancy force and advection, and the patch can be labeled as a plume [27]. The receiving-water state is dominated by wind and tidal flow; however, swell waves are also relevant, as they influence the coastal longshore current [28]. The region of interest (ROI) is also characterized by northwesterly alongshore wind-driven currents that induce an upwelling circulation pattern, typical for summer months, while southwesterly wind causes downwelling flow mainly in autumn and winter [19; 29]. Moreover, the coastal water mass stratification primarily depends on the salinity and water temperature range, which changes seasonally and can limit the plume's ascension. For example, where significant density stratification exists, the sewage plume will face an energy barrier to rise to the ocean surface and therefore will take more time to ascend to the surface or remain submerged at the intermediate location, whereas on a non-/weakly-stratified water column, the mixed seawater and effluent plume freely ascends to the surface [23], becoming visible on satellite images. The Aveiro coastal lagoon's hydrodynamics depend mainly on the semidiurnal tide flow [30; 31]. Its main tributaries are _valas_ de Mira, and the rivers Vouga, Antua, Caster, and Boco. Furthermore, the coastal lagoon's outflow is expected to interact with the marine outfall plume, as it is located near the Sao Jacinto marine outfall [19]. Henceforth, the coastal system presents two sources of nutrient load: the marine outfall discharge and the lagoonal water inflow from Ria de Aveiro.