Modeling of the Spread of an Effluent and the Overturning Length Scale near an Underwater Source in the Northern Adriatic

The spread of sewage in a coastal sea is a complex phenomenon that includes mechanisms that act on different environmental scales. In this work, a review of different mechanisms will be presented, all mostly confined to the hydrodynamic conditions of sewage that leaves a treatment plant and travels to the sea through sewage pipes (first stage), spreads into the sea through the diffuser orifices in conelike structures (second stage, initial dilution), forms a patch of pollutant around the source (third stage), and dilutes further with the motion of ambient fluid, through turbulent diffusion and advection, that stretches the initial patch (fourth stage, secondary dilution). This process is also called turbulent dispersion, in which mixing and dilution take place.1 In a study of the spread of sewage from two submarine diffusers off the town of Piran and from a single diffuser off the town of Izola (see Figure 1 for locations), the sewage near field was surveyed with a conductivity, temperature, and depth (CTD) probe in a very shallow sea (depths less than 21 m). The hydraulics of the discharge system were also studied,2 because the flow rates and buoyancy fluxes through the orifices of the diffusers are the initial conditions for the modeling of the initial dilution of sewage in the sea.3 A review will be presented of numerical solutions of the dilution of the initial phase, when a conservative pollutant is rising toward a layer of neutral buoyancy. This process takes place on a scale of 20 m. Formation of the initial patch of effluent above the sewage diffusers will be estimated through the space distribution of the overturning length scale on a slightly larger scale (up to 500 m). Secondary dilution, which mainly depends on advection, will also be examined for a specific, yet typical, case of wind-driven circulation when it is supposed that the effluent reaches the surface layer, if released outside the Bay of Koper near the center of the Gulf of Trieste. It is supposed that, during the initial dilution, the pollutant is a conservative one, since this process takes less than 1 min, while in the case of secondary dilution, a decay of faecal coliforms on a time scale of hours is supposed. Small wiggles on the vertical profiles of temperature and salinity were observed on vertical CTD profiles in a near field above the submarine source of sewage. They resulted in small wiggles on the vertical profiles of density. Many of them could not be explained simply by temperature and conductivity mismatches due to the different dynamic responses of the probes.4 It is suspected, however, that they are related to the presence of a turbulent sewage fluid, already mixed with the surrounding water, that is spreading horizontally at a level of neutral buoyancy. The detection of sewage that may spread horizontally in a layer that is less than 0.5-m-thick is a key problem in field observations when its depth is not known in advance. There are numerous mechanisms that cause these small vertical fluctuations of density, such as internal waves5 with their breaking, surface wave breaking,6 wind-driven Langmuir cells over the ocean,7 patchy turbulent mixing at the thermocline,8 slope turbulent gravity currents and episodic convective plumes,9 surface cooling and convection overturns,10 and intrusions and underflows of a denser river fluid.11 Different turbulent processes at the sea surface expand deeper into a water column,12 and a shallow sea may be filled with them. Most * Corresponding author. E-mail: [email protected]. Figure 1. Top: Location map of the sewage outfall off Piran composed of two pipes that end with diffusers above which the near field was explored (larger dashed rectangle) and an outflow of another small town (Izola) little more than 200 m offshore that was also surveyed (smaller rectangle). The dotted arrow represents the Sirocco (Jugo) wind that blows along the axis of the Adriatic Sea, while the Bora (Burja) wind (dashed arrow) blows along the axis of the Gulf of Trieste. Bottom: two parallel pipelines of the outfall off Piran that end with diffusers, which were deflected westward at the time of the CTD surveys (1996-1998). Diffusers are 2.6 km away from the nearest land point. 1491 J. Chem. Inf. Model. 2005, 45, 1491-1501