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Fig. 9. (a) Salinity transversal sections (psu units) at 20, 40, 60, 80, 100 and 120 m downstream from the middle point of the diffuser; (b) Salinity horizontal sections (psu units) at 2, 4, 6, 8, 10, and 12 m depth; Salinity longitudinal sections (psu units) from -20 m West to 100 m East.

waters at the same depth, rising to the water surface due to the relatively weak stratification, low currents and shallow waters.

In the 20 m transversal section and on the several horizontal sections in the region close to the diffuser it is possible to observe the plume rising from near the bottom to the surface, in accordance with Fig. 5 (see that the East end of the diffuser is over the first transect). South from the diffuser, downstream, there is also evidence of the presence of the effluent plume at the surface, with salinity decreasing to the edges.

Major differences in salinity between the plume and surrounding waters at the surface was observed to be about 0.4 psu in the first two sections, decreasing to about 0.15 psu in the third and fourth sections, and being less than 0.1 psu in the fifth section, finally being almost equal to that of background waters at 120 m distance from the diffuser.

Salinity anomalies of the same order were found by Washburn et al. (1992) and Petrenko et al. (1998). Vertical profiles of salinity collected by Petrenko et al. (1998) at the center and over the western end of the diffuser, where the highest effluent concentrations were found, indicated differences of 0.2 psu. Typical salinity anomalies in the plume of the order of 0.1 psu were observed by Washburn et al. (1992).

The effluent plume was detected from close to the surface (at minimum depths around 1.5 m) to nearly 8 m depth from the first to the fourth section with clearly decreasing thickness downstream. A sharp difference in salinity at the effluent plume lateral edges is clearly visible.

The form of the wastefield spreading (almost centered in the survey area) indicates that the sampling strategy designed was very successful, even for a surfacing plume. A surfacing plume, several times more diluted than a submerged plume, surrounded by low salinity surface waters, with its own weak signals, could be further blurred by the background signals (Petrenko et al., 1998).

The plume exhibits a considerably more complex structure than the compact shape of the classical picture of the buoyant plume but is not so patchy as in previous studies, perhaps because of improvements in horizontal and vertical resolution (Faisst et al., 1990; Petrenko et al., 1998; Jones et al., 2001; Carvalho et al., 2002).

Roberts et al. (1989) laboratory experiments on multiport diffusers in density-stratified perpendicular crossflows show that at low current speeds (F « 0.1) the flow has the normal plume-like pattern with the plume bent downstream. At higher current speeds (F «10) the plume cannot entrain all of the incoming flow while maintaining the free plume pattern and the base of the wastefield stays at the nozzle level. This is known as the forced entrainment regime, which occurs when the Froude number F defined above exceeds a value which lies somewhere between 1 and 10. The rise height and thickness of the wastefield decrease with increasing current speed in the forced entrainment regime. Our in situ observations and measured value of the Froude number seem to be in agreement with these experiments. The observations indicate that the plume is being swept downstream and not attached to the lower boundary, a regime that lies between the two mentioned, in agreement with the behavior expected for a measured Froude number F = 1.18.

3.2 Dilution estimation

Dilution was estimated empirically using temperature and salinity and their representation on a TS - diagram, with initial mixing lines between sewage effluent and receiving waters. Details of this method may be found in Washburn et al. (1992) and Petrenko et al. (1998). When sewage effluent (with temperature Te and salinity Se is discharged, it starts mixing with receiving waters (with temperature Ta and salinity Sa) at the port level. The temperature and salinity of the mixed water mass, respectively Tm and S m , correspond to a point of the mixing line connecting the effluent and the ambient TS - points. The characteristics Tm and S m vary according to the dilution factor between the effluent and the receiving waters. For a given dilution S , Tm and S m are equal to (Fischer et al. 1979)

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