**5. Acknowledgment**

18 Will-be-set-by-IN-TECH

*C*0. In the case of variability of the

(*C*<sup>0</sup> − *Ca*0), which in

, (29)

 <sup>1</sup> *S* 

East (m)

0 50 100 150

 *<sup>S</sup>*−<sup>1</sup> *S* + <sup>1</sup> *S* 

*<sup>S</sup>* <sup>=</sup> *<sup>C</sup>*<sup>0</sup> <sup>−</sup> *Ca*<sup>0</sup> *C* − *Ca*

where *Ca*<sup>0</sup> is the background concentration of substance *X* in ambient water at the discharge

simple terms means that the increment of concentration above background is reduced by the dilution factor *S* from the point of discharge to the point of measurement of *C*. Using salinity distribution at depths of 1.5 m and 3 m we estimated dilution using Equation 29 (see the contour maps in Fig. 13). We assumed *C*<sup>0</sup> = 2.3 psu, *Ca*<sup>0</sup> = 35.93 psu, *Ca* = 36.008 psu at 1.5 m depth and *Ca* = 36.006 psu at 3 m depth. The minimum dilution estimated at the depth of 1.5 m was 705 and at the depth of 3.0 m was 1164 which is in accordance with Portuguese legislation that suggests that outfalls should be designed to assure a minimum dilution of 50 when the plume reaches surface (INAG, 1998). (Since dilution increases with the plume rising we should expect that the minimum values would be greater if the plume reached

Through geostatistical analysis of temperature and salinity obtained by an AUV at depths of 1.5 m and 3 m in an ocean outfall monitoring campaign it was possible to produce kriged maps of the sewage dispersion in the field. The spatial variability of the sampled data has been analyzed and the results indicated an approximated normal distribution of the temperature and salinity measurements, which is desirable. The Matheron's classical estimator and Cressie and Hawkins' robust estimator were then used to compute the omnidirectional variograms that were fitted to Matern models (for several shape parameters) and to a Gaussian model. The performance of each competing model was compared using a split-sample approach. In the case of temperature, the validation results, using a two-dimensional ordinary block

North (m)

−150

−100

−50

0

50

depth. This expression in 29 can be arranged to give *C* = *Ca* +

background concentration of substance *X* in ambient water the local dilution is given by

which can be rearranged to give *C* = *Ca*

surface (Hunt et al., 2010)).

North (m)

−150

**4. Conclusion**

−100

−50

0

50

East (m)

0 50 100 150

Fig. 13. Dilution maps at depths of 1.5 m (left) and 3 m (right).

This work was partially funded by the Foundation for Science and Technology (FCT) under the Program for Research Projects in all scientific areas (Programa de Projectos de Investigação em todos os domínos científicos) in the context of WWECO project - Environmental Assessment and Modeling of Wastewater Discharges using Autonomous Underwater Vehicles Bio-optical Observations (Ref. PTDC/MAR/74059/2006).
