Fig. 10. Temperature-Salinity (TS -) diagram of data from Section 1.
Fig. 10. Temperature-Salinity (TS -) diagram of data from Section 1.
The TS - points in the plume located off the initial mixing lines have already started to mix with waters above 11 m depth. According to these results, effluent dilutions were then at least 30:1. This value is probably a lower bound of the dilution since, in reality, mixing continued to occur up to surface. However, with this method no further dilution can be inferred.
An oceanographic campaign was performed on July 30, 2002 to study the shape and dilution of the S. Jacinto outfall plume using Isurus AUV.
Our results demonstrate that AUVs can provide high-quality measurements of physical (and probably optical) properties of effluent plumes in a quite effective manner. An efficient sampling strategy, enabling improvements in terms of resolution of time and space scales and undersampling, demonstrated that effluent plumes can be clearly traced using naturally-occurring tracers in the wastewater.
In order to reduce the uncertainty about plume location and to concentrate the vehicle mission only in the hydrodynamic mixing zone, outputs of a near-field prediction model, based on in situ measurements of current speed and direction and density stratification obtained in real-time, were used to specify the AUV mission.
A built-in application adaptively specified the AUV monitoring field transects according to the environment conditions, in real-time, taking into account the outputs of the model and the vehicle navigation requirements.
A data processing system was created, applying the Least Squares Collocation Method (LSCM) technique, in order to map effectively the dispersion of the effluent using the AUV data. LSCM results for salinity enable the effluent plume to be identified unambiguously and its dispersion downstream to be observed. The effluent plume appeared as a region of lower salinity compared to surrounding ocean waters at the same depth, rising to the water surface due to the relatively weak ambient stratification and relatively weak low currents. Dilution was estimated using temperature-salinity, by means of a (TS -) diagram. The analysis demonstrated that effluent dilutions were at least 30:1 in this study. Dilutions estimated with the TS - method represent lower bounds of dilution, specially for surfacing plumes.
If artificial tracers had been used, better estimates of dilution would certainly have been obtained. However, since the present monitoring methodology is considerably less expensive and more practical for routine monitoring (not forgetting the negative impacts of releasing fluorescent dyes or other contaminant components in the effluent) further developments with the present system will certainly be justifiable.
Spectral fluorescence methods provide some promise for observing potentially-unique characteristics of effluent plumes (Petrenko et al., 1997). In the near future it would be interesting to Isurus AUV to test the potential of fluorescence measurements as a non-invasive, real-time technique to detect sewage fields in the coastal environment. AUVs also appear to be quite promising for studying the patchiness problem, in spite of several limitations that must be overcome in the future, such as predicting variability over broad time and space scales.
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