Emilia Arasaki and Jayme Pinto Ortiz
Escola Politécnica da Universidade de São Paulo, Laboratório de Mecânica dos Fluidos
Av. Prof. Mello Moraes, 2231 - CEP 05508-900 São Paulo (SP) BRAZIL
phone (55)(11)3818 5335; Fax (55)(11)3813 1886
Escola de Engenharia Mauá—São Caetano do Sul—SP-BRAZIL
E-mail: earasaki@usp.br ; jportiz@usp.br
Abstract: The coastal areas are polluted by a varied dejects and part of them are disposed through submarine outfalls, admitting that ocean have a large capacity of dilution and dispersion of the effluent. A better procedure would be to identify environmental impacts and to decide strategies for minimizing them through applying technology in this field.
The subject is unknown and this work (in preliminary phase) focuses the methodology for the detection of sewer plume in marine environment, without the previous knowledge of its accurate dimension and the area of contamination.
Keywords: submarine outfall, sampling design, coastal waters, marine pollution
Studies evaluating the dispersion of effluent through submarine outfalls in Brazilian coastal areas are rare. The first monitoring data, described by Berzin (1992) was made around the Santos submarine outfall (São Paulo State), during 10 years in operation. The author verified the efficiency of the oceanic sewage disposal, analyzing physical, chemical and biological parameters of the last four field studies (1976, 1979, 1982 and 1986). Other authors (De Luca et al., 1992) obtained around the Ipanema submarine outfall (Rio de Janeiro State) nutrient concentrations, dissolved oxygen, particulate matter and other physical-chemical parameters which were measured monthly for 15 months. The results indicated steady state conditions.
Both surveys shows a long period of observations in situ to evaluate levels of pollution, related to human activities.
Another monitoring was described in the region of Ipanema submarine outfall by Souza & Roldão (1997), describing the study of effluent dilution. It was made by continuous injection of a known concentration of dye tracer, but this method requires a constant volumetric flux of the domestic sewage.
The measurement of anthropogenic disturbance are a compromise between scientific ideal and financial and logistic support. Oceanographic campaigns are essential for monitoring but, considering the costs estimated as 10% of the total implantation of one sea outfall, approximately US$ 0,5 to US$1 million/year (Berzin, op. cit.), it’s necessary to evaluate the benefit-cost.
It is important to emphasize that beside the difficulty of getting field data is the necessity of trustworthy data. Indeed, not only the point density, but even the total amount of information, are inevitable to register the real world (Burrough, 1992). Consequently, a minimum number of oceanographic stations becomes essential, not only to delimit the area of influence of the emissary, but even to define space-time intervals that allows the attainment of a representative base of data.
In this paper is presented a method using strategies for localize a plume, considering the available time, financial and logistic support to realize the sampling.
Praia Grande, in south coast of São Paulo State (Figure 1), presents a great flow of tourists (estimated in 253,755 inhabitants), at times of vacation and Carnival (Marcellino & Ortiz, 2000). To take care of this demand, SABESP (São Paulo State Basic Sanitation Company), it is expanding sewer drain network along the coast, trying to diminish the huge environmental pollution.
The network of sanitary sewers is currently made by two independent subsystems of remove, transport and preliminary treatment in Pre-Conditioning Station, preparing the sewer for a disposal via submarine outfall. The chosen submarine emissary for the study was the subsystem 1, called “Forte de Itaipu”, situated in a sheltered area and near the fort with same name.
Although the constant monitoring of CETESB (Environmental Sanitation Technology Company), attached to the State Environmental Secretariat, high values of faecal coliforms are detected during the summer (December to March).
It is a high density polyethylene tube, 1 m in diameter, externally attached with concrete blocks, 3300 m in length and 174 diffusers (each diffuser with 6,5 cm in diameter) located in the last 435 m. A preliminary calculation carried through by SABESP indicated that the diameter of the sewer plume would be in order of 144 to 192 m.
The strategy requires the exact localization of submarine outfall (approximate position Lat. 24° 3'S; Long. 046° 26,9'W) that was made by sounding with sidescan sonar, an equipment on board of R/V “ORION” (Brazil Navy). Gotten the imagery of the emissary, the ship was located with DGPS (Differential Global Positioning System) on the area of the diffusers. To map the plume it was used a methodology called in military jargon as “research of danger in position doubtful" (Barahona Fernandes, 1971). It consists of a kind of searching that tries to cover the area in squares. This methodology define the sampling plan in two networks of distinct oceanographic stations. The first one with 6 radial routes and 6 stations in each one (50 m apart from each other) in an area of 0,09 km2, under influence of the diffuser (approximate position Lat. 24° 3'S; Long. 046° 26,9'W). In this first one, each of the 36 sampling points (located with DGPS) collected abiotics data with the aid of a CTD (conductivity—temperature—depth), on board “ACHERNAR” , launch of the São Paulo State Port Administration. Measures of currents (during 24 hours) had been taken with the R/V “ORION”, besides the diffuser. A second network (with total measurement area of 1 km2), around the same submarine outfall, was used as larger control area.
Sampling for nutrient determination (nitrate, nitrite, phosphate and silicate) was made in some points with the help of a Van Dorn sampler. The analytical methods applied for nutrient determination are described in Grasshoff et al. (1983) and Tréguer & Le Corre (1976). The samples were frozen to be analyzed at the chemical laboratory (Laboratório de Química do Instituto Oceanográfico da Universidade de São Paulo), using the AutoAnalyzer II – Technicon. Total faecal coliforms are sent to microbiology laboratory ( Laboratório de Microbiologia Ambiental do Instituto de Ciências Biomédicas da Universidade de São Paulo), where the samples were analyzed following the recommendations of APHA (1995).
After this sampling, another imagery was made utilizing a 95 kHz multibeam echo sounder on board of R/V “TAURUS”(Brazil Navy). This equipment is capable of mapping the seabed at depths between 3 and 1000 m below the transducer. Data acquisition of digital bathymetry is integrated with position data and computerized map processing techniques. Seabed images similar to those produced by shallow towed sidescan sonars can be produced in real time, but with correct geometric registration (Kongsberg-Simrad A/S, operator manual).
To evaluate the influence of the sewer on marine biota, otter trawls had been used for complementing the sampling of biological data. The use of this fishing-net aimed to capture demersal fish and benthic macroinvertebrates, to observe possible histopathological alterations of the gills, liver and kidneys caused by the presence of faecal coliforms and other chemical contaminants.
Liver dissections were made on board the launch and fixed immediately in 10% neutral buffered formalin. In laboratory, the material was dehydrated in an alcohol series, embedded in paraffin and other processes, whose method is described in Schreck & Moyle (1990). For analysis of anomalies the method used is edited in Hibiya (1982).
The data had been collected in two campaigns (April/99) and the contribution of Brazil Navy for the logistic support permitted a reduction in the costs of oceanographic campaign. Imagery obtained with sonar on board R/V “TAURUS” shows that the submarine outfall presents a curve next to the shoreline (Fig. 2). It’s necessary a scuba-diving inspection to confirm this. Preliminary results indicated that the water quality was good, ideal for recreation. All the values of total faecal coliform (highest value = 230/100ml; lowest value < 20/ 100 ml) are in accordance with Brazilian environmental law. This can be explained by the small quantity of sewers throw during the autumn. Physical data were measured along the water column and processed by the Brazil Navy. Values of salinity (surface to mid-depth, approximately 33 PSU) seem to indicate the detection of the effluent. The salinity was best for detecting location of the wastewater field that the others directly measured CTD parameters (Dalkey & Shisko, 1996), like the pressure (db), temperature (℃), conductivity (S/m), density (kg/m3) and sound velocity (m/s). Directions and currents velocities were variable during sampling.
In general, values of nutrients were similar or lower than expected for unpolluted areas, despite Praia Grande is near Santos Bay. In this region, concentrations obtained by Braga et al. (2000) presented high values (example, nitrate > 90 mml/l) than Praia Grande (highest value for nitrate =2,71 mml/l). The dissolved inorganic nitrogenous compounds are directly related to the primary production and to the decomposition process (Braga, op. cit.).
In relation to the biological data, juveniles of banded croaker (Paralonchurus brasiliensis, Sciaenidae), adults and juveniles of corocoro grunt (Orthopristis ruber) and a type of weakfish (Sciaenidae) were more numerous. The histology analysis did not finish due to the great volume of material to process. But only the collect of tubes of polychaeta Myriochele cf. heeri (Oweniidae) already indicate a highly polluted area. The presence of this specie shows that the sediment contain high concentrations of organic material.
The next stage will be the analysis of the spatial distribution of the physical data (gotten to each 0.5 second with CTD), trying to show the bacterial decay along the water column.
(1) Values of nutrients and faecal coliform presents sign that the preliminary treatment of sewage permit to take care of the local population;
(2) The sediment seems to be contaminated with organic compounds deposited by discharged dejects;
(3) Although references about monitoring the wastewater plume presents long temporal and spatial series, the developed methodology shows that it’s possible to analyze impacts with short series, that is, a minimum number of oceanographic stations that contain a representative base of data. The monitoring methodology can also complemented with results of pathogenic anomalies in fishes or the use of indicator organisms in organic pollution incidents like around submarine outfalls.
Diretoria de Hidrografia e Navegação (Brazil Navy) and CNPq (National Council for Scientific and Technological Development), who support this work.
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Fig.1 Brazil’s map shows Praia Grande, São Paulo (in square). The approximate area of submarine outfall is located in rectangle.
Fig.2 Imagery reveals an accentuate curve of submarine outfall.