Dye tracer technique is well established and of wide application for assessment of outfalls and for delineation of near field and far field extensions. Common goals of a tracer study include the measurement of the dilution factor, estimation of the dispersion coefficients, measurement of the effluent discharge and calibration of a contaminant transport model. This paper presents a brief review of the methods involving the use of dye tracer for outfall assessment and illustrates the methods of slug release and continuous injection based on two real cases of campaigns carried out on Brazilian coastal waters. Slug injection on the surface of the water body was used for preliminary dispersion studies aiming at outfall positioning. During the operational phase of an outfall, the continuous injection of dye tracer was used to determine effluent dilution in different seasons. In coastal waters of Rio de Janeiro city, sea current pattern, tidal modulation and thermal stratification explained the main features of the dilution field.

The alternative use of effluent turbidity to determine the dilution field of a domestic marine outfall located in the city of Rio de Janeiro was evaluated through a field work comprising fluorescent dye tracer injection and tracking with simultaneous monitoring of sea water turbidity. A preliminary laboratory assessment was carried out with a sample of the outfall effluent whose turbidity was measured by the nephelometric method before and during a successive dilution process. During the field campaign, the dye tracer was monitored with field fluorometers and the turbidity was observed with an optical backscattering sensor interfaced to an OEM data acquisition system. About 4 thousand samples were gathered, covering an area of 3 km × 3 km near the outfall diffusers. At the far field – where a drift into the coastline direction was observed – the effluent plume was adequately labeled by the dye tracer. The turbidity plume was biased due to the high and variable background of sea water. After processing the turbidity dataset with a baseline detrending method, the respective plume presented high correlation with the dye tracer plume drawn on the near dilution field. However, dye tracer remains more robust than effluent turbidity.

RESUMO Um estudo de campo foi realizado em um trecho de aproximadamente 70 km do rio Piabanha, localizado entre Petrópolis e Areal, na região serrana do Estado do Rio de Janeiro, para estimar as características de transporte e dispersão de contaminantes usando técnicas de traçadores fluorescentes. A análise dos resultados de dois trabalhos de campo, realizados no outono de 2012, permitiu determinar os valores de tempo de trânsito, vazão líquida, velocidade média e coeficiente de dispersão longitudinal. Utilizou-se os dados de vazão e velocidade média obtidos nos ensaios com traçadores para calibrar o modelo hidrodinâmico unidimensional Hydrologic Engineering Center’s River Analysis System (HEC-RAS). Considerando que parâmetros hidráulicos obtidos por métodos clássicos em rios com geometria muito irregular, como a do rio Piabanha, não são adequados para representar as características médias de um trecho, os ensaios com traçadores representaram uma boa alternativa para obtenção de parâmetros necessários a modelos hidrodinâmicos. Os resultados obtidos pelo modelo HEC-RAS apresentaram ótima concordância com os dados obtidos por meio dos ensaios com traçadores fluorescentes.