Radioactive and dye tracer studies for the NWNT sewerage outfall, Hong Kong, and comparison to near-field modelling

A monitoring programme for the North West New Territories (NWNT) sewage outfall, located in the Urmston Road channel in Hong Kong, was completed in 1996. This included three surveys measuring effluent behaviour and oceanographic conditions near the outfall. Radioisotopes gold-198 (γ energy 0.42 MeV, half-life 2.7 days) and tritium (as tritiated water, HTO, β energy 0.018 MeV, half-life 12.3 years) were used to trace the effluent discharging from the outfall during both a wet season and dry season survey. The effluent was also simultaneously tagged with Rhodamine dye which was detected with fluorometers. The gold-198 was generally traced in real time using submersible detectors, while the tritium was measured in the laboratory from sea water samples using liquid scintillation techniques. The Radioisotopes and dye measurement techniques were progressively refined over the surveys and a reliable equipment arrangement and sampling procedure was established