The results of tests with a sodium chloride tracer in a shallow maturation pond in Brazil are presented. The pond – a full-scale effluent treatment unit for around 250 p.e – was 0.80 m deep with a length/width ratio of 4.3:1, and had accumulated sludge for 12 years to about 40% of the total volume. The pond is subject to daily cycles of thermal stratification/destratification, as shown by vertical profiles of temperature, dissolved oxygen, pH and redox potential, and by Computational Fluid Dynamics simulations. Four tracer tests were undertaken. There was a tendency to complete mixing with high dispersion, and daily vertical cycles were evident in all tracer response curves, a distinctive feature of these studies. However, the reliability of the hydraulic retention time calculation, and the short-circuit and mixing indicators, is doubtful, primarily because of the liquor's high density. The study's results enabled practical consideration of the use of saline tracers in pond systems. Sodium chloride acquisition and semi-continuous sensor measurements of effluent concentrations (electrical conductivity) are both easy, but the large amount of salt required (around 300 kg for a 125 m3 pond) and solution density are limiting factors, and can yield unrepresentative results, making this tracer potentially impractical, especially for larger ponds. However, the diurnal cycles of effluent tracer concentration were important in this study and assisted in verification of hydraulic behavior in the liquid column, associated with the stratification and mixing patterns.

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