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The tank cascade system (TCS) in Sri Lanka is one of the most advanced waterconveyance mechanisms among the medieval hydraulic civilizations in the world. In this study, temporal and spatial variations of the hydrogeochemistry of a small cascade system were investigated. A cascade system at Malagane in the Deduru Oya basin, located in the north-west of Sri Lanka, was selected. Measurements of pH, electrical conductivity (EC), nitrates, nitrites, phosphates, sulphates, alkalinity, chloride, Na, K, Ca, Mg, Fe and Mn were carried out fortnightly throughout the year, to understand the nutrient dynamics in TCSs. Most parameters show a decreasing trend when water passes through 'Thaulla', which is located in the upper peripheral region of the tank. However, phosphate behaves differently, probably due to human activities that dominated along the tank bund. The results suggested that Thaulla, which abounds with numerous hydrophytes, acts as an active 'constructed wetland' and efficiently removes pollutants in order to sustain the system. The water quality monitoring described in this paper provides evidence for the wetland function of Thaulla in the ancient irrigation hydraulic system in Sri Lanka.

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