Preliminary Field Data of Dissolved Oxygen Profiles in Padaviya Reservoir, Sri Lanka

Abstract

AttachedDissolved oxygen (DO) concentration is one o f the most important physicochemical parameters in aquatic systems that could significantly influence the overall quality o f water. Dissolved oxygen profiles describe the variation o f DO concentration along the gradient o f the water column. Understanding the variation o f DO in aquatic systems is important as it relates to other chemical characteristics o f the system. Although DO profiles o f lentic systems in other countries have been extensively studied, similar studies o f reservoirs o f Sri Lanka are limited. Therefore, the present study focused on the DO distribution in the Padaviya reservoir in the North Central Province o f Sri Lanka. A preliminary study was carried out on 23rd January 2016, at 25 locations covering the entire reservoir using a field portable DO meter equipped with a probe having a 20 m cable. The probe was deployed to water body and data were collected at two feet depth intervals. According to field data, the average DO value at the surface o f the reservoir was 8.1 (± 1.0) mg L'1 and varies in the range o f 9.2 to 4.6 mg L'1. The maximum surface DO concentration was observed at the middle section o f the reservoir which has an average depth about 8.84 m and the minimum surface DO concentration was observed at the two water inlets o f the reservoir which were shallow. The results suggest that the system contains a clinograde type DO profile with steep decline o f DO after 4.57 m in Padaviya reservoir. The epilimnion, the upper most water column o f the reservoir, had a DO value o f ~ 8.1 mg L'1 and the hypolimnion, the bottom stratum o f the water column, exhibits anoxic condition. The most important observation was the abrupt DO depletion between approximately 4.57 to 5.18 m. At this depth, DO values decreased from ~ 6.5 to ~ 0.4 mg L'1 within 0.61 to 0.91 m. This phenomenon could not be satisfactorily explained with the current data. It could be assumed that DO is consumed by the aerobic microbial mediated oxidation o f organic matter and by the oxidation o f chemical species such as Fe+2, Mn2+ and NO2 Further studies are underway with more data to explain these oxygen profiles in the Padaviya reservoir.