Abstract
Lakes, reservoirs, rivers, and aquifers are important freshwater sources for basic human needs such as drinking, sanitation, and agriculture. The anthropogenic influences on the natural environment, especially on freshwater resources, have increased dramatically during the last few decades. Eutrophication and pollution are major threats to many of these water bodies. There are thousands of man-made reservoirs, which are centuries old in Sri Lanka, and only a handful of them have been extensively studied and monitored. This study investigates the spatial and seasonal variations of water quality in Padaviya Reservoir by studying the vertical distribution of physical parameters and inorganic nitrogen species: ammonia, nitrite and nitrate, reactive phosphate, and dissolved oxygen. Padaviya Reservoir, which is an ancient man-made irrigation reservoir, has never been studied in detail to assess its water quality. Sharp chemical gradients for ammonia, nitrite, nitrate, reactive phosphate, and dissolved oxygen were observed between surface and bottom waters of the reservoir, suggesting that it does not overturn completely. The temperature difference is between the surface and bottom waters of about 2°C, which is not large enough to cause thermal stratification. The most probable reason for the stratification is extensive photosynthesis at surface waters with subsequent decomposition of the organic material at the bottom.
Highlights
Sediments in reservoirs play an important role by acting as both a nutrient source and sink
Sri Lanka is a tropical island in the Indian Ocean, southeast of the Bay of Bengal, with mean temperatures of 17°C in the central highlands to 27°C in lowlands. e rainfall distribution throughout the island is dominated by two monsoons: southwest and northeast, prevailing from April to September and October to March, respectively [31, 32]. e dry climate zone, which receives less than 1500 mm of rainfall per year, is benefitted by a sophisticated system of reservoirs connected with canals resembling a cascade system, which was built in the ancient times to supply freshwater for basic human needs and most importantly for agriculture [33,34,35]
It is widely believed to have been constructed during King Mahasena’s reign from 274 to 301 A.D. It has been extensively renovated and expanded since and the most recent restoration was completed in 1954 giving its current shape and a maximum water holding capacity of approximately 0.1 km3. It is built by the impounding seasonal streams of Mora Oya and Mukunu Oya, to create a water spread area of 56.6 km2 with a catchment area of approximately 270 km2 [49, 50]. e surface water level of the reservoir dramatically decreases in the dry season, and the littoral zone cover terrestrial plants and grass that turn into feeding grounds to free range cattle and herds of elephants. e average depth of the reservoir is around 8.8 m, and the depth increases towards the embankment
Summary
Sediments in reservoirs play an important role by acting as both a nutrient source and sink. Is study investigates the water quality in Padaviya Reservoir by studying the vertical distribution of nitrogen species (nitrate, nitrite, and ammonia), reactive phosphate, dissolved oxygen, conductivity, and pH. E average reactive phosphate concentration at surface and bottom waters was 0.18 ± 0.06 mgL−1 and 1.99 ± 0.66 mgL−1, respectively.
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