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Seasonal characteristics of surface water quality in the wastewater catchment system of an urbanizing basin

Kimura, Masaomi, Noda, Keigo, Makino, Tatsuya, Yamagata, Hijiri, Douangsavanh, Somphasith, Keokhamphui, Keoduangchai, Hamada, Hiromasa, Kiguchi, Masashi, Iida, Toshiaki, Oki, Kazuo
Paddy and water environment 2018 v.16 no.3 pp. 519-531
basins, drainage channels, dry season, marshes, nitrogen, nutrient balance, nutrient content, paddies, phosphorus, rivers, runoff, suburban areas, surface water, surveys, total organic carbon, urban areas, urbanization, wastewater, wastewater treatment, water quality, watersheds, wet season, Laos
Vientiane, Lao PDR, has been subject to extensive and ongoing urbanization plans, including development of natural marshes and residentialization of paddy fields into suburban areas, despite natural marshes playing a vital role in treating wastewater from urban areas. Therefore, it is important to understand the current situation regarding the nutrient balance in these natural wastewater treatment systems to predict future conditions and design appropriate measures against water quality deterioration. However, limited data are available in Vientiane on the hydrological characteristics of water and nutrient runoff that flow into marshes through drainage canals. In this study, we conducted a periodic survey of drainage canals and Mak Hiao River in the wastewater catchment system surrounding Vientiane during the rainy and dry seasons. We monitored the discharge of surface water at 21 observation sites and analyzed water quality of nitrogen, phosphorus, and total organic carbon. These observations revealed that the concentrations of dissolved nitrogen and phosphorus were significantly higher at sites in urban areas, followed by sites in the main river basin and those in agricultural areas. Dissolved nitrogen and phosphorus concentrations varied with runoff discharge, especially in urban and river basin sites, with lower concentrations in the rainy season and higher concentrations in the dry season. On the other hand, we found no significant differences between the rainy and dry seasons in nutrient concentrations in the agricultural basin. Finally, we proposed measures to counteract the deterioration of water quality during dry seasons and simulated the impact of these measures.