[A-HW22] 流域生態系における物質輸送と循環：源流から沿岸まで

[AHW22-07] Influence of recharge and irrigation return flow on water quality in an intensively irrigated agricultural watershed, Kala Oya, Sri Lanka

★Invited Papers

*Meththika Vithanage¹、G.M.H.M. Muhandiram²、W.LG.D. Perea²、E.A.N.V. Edirisinghe²、B.C.L. Athapattu³、Morihiro Maeda⁴ (1.Ecosphere Resilience Research Center, Faculty of Applied Sciences, University of Sri Jayewardenepura、2.Department of Civil Engineering, Faculty of Engineering Technology, The Open University of Sri Lanka、3.Isotope Hydrology Section, National Atomic Energy Authority, Sri Lanka、4.Graduate School of Environmental and Life Science, Okayama University)

キーワード：Water quality index, Groundwater recharge, Stable isotopes

The dry zone of Sri Lanka, basically driven by the ancient hydraulic structures for rainwater harvesting and management of irrigation return flow, is heading towards a water scarcity due to the increase of extreme events and mismanaging the tank cascade system. Kala Oya basin is an intensively irrigated area for rice cultivation in the north central Sri Lanka, which flows 150 km from north mid mountain range to northwestern coast of Sri Lanka. The study objectives were to assess the water quality in the Kala Oya basin, in order to understand the influence of precipitation and irrigation return flow on water quality as an indication of water management. Water samples were collected from 48 locations (20 surface water, 14 shallow groundwater and 14 deep groundwater) twice in 2019. Water Quality Index (WQI) was calculated using Weighted Arithmetic method by considering various physical and chemical parameters pH, EC, COD, potentially toxic elements, ¹⁸O and ²H and major ions. The EC of samples varied from 132.2 to 1298 µS/cm in wet season, 98.1 to 3784 µS/cm in dry season. The results show that ranges of hydrogen and oxygen isotopic composition of deep groundwater increased in the dry season than that of the wet season. Depleted isotopes in the upper part of the basin indicated direct recharge into deep groundwater by higher elevation precipitation experiencing. Intermediate isotope values in the mid basin indicated the possible recharge through mixing. The enriched deep water in the lower basin indicated the limited rain recharge but recharge with irrigation return flow. For the sustainable water management of the Kala Oya basin, it is essential to rehabilitate the ancient tank cascade system in an appropriate manner, which will increase the water quality, improve recharge and reduce the mixing of irrigation return flow.