A study of the groundwater cycle in Sri Lanka using stable isotopes

The characteristics of the groundwater cycle were researched using stable isotope technology in western Sri Lanka where climatic conditions change greatly within a relatively short distance. The effects of local climate, surface water and topography on the groundwater cycle in the study area with similar geological conditions were investigated. Sri Lanka can be divided spatially into a dry zone, an intermediate zone and a wet zone, and also temporally into the rainy season and the dry season. The zonal characteristics of the groundwater cycle were also elucidated using stable isotopic technology. As an input delta diagram of precipitation in the study area, there are obvious seasonal changes in the isotopic composition and a magnitude effect, both in the wet, zone and dry zone. In the wet zone, the slope of the regression line between delta D and delta O-18 and deuterium excess is close to 8 and 10, respectively. However, in the dry zone, the slope of the regression line between delta D and delta O-18 and deuterium excess is much less than 8 and 10, respectively. In the wet zone, there is an obvious seasonal change in the isotopic composition of groundwater. The groundwater was recharged by precipitation during the whole year. The isotopically lighter groundwater was found at the valley bottom in the rainy season there. Under the very heavy precipitation conditions, the slope of the regression line between delta D and delta O-18 and deuterium excess for groundwater was close to 8 and 10, respectively. In other cases, the slopes of the regression lilies are less than 8. In the dry zone, the groundwater was recharged by precipitation only in the rainy season. The isotopically lighter groundwater was found on the ridge of the valley in the rainy season. The slope of the regression line between delta D and delta O-18 and deuterium excess for groundwater was much less than 8 and 10, respectively. Copyright (C) 1999 John Wiley & Sons, Ltd.