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

The characteristics of the groundwater cycle were researched using stable i sotope technology in western Sri Lanka where climatic conditions change gre atly within a relatively short distance. The effects of local climate, surf ace water and topography on the groundwater cycle in the study area with si milar geological conditions were investigated. Sri Lanka can be divided spa tially into a dry zone, an intermediate zone and a wet zone, and also tempo rally into the rainy season and the dry season. The zonal characteristics o f the groundwater cycle were also elucidated using stable isotopic technolo gy. As an input delta diagram of precipitation in the study area, there are obv ious seasonal changes in the isotopic composition and a magnitude effect, b oth in the wet, zone and dry zone. In the wet zone, the slope of the regres sion 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 regressio n line between delta D and delta O-18 and deuterium excess is much less tha n 8 and 10, respectively. In the wet zone, there is an obvious seasonal change in the isotopic compos ition of groundwater. The groundwater was recharged by precipitation during the whole year. The isotopically lighter groundwater was found at the vall ey bottom in the rainy season there. Under the very heavy precipitation con ditions, the slope of the regression line between delta D and delta O-18 an d 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 dr y zone, the groundwater was recharged by precipitation only in the rainy se ason. The isotopically lighter groundwater was found on the ridge of the va lley 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 an d 10, respectively. Copyright (C) 1999 John Wiley & Sons, Ltd.