Isotopic variations in natural water and groundwater recharge conditions in the Kalaoya basin, Sri Lanka

Stable isotope (¹⁸O & ²H ) composition of surface water and groundwater was measured and analyzed to interpret the recharge mechanism of groundwater in the upper Kalaoya basin of North Central Sri Lanka. Samples were collected during the latter part of the dry season (September, 2012). Isotopic data of most of surface water and shallow groundwater samples fall along the evaporation line in the δ²H vs. δ¹⁸O plot. The δ ¹⁸O and δ²H in surface water varied from -4.42 to -0.07 ‰ and -23.94 to -0.50 ‰ respectively. Shallow groundwater is characterized by lighter isotopes (δ¹⁸O, -7.41 to - 2.27 ‰ and δ²H, -48.15 to -12.72 ‰) compared to surface water. Deep groundwater is much more depleted than other studied water (δ¹⁸O, -7.32 to -4.06 ‰ and δ²H, - 47.28 to -25.14 ‰). Heavier isotopes of surface water and shallow groundwater indicated the high evaporation conditions of the area. Also the data indicated three main recharging mechanisms of groundwater. The deep groundwater with most depleted isotope values (δ¹⁸O <-7.11 ‰) in the uppermost part of the basin indicated that the rain experiencing in the higher elevations of Kalaoya basin would recharge the upper most area directly. Intermediate isotope values (-6.79 ‰ < δ¹⁸O <-6.43 ‰) are common in the middle part of the basin suggesting higher contribution of shallow groundwater and surface water for the deep groundwater recharge. Relatively enriched isotope values (-6.07 ‰ < δ¹⁸O <-4.06 ‰) in the lower part of the basin show the higher contribution of surface water for the groundwater recharge. Enriched isotopic compositions of the stream water from Mahaweli diversion schemes indicated that they are distinctly different from water in the Kalaoya basin. Present study revealed that longterm monitoring of isotope indices of water could facilitate precise assessment of the groundwater recharge mechanism in the Kalaoya basin. (author)