Integrated Assessment Of Groundwater Recharge In The North Kelantan River Basin Using Environmental Water Stable Isotopes, Tritium And Chloride Data

Estimation and understanding of groundwater recharge mechanism and capacity of aquifer are essential issues in water resources investigation. An integrated study of environmental chloride content in the unsaturated zone using chloride mass balance method (CMB) and isotopic analyses of deuterium, oxygen-18, and tritium values range in the alluvial channel aquifer profiles (quaternary sediments) of the North Kelantan River basin has been carried out in order to estimate and understand groundwater recharge processes. However, the rate of aquifer recharge is one of the most difficult factors to measure in the evaluation of ground water resources. Estimation of recharge, by whatever method, is normally subject to large uncertainties and errors. In this paper, changes in stable isotopic signatures in different seasons and tritium analysis of the sampled groundwater observed at different depth in the aquifer system were evaluated. Stable isotope data are slightly below the local meteoric water line (LMWL) indicating that there is some isotopic enrichment due to direct evaporation through the soil surface which is exposed prior or during the recharging process. The overall data on water isotopic signatures from boreholes and production wells (shallow and relatively deep aquifer system) are spread over a fairly small range but somewhat distinct compared to river water isotopic compositions. Such a narrow variation in isotopic signatures of the sampled groundwaters may suggest that all groundwater samples originated from the same area of direct recharge predominantly from rainfall and nearby rivers. Environmental tritium data measured in groundwater at different depths and locations together with a medium-term of limited monthly rainfall collections were used to investigate the groundwater age distributions (residence times). The existence of groundwater in the aquifer system (sampled wells) is predominantly designated as modern (young) water that has undergone recharged occurring after 1953. Groundwater age data together with other additional information related to the wells bore could then be applied to translate into semi-qualitative estimation of long term average groundwater recharge rate within the aquifer system (mm/y). Environmental isotopic data suggest recharge rate in a range 11 mm/y to 1270 mm/y with an average of 261.5 mm/y that corresponds to 10.5 % of the total annual rainfall. Recharge estimation obtained by isotopic approach was found smaller than the amount of recharge rates calculated based on CMB methodology in the unsaturated zone ranged between 155 mm/y to 966 mm/year. These data correspond to the average of 484.3 mm/y or 19.4 % of the total effective annual rainfall. Spatial variation of the predicted groundwater recharge map from tritium dating method is established in this preliminary study. Accurate estimation of groundwater recharge and further assessment of its source are useful and recommended for proper sustainable management and utilization of groundwater resources in this basin. (author)