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This paper discussed the use of stable isotope compositions of precipitation (δ2H and δ18O) to understand its characteristics and the parameters controlling meteorological conditions in Malaysia. The meteoric water line for Malaysia, or MMWL established in this study are δ2H = (7.43 ± 0.05)δ18O + (7.33 ± 0.11) (n = 595, r2 = 0.97) for individual values, and δ2H = (7.07 ± 0.20)δ18O + (4.90 ± 0.75) (n = 51, r2 = 0.96) for annual precipitation weighted means. No major variations in local meteoric water lines established between the stations, except for one (i.e., Cameron Highlands), which was found to be significantly different due to its high deuterium excess, or d value, attributed to moisture recycling (re–evapotranspiration) from the surrounding dense rainforest area. The precipitation moisture came from the two monsoon systems; the Northeast monsoon (NEM) and Southwest monsoon (SWM). They differ only slightly in their isotope signatures, where the NEM (winter monsoon) is 0.38‰ more negative than the SWM (summer monsoon), and the rainfall amount and temperature have a relatively weak influence on the stable isotopes in precipitation. Rainfall δ18O is poorly correlated with the monthly rainfall amount and temperature, with very low r2 values (–0.4‰/100 mm; 0.09 and 0.7‰/°C; 0.06, respectively). The seasonal variability of isotopic compositions of precipitation in this region is controlled by the insubstantial but conspicuous inverse correlation amount effect. The seasonal patterns of δ2H and δ18O are evident, with higher values observed during the NEM and lower values observed during SWM. However, the temporal variations of rainfall δ2H and δ18O during NEM and SWM are almost indistinguishable due to local climatic conditions, prevailing atmospheric circulations, and sources of precipitation. The apparent sign of El Niño and La Niña events (2015 and 2017, respectively) of the El Niño–Southern Oscillation (ENSO) cycle were successfully identified from this study, indicated by the heavier δ18O from the peaks in the annual precipitation amount versus δ18O annual precipitation weighted mean plot, while La Niña corresponds to the lighter values of the rainfall isotopes. [ABSTRACT FROM AUTHOR]

The groundwater from shallow alluvial aquifers of northern Kelantan, Malaysia, was analysed for stable isotopes (δ18O and δ2H) during northeast and southwest season to assess the dynamics of the recharge processes. The groundwater shows quite a large variation in isotopic signatures (18O ranges from -7.5 to 0.62 ‰ for northeast (NE) and from −7.4 to −4.98 ‰ during southwest (SW)). These values reflect the contribution of different recharge sources in the aquifer. During NE season, the average isotopic signature of groundwater (δ18O: −5.30‰) is similar to the rainfall (δ18O: −4.13‰) indicating that rainfall is the main source of recharge compared to the other sources. The SW season shows an isotopically distinct signature due to contribution from surface water bodies (river sources) and enriched irrigation return-flow. Interestingly, the groundwater average isotope signature in SW season (δ18O: −6.40‰) is very close to the surface water (δ18O: −6.47‰), indicating significant contributions from surface water bodies of the area. Overall, the isotope data suggests that shallow groundwater is very dynamic during SW season whereas in NE the impact of surface manifestations is less. The study highlights the significance of further studies to accurately quantify the recharge volumes from different sources that will help to model and manage the aquifers both qualitatively and quantitatively. Moreover, the quantification of irrigation return-flow is very essential component keeping in view the contaminant transport from the agricultural areas. [ABSTRACT FROM AUTHOR]