Isotope Hydrology is the use of isotopic tools and nuclear techniques to study water cycles. It was introduced just after the Second World War (Aggarwal et al. 2005). Stable isotope compositions in natural waters are typically measured using double–focused magnetic sector field mass spectrometers, which Alfred Nier initially designed in 1947 (Gourcy et al. 2005). His design remains the basis of stable isotope mass spectrometry, although modern instruments are equipped with multiple collectors for simultaneously measuring several isotope ratios.
Environmental isotopes are naturally occurring isotopes of elements found in abundance in the environment. However, only a few are of practical importance (H, C, N, O, and S) because they are the principal elements of hydrological, geological, and biological systems (Clark and Fritz 1997). The environmental isotopes can be divided into stable isotopes and radioisotopes; however, this article only discusses the latter. In a hydrological cycle, surface water and groundwater are derived from meteoric waters (atmospheric moisture and precipitation) evaporated from the ocean (Mook 2001; Mook 2006). Environmentally stable isotopes are ideal tracers of water and solutes in the environment, as they are incorporated in the water molecules and relatively conservative in their reactions with geological material. They can also be used to monitor processes (e.g., mixing and evaporation). Their behaviors and variations reflect the origin of and the hydrological and geochemical processes experienced by natural water bodies (Gonfiantini 1998), which is especially true of hydrogen (protium, deuterium, and tritium) and oxygen (oxygen–16, oxygen–17, and oxygen–18) isotopes in water, where the meteoric waters retain their distinctive isotopic fingerprint until they mix with waters with different compositions (Kendall and Caldwell 1998).
Isotope ratios of deuterium or 2H/1H (δ2H) and oxygen–18,18O/16O (δ18O) in precipitation are closely related, lying on a single line known as the Meteoric Water Line (MWL). Initially, the relationship between δ2H and δ18O in precipitation and non–marine surface waters, established from ~ 400 samples distributed from all over the world, is defined as:
δ2H= 8δ18O + 10‰ SMOW (Craig 1961)
and known as the Global Meteoric Water Line (GMWL). Later, based on the long–term annual weighted means of δ2H and δ18O values from 219 International Atomic Energy Agency and World Meteorological Organization (IAEA/WMO) stations network, the line was redefined as:
δ2H= 8.20 (±0.07) δ18O + 11.27 (±0.65) ‰ VSMOW (Rozanski et al. 1993)
In order to determine the mean of freshwaters isotopic composition globally, Craig's (1961) equation describes distinctively the estimation of locus of the points, which then was also supported by Rozanski’s et al. (1993) equation. However, developing a local meteoric water line for a specific study area is very important as δ2H–δ18O relationship in precipitation varies spatially and region–dependent. Besides, identification of surface water and groundwater sources and effects of evaporation on water bodies can be obtained from the best–fit Local Meteoric Water Line (LMWL) which manifests the isotopic input functions for hydrological studies (Liu et al. 2014). Furthermore, meteorological factors (temperature, atmospheric humidity, precipitation amount, initial conditions and trajectory of the water vapor, rainout history of the air mass, secondary evaporation during rainfall and evapotranspiration effect) as well as environmental factors (geographical location, atmospheric circulation, weather system, and terrain) also affected the isotopic composition of local precipitation (Craig 1961; Peng et al. 2004; Hughes 2013; Jia et al. 2019).
Delineation of the deuterium excess in global precipitation using the d value was pioneered by Dansgaard (1964). The value d is defined for a slope of 8 and calculated for any precipitation sample using:
d = δ2H – 8δ18O (Dansgaard 1964)
Primary information on the initial moisture source and secondary processes such as evaporation during precipitation and recirculation of moisture from large continental water bodies can be derived from deuterium excess (Dansgaard 1964; Araguas–Araguas et al. 1998; Araguas–Araguas et al. 2000; Gat 2000). According to Jacob and Sonntag (1991), evaporation of raindrops under the cloud base and warm and dry conditions can be associated with low deuterium excess, while low humidity during evaporation at the moisture source (for example, sea surface) and evapotranspiration can be translated to high deuterium excess values (Hughes 2013).
International Atomic Energy Agency (IAEA) and World Meteorological Organization (WMO) started The Global Network of Isotopes in Precipitation (GNIP) program in 1958. The objective of the program was to collect spatial data of isotopic composition in precipitation globally to determine its spatial and temporal variations. GNIP also has strengthened its role to monitor and observe the network of stable hydrogen and oxygen isotope data for hydrological studies although its original goal was monitoring the atmospheric thermonuclear test fallout via the determination of the radioactive hydrogen isotope (tritium) since 1970s. As a result, global isotope data for hydrological studies such as water resources investigation, planning, conservation, and development have been derived from GNIP for the last 50 years. Therefore, over the past decade, more advanced scientific disciplines have leveraged the invaluable and exceptional isotope database for multiple applications which cover (i) validating and enhancing atmospheric circulation models, (ii) investigating global, regional, spatial, and temporal climate, (iii) examining the interactions of water between atmosphere and biosphere, or as an input to hydrological studies, and (iv) delivering baseline information for the authentication of commodities (such as food and plants), ecology, e.g., for tracking migratory species (birds, fish, and butterflies) and for forensic purposes (Baisden et al. 2016; IAEA 2019).
The lack of localized detailed investigations related to the use of stable isotopes, i.e., deuterium (2H) and oxygen–18 (18O) in water resources study conducted previously in Malaysia are mainly due to insufficient knowledge in isotope technique, unavailability of Isotope Ratio Mass Spectrometer (IRMS), and the lack of a database of stable isotope compositions of precipitation to act as a reference point. In light of the abovementioned facts, this study aims to develop a δ2H and δ18O database of precipitation to elucidate various components of the water cycle and groundwater evolutions. Stable isotope tracers of water can be used for this purpose due to their unique ‘fingerprint’ of sources often preserved within the subsurface. The precipitation process has a geographically–specific isotopic fingerprint inherited by the local groundwater (Kortelainen 2011). By collecting information, establishing a database on stable isotopes of hydrogen and oxygen of meteoric water, and analyzing the groundwater and surface water together with the hydrogeological information, an evaluation can be made to define the source and origin of groundwater, surface water, and groundwater interactions and groundwater dynamics. Recharge, sources of groundwater salinity, and the mass balance in the study area can also be determined (Yeh et al. 2009).
In Malaysia, isotope hydrology techniques were introduced in the early 1980s. The Malaysian Meteoric Water Line (MMWL) was established in 1981 with 3 rainfall stations covering only the northern part of Peninsular Malaysia. There were no stations in East Malaysia or Borneo Island. The study continued in the 1990s until mid–2000s, with irregular sampling intervals resulting in intermittent data. The number of rainfall stations also varied during the entire study period, with only 5 stations remained by the end of the study, mostly located on the upper half of Peninsular Malaysia. Overall, the Malaysian Meteoric Water Line was established as:
δ2H = 8δ18O + 13.255 (Ayub 2006)
Efforts to strengthen the cooperation of the Malaysian Nuclear Agency (Nuclear Malaysia), the national nuclear research institute, formerly known as the Malaysian Institute for Nuclear Technology Research (MINT), with GNIP/IAEA was made possible by the former Director–General of Malaysian Nuclear Agency. Subsequent correspondence between the Division of Physical and Chemical Sciences, Isotope Hydrology Section (NAPC, IHS) and the Malaysian Nuclear Agency revealed the existence of a national precipitation isotope sampling network. However, these efforts were not pursued for several years starting mid–2000s and sampling activities resumed only in 2013. On the IAEA side, historical data was compiled, and precipitation isotope data from Malaysia were reviewed to identify potential overlap, and activities to be undertaken by IAEA alongside the Malaysian Nuclear Agency were recommended.
Unlike the mid–latitudes and high latitudes, this study is vital as the relation between the isotope signature of precipitation and climate is not well understood vis–à–vis the tropics (Araguas–Araguas et al. 1998). Consequently, it will result in a valuable database for hydrological studies in Malaysia and also Southeast Asia. Marryanna et al. (2017) highlighted the importance of stable isotope signals in precipitation water in their effort to conserve the tropical rainforests in Peninsular Malaysia. Stable isotope signals in precipitation water serve as indices of site–dependent rainfall and climate characteristics. These records can also serve as basic information for regional research and tools for monitoring possible climate change. The standard parameters related to climate change are surface air temperature, sea level, sea surface temperature, Arctic Sea ice, precipitation, and extreme weather occurrences. However, changes in Arctic Sea ice are irrelevant in the context of a tropical country. These indicators are distinguished by high temperature, high rainfall, dry spell, thunderstorm, and strong winds in Malaysia. According to Daniel Tang (2019), the accuracy of the review on temperature increase, rainfall variation, sea–level rise, and future projections on climate change is dependent on the accuracy of the data and simulation models adopted in the literature. Thus, the isotope technique can serve as a potential tool for defining the effect of climate change and help develop mitigating measures.
Poor correlation between δ18O of rainfall and daily precipitation (amount effect) is consistently reported in studies conducted in East Malaysia. A study of extensive–scale climatic controls on rainfall at Gunung Mulu, Sarawak in northern Borneo by Cobb et al. (2007) showed that seasonal to interannual changes in large–scale precipitation is reflected in the δ18O of rainwater, where the climate is dominated by monsoonal variability, and the precipitation is highly correlated to the Southern Oscillation Index (SOI). The study also found a low correlation (r = 0.05) between rainfall and precipitation, known as the local “amount effect” caused by the fractionation from the convective rain. On regional scales, the moisture transport path mainly determines δ18O in rainwater (depleted or enriched) characteristics, degree of the rainout, and orographic effect. Again, a relatively weak but significant (r=–0.19) inverse correlation between rainfall δ18O and daily precipitation was demonstrated when investigating the variability of northern Borneo rainfall δ18O and its response to local and regional climate variations. However, the weak amount effect increased with increasing temporal averaging or running average (Moerman et al. 2013).
For the first time, besides developing a database of δ18O and δ2H and local meteoric water lines from monthly precipitation samples collected at the GNIP network stations throughout Malaysia, the moisture sources of the precipitation between monsoons will also be isotopically characterized and differentiated. The seasonal and temporal behaviors of the isotopes and the relationship between isotope composition, temperature, and precipitation amount are also explained in this study. The need for reliable and detailed local MWL in Malaysia served as an impetus to the publication of this article.