Abstract

This study investigates the spatio-temporal variation of stable isotopes (δ¹⁸O, δ²H), deuterium excess (d-excess) in rainwater and surface water in the Mekong Delta, Vietnam to identify hydrological processes and freshwater–seawater interaction. The δ¹⁸O and δ²H of the rainwater samples from five sites experienced seasonal change, with an average δ¹⁸O and δ²H of -5.35‰ and -35.74‰, respectively. D-excess ranged from -13.25‰ to 26.09‰ with an average of 7.07‰, suggesting mixing of moisture sources and post-depositional evaporation. Surface waters were generally enriched (-4.23‰ for δ¹⁸O; -30.16‰ for δ²H), with inland river waters the most depleted (-6.17‰ for δ¹⁸O) and coastal waters the most enriched (-1.60‰ to -1.87‰ for δ¹⁸O), due to effects of tides and evaporation. Isotopic concentrations became more positive closer to the coast, and significant and positive relationships were observed between chloride and δ²H (r = 0.74) and δ¹⁸O (r = 0.60; p < 0.001). Rainwater presented the highest d-excess values, indicating a greater evaporative influence on surface waters. These observations suggest that the isotope-salinity relation may support to understand the influences of the mixing of water sources, seasonal recharge, and saltwater intrusion. The results also indicate that stable isotopes could be recommended as reliable hydrological indicators in some deltaic regions subjected to clearly distinct climatic and anthropogenic influences for analyzing hydrological processes and providing insights into water management.

1 INTRODUCTION

The Mekong Delta is one of the most productive and populous in the world, commonly called "the rice bowl" of Southeast Asia (Chapman and Darby 2016; Ngoc Que Anh et al. 2019; Smajgl et al. 2015). Therefore, water resources are crucial in agriculture, aquaculture, household use, and the region's ecological balance. Nevertheless, the Delta has also increasingly come under pressure from climate change (Smajgl et al. 2015), changes in the upstream hydrology (Dang et al. 2018), land use (Nguyen et al. 2023), and seawater intrusion (Eslami et al. 2021), which have presented serious challenges to water sustainability.

Oxygen (δ¹⁸O) and hydrogen (δ²H) stable isotopes in the water cycle are a good indicator of hydrological processes, including evaporation, condensation, and mixing (Gat 1996; Scholl et al. 2015; Aizen et al. 2005; Liu et al. 2010; Klaus and McDonnell 2013). These isotopic endmembers are natural tracers with which water sources, water migration, and environmental changes can be traced during the hydrological cycle (Clark and Fritz 2013). Therefore, stable isotopes have previously been employed to investigate water cycling on a local and regional scale (Ludvigson et al. 2023; Yao et al. 2021; Giustini et al. 2016; Yuan et al. 2020; Lee et al. 2021). This is especially true in complex settings such as coastal deltas (Han et al. 2015; Han and Currell 2018; Kanagaraj et al. 2018). In the Mekong Delta, the physical distribution of δ¹⁸O and δ²H in space and time is one of the critical factors in understanding the regional hydrological process. Rain isotopic composition changes seasonally, influencing air-mass flow patterns, precipitation quantity, and high air temperature (Duy et al. 2018). During this time, surface water, such as upstream rivers, inland canals, and coastal estuaries, is influenced by the combined effect of precipitation, river discharge, evaporation, and seawater intrusion (Dang et al. 2018).

Previous studies have indicated significant tendencies in which the Mekong Delta precipitation has pronounced seasonal variations with depleted δ¹⁸O and δ²H values during the wet season and enriched values during the dry season (An et al. 2018; Tran et al. 2019). The spatial gradients of surface water isotopic compositions range from almost pristine freshwater in the upstream part of the Mekong Delta, to more saline and evaporation-influenced water on the coast (Tran et al. 2019). In the inland region, the isotopic composition of surface water is a mixture of local rainfall inputs, evaporation, and shallow groundwater exchanges (Duy et al. 2019). However, despite these valuable findings, a holistic understanding of how spatial (upstream–inland–coastal) and temporal (seasonal) stable isotopic characteristics of the overall water resources of the Mekong Delta are presently limited.

Most importantly, the isotopic links between rainfall and surface water and the key environmental influences, including evaporation strength, hydrodynamic variability, and seawater invasion, are poorly understood. A better understanding of these isotopic trends is essential in the context of sustainable water management, particularly since climate variability, land subsidence, and upstream development are all having an increasing influence on the availability of freshwater across the Delta (Minderhoud et al. 2017; Eslami et al. 2021; Dang et al. 2018).

Therefore, the purpose of this study is to

1. Investigate the spatial and seasonal variation of δ¹⁸O and δ²H in rainfall and surface water along the upstream, inland, and coastal areas of the Mekong Delta;
2. Compare the isotopic characteristics of rain to surface waters; and
3. Determine the principal hydrological and environmental parameters influencing the stable isotopic distribution.

2 MATERIALS AND METHODS

2.1 Study area

In this study, we investigated the Mekong Delta (MD) and two areas in the southeastern region, including Ho Chi Minh City and Tay Ninh province, where both agriculture and aquaculture are crucial for local livelihoods and the regional economy (Figure 1). The MD consists of a complex network of rivers, canals, rice paddies, and coastal estuaries. The Delta is in a tropical monsoon climate where the wet season between May and November, and the dry season starts from December to April. Fresh water resources are distributed uniformly within the Delta and are influenced by various controlling factors. For example, the upstream regions are affected mainly by upstream discharge from the Mekong River Basin, while downstream regions are influenced by river-tidal interaction and local precipitation. In addition, change in rainfall pattern (Dang et al. 2020; Minh et al. 2024), reducing of inflow from upstream (Tang et al. 2023), tidal regime and climate change (Bui et al. 2017) have resulted in extreme drought (Nguyen et al. 2021; Phan et al. 2020) and severe saltwater intrusion (Loc et al. 2021) into the surface water system causing serious freshwater scarcity in the coastal zones influencing millions of local residents.

2.2 Sample collection

In this study, rainwater samples were collected bimonthly in dry and wet seasons at five monitoring sites in the Mekong Delta including Thu Duc City (Ho Chi Minh City), Dau Tieng Reservoir (Tay Ninh province), Chau Doc City (An Giang province), Rach Gia City (Kien Giang province), and Lich Hoi Thuong (Soc Trang Province) which represent the rainfall pattern in the Mekong Delta. The rainwater was collected by a standard collector installed according to methods described by the International Atomic Energy Agency (IAEA) to reduce post-collection evaporation (Gröning et al. 2012). The collected samples were introduced into 50 ml high-density polyethylene (HDPE) bottles with hermetic caps and maintained at 4°C as sampled after the rain shower, until analysis. Surface water samples were obtained from rivers, canals, and estuaries in high flow (wet season) and low flow (dry season) seasons at two main inflow stations, including Tan Chau and Chau Doc, as well as surface water in inland floodplain area and coastal area of the Mekong Delta (Figure 1). Subsequent samples were collected from 2 m depth using pre-cleaned HDPE bottles to minimize contamination. Physical parameters such as salinity, temperature, and electrical conductivity (EC) were measured in situ at each sampling site with a portable multi-parameter meter.

Figure 1 Sampling locations of rainwater and surface water in the Mekong Delta, Vietnam.

2.3 Isotopic measurements

The oxygen (δ¹⁸O) and hydrogen (δ²H) stable isotope ratios in water samples were examined utilizing two laser-based spectroscopy systems, specifically the Los Gatos Research LWIA-24D laser spectrometer(Anh et al. 2023). All measurements were calibrated to international standards, particularly the Vienna Standard Mean Ocean Water (VSMOW2), which is supplied by the Isotope Hydrology Laboratory of the International Atomic Energy Agency (IAEA) located in Vienna, Austria (Anh et al. 2023). Calibration was conducted using laboratory working standards, namely LGR 1C, LGR 2C, LGR 3C, and LGR 4C provided by Los Gatos Research. The isotope ratios are expressed in delta (δ) notation in per mil (‰) relative to the VSMOW standard, following this Equation 1: 

(1)

Where:

The analytical precision achieved was ±0.1‰ for δ¹⁸O, and ±1‰ for δ²H. Each sample underwent triplicate measurements, resulting in standard deviations of 0.1‰ for δ¹⁸O and 0.3‰ for δ²H. Furthermore, the deuterium excess (d-excess), which serves as an indicator of non-equilibrium fractionation processes, was computed using Equation 2 as established by Dansgaard in 1964 (Dansgaard 2012): 

d-excess

(2)

Salt concentration was determined directly in the field or in the laboratory from the Electrical Conductivity (EC) readings, which were adjusted by comparison with standard solutions. The relationship between δ²H and δ¹⁸O was then depicted and compared to the international standard Global Meteoric Water Line (GMWL) and a locally established Local Meteoric Water Line (LMWL) for the Mekong Delta. Differences from the meteoric line were interpreted as resulting from evaporation and seawater mixing. Maps of spatial distribution were developed through isotopic and salinity data interpolation via Geographic Information System (GIS) mapping methods.

3 Results

3.1 Variations of δ¹⁸O and δ²H in rainfall

The spatial variability in the isotopic composition (δ¹⁸O and δ²H) of rainwater obtained at five stations in the Mekong Delta and Southeastern Vietnam, including Ho Chi Minh City, Dau Tieng Reservoir, Chau Doc City, Rach Gia City, and Lich Hoi Thuong (Soc Trang Province) provides direct information about the control of hydrological and climatic conditions on the isotopic signatures of rainwater.

The statistical summary of stable isotopic compositions (δ¹⁸O and δ²H), deuterium excess (d-excess), and precipitation amount (P, mm) for all rainfall samples across the study area provides an integrated overview of regional hydrological and climatic patterns as presented in the Table 1. The average δ¹⁸O and δ²H values of rainfall for all sites were -5.35‰ and -35.74‰, indicating that the rain is moderately depleted in heavy isotopes compared to the VSMOW standard. This reduction suggests that the typical characteristics of monsoonal rain, such as more dominance of large-scale convective processes and moisture recycling in the heavy rain season, lead to the above reduction effect. The high standard deviations of δ¹⁸O (2.72‰) and δ²H (16.08‰) demonstrate considerable variation in rainfall events related to diverse moisture sources, modification due to evaporation during rainout, and variable intensity of precipitation events.

Table 1 Summary of stable isotopes and rainfall amount in mm of rainwater in the study area.

The δ¹⁸O values ranged from -10.40‰ to 1.12‰, and δ²H values of the water samples in Ho Chi Minh City varied between -73.15‰ and 2.85‰. The average values were -5.66‰ for δ¹⁸O and 36.14‰ for δ²H, respectively. The standard deviations of 3.06‰ for δ¹⁸O and 18.61‰ for δ²H demonstrate moderate variability, implying a rainwater system derived from seasonally distributed tropical precipitation, with occasional evaporative enrichment brought on by urban effects or short dry periods (see Figure 2). The relatively enriched stable isotopes also indicate some post-precipitation evaporation for a few rain events.

Figure 2 Stable isotopes (δ¹⁸O and δ²H) of rainwater in the Mekong Delta, Vietnam.

At the Dau Tieng Reservoir (Tay Ninh), values of δ¹⁸O varied from -8.69‰ to -0.43‰, and δ²H values ranged from -60.94‰ to -8.39‰. The means of -4.35‰ and -34.25‰ δ¹⁸O and δ²H values and their standard deviations (2.59‰ and 15.42‰) suggest a small degree of isotopic enrichment compared to the coastal areas. These generalizations suggest considerable evaporation during the dry season, especially from the open water of the reservoir, yet with little overall seasonal difference. In Chau Doc City (An Giang), isotopic distributions are, however, more extensive with δ¹⁸O values from -8.22‰ to 0.63‰ and δ²H values from -45.83‰ to 1.11‰. The average isotopic compositions were -5.74‰ for δ¹⁸O and -37.64‰ for δ²H, with standard deviations of 2.69‰ and 13.58‰. These values indicate a combination of different moisture sources and evapo-concentrated floodplain water, particularly enhanced during dry periods when floodplain evaporation and atmospheric recycling processes are improved. At Rach Gia City (Kien Giang), the isotopic composition of the water was lighter than observed, and the δ¹⁸O and δ²H values ranged from -8.55‰ to -3.57‰ and -56.89‰ to -18.49‰, respectively. Values for the average of δ¹⁸O and δ²H are -6.32‰ and -39.53‰, respectively. The largest isotopic range was observed in Lich Hoi Thuong (Soc Trang province), where the minimum to maximum δ¹⁸O and δ²H values were respectively -9.28‰ to 1.98‰ and -63.40‰ to 4.58‰. The average values amount to -5.64‰ (δ¹⁸O) and -37.45‰ (δ²H), the latter indicating relatively depleted conditions, while the high-end isotopic enrichment indicates strong evaporation effects. The standard deviations of 2.33‰ (δ¹⁸O) and 13.73‰ (δ²H) confirm a low variability associated with hydrological seasonality. The δ¹⁸O versus δ²H plot of all rainwater samples indicates that data points fall slightly below GMWL, suggesting that rain is mainly related to regional convective precipitation processes and with moderate post-depositional secondary evaporation in the atmosphere or after the precipitation event. Inland sites (e.g., Ho Chi Minh City and Dau Tieng Reservoir in Tay Ninh province) lay closer to the GMWL, suggesting less evaporation than in more coastal sites (Tran De in Soc Trang and Rach Gia City in Kien Giang province) where isotopic enrichment due to evaporation is more pronounced.

3.2 Spatial distribution of δ¹⁸O and δ²H in surface waters

Seasonal surface water samples of five hydrological groups as presented in Table 2, including river water in the dry season 2023 (SWD2023), in the rainy season 2023 (SWR2023), coastal seawater (SEAWATER2023), coastal surface water in the dry season 2023 (CSWD2023), and in the rainy season (CSWR2023). Representative members of the SWD2023 and SWR2023 groups (inland freshwater) showed the most negative isotopic signals. The average δ¹⁸O values were -6.17‰ for SWD2023 and -5.76‰ for SWR2023, and the average δ²H values were -43.84‰ and -46.44‰, respectively. These findings confirm the control of meteoric water input, with little contribution by evaporation, especially in the rainy season, when river discharge is high, and dilution by precipitation is significant. On the other hand, the SEAWATER2023 samples yielded the most enriched isotopic compositions, with mean δ¹⁸O and δ²H values of -1.87‰ and -9.45‰, respectively. The isotopic composition of these water sources is consistent with the average for oceanic water, suggesting minimal, if any, dilution or evaporation effects and the material was largely from open ocean water. The CSWD2023 and CSWR2023 clades from estuarine habitats on the coast fell between the two groups from freshwater environments regarding isotope values. The average δ¹⁸O was -1.60‰ (CSWD2023), and -4.00‰ (CSWR2023), and the mean δ²H was -8.74‰ and -26.90‰, respectively. These values indicate dynamic tidal mixing between riverine freshwater input and seawater, and the seasonal differences are attributed to variations in precipitation, river flow, and evaporation. The standard deviations for the two isotopic terms indicate a fair degree of variability, emphasizing the transitional character of coastal surface waters.

Table 2 Summary of stable isotopes and d-excess of surface water in the study area.

A scatter plot of δ¹⁸O and δ²H surface water samples (see Figure 3) indicates that inland river water samples (SWD2023, SWR2023) are clustered within a close line slightly to the evaporatively shifted meteoric water line, reflecting recent precipitation origin mixing with surface water evaporation. Coastal water samples (CSWD2023, CSWR2023) tend to have more enriched δ¹⁸O and δ²H compared to inland river waters and seem to be controlled by the combined effects of tidal mixing, enhanced evaporation given onshore conditions, and the seasonal saltwater intrusion (Tran et al. 2019). The marine sources (SEAWATER2023) are plotted at the extreme upper part of the isotope range, reflecting the most significant enrichment of the marine component as shown in Figure 3.

Figure 3 Stable isotopes (δ¹⁸O and δ²H) of surface water in the Mekong Delta, Vietnam.

3.3 d-excess in water resources

The deuterium excess (d-excess), defined as d-excess = δ²H – 8 × δ¹⁸O, is a reliable water tracer and can be effectively utilized for tracing the origin of moisture and hydrological dynamics of the Mekong Delta (Tran et al. 2019). Rainwater and surface water were gathered at five sampling spots of the Mekong Delta and Southeastern Vietnam from 2023 to 2024 as follows: Ho Chi Minh City, Dau Tieng Reservoir (Tay Ninh); Chau Doc City (An Giang); Rach Gia City (Kien Giang); and Lich Hoi Thuong (Soc Trang).

Spatial variability of d-excess values was very well observed with rainwater samples as presented in Figure 4. Overall, the rainwater samples had higher (more positive) d-excess values (7.07‰) in comparison to surface water samples (3.71‰), suggesting limited post-precipitation alteration. Precipitation in coastal locations, including Ho Chi Minh City (average d-excess: 10.10), Lich Hoi Thuong (8.50), and Rach Gia (8.00) indicated considerable oceanic moisture sources related to the Southwest monsoon regime. However, they were marginally lower than the commonly accepted maritime reference (10–12‰), attributed to partial evaporation during rainout. The high d-excess values (>26.09‰ in Ho Chi Minh City) reflect the occasional input of highly fractionated atmospheric moisture. In contrast, the inland Dau Tieng Reservoir (Tay Ninh) was characterized by a relatively lower rainwater d-excess (0.59‰) varying from -11.56‰ to 11.04‰, indicating intense second evaporation and terrestrial moisture recycling processes that affect the isotopic composition. Chau Doc City (An Giang), situated alongside the Mekong River, had a relatively high mean d-excess value of 8.31‰ (±10.30‰), reflecting substantial sub-cloud evaporation processes, atmospheric moisture recycling, and contributions from local convective rainfall sources. These results indicate that geographic location, hydrological process, and atmospheric state affect the isotopic composition of rainwater in each region.

Figure 4 δ¹⁸O vs. d-excess of rainwater in the Mekong Delta, Vietnam.

Water samples from surface water show generally lower d-excess values than rainwater and significant variability, indicating substantial post-depositional evaporation and mixing, as illustrated in Figure 5. The surface water samples in the inland areas (SWD2023 and SWR2023) have a mean d-excess ranging from -0.59 ‰ in the rainy season (SWR2023) to 5.55‰ in the dry season (SWD2023), indicating a complex mixture of river inputs and evaporation. The surface water samples from coastal areas have shown more positive mean d-excess (4.06‰ – 5.11‰), indicating the combined influence of evaporative enrichment and saline intrusion processes characteristic of estuarine and coastal environments. A wide variation of d-excess from seawater (-17.89‰ – 19.12‰), indicating strong evaporative effects as well as mixing with meteoric inputs and possible brackish water influence in the estuarine environment.

Figure 5 δ¹⁸O vs. d-excess of surface water in the Mekong Delta, Vietnam.

In comparison, rainwater generally displays higher d-excess values than surface water, especially in coastal areas, since evaporation strongly enriches isotopically. The surface water in land areas shows a predominantly land-based signal, differing from the more oceanic signatures evident in coastal areas. An indication of complex river-atmosphere relations Chau Doc City (An Giang) shows the highest variability of the d-excess values in rain and surface water. These results highlight that atmospheric fingerprints are preserved in rainwater, while surface waters integrate a mixture of evaporation and residence time.

3.4 Relationship with salinity

The isotopic composition and salinity of water samples were highly positively correlated with 0.74 for δ²H and 0.60 for δ¹⁸O (p < 0.001), demonstrated as an increase of salinity leading to less seawater depletion, δ¹⁸O and δ²H as demonstrated in Figure 6. The stable isotope data demonstrate the potentially significant influence of saltwater intrusion on the hydrology of the Mekong Delta. The depleted isotopic values of SWD2023 and SWR2023 imply a freshwater influence in low salinity regions, whereas the enriched isotopic values of SEAWATER2023 indicated a marine source in the coastal areas. The intermediate isotope compositions of CSWD2023 and CSWR2023 reflect a mixing zone through which freshwater from rivers and/or precipitation comes into contact with increasing seawater levels. These results are essential for understanding the magnitude of saline intrusion, especially in response to climate change and sea-level rise, as both cause increased sea incursion to deltas. The d-excess variability among groups indicates evaporation modifies isotopic composition, particularly in brackish water. Further studies that take account of other environmental parameters, like temperature and humidity, are warranted to explain the mechanisms that play a role in d-excess. Furthermore, stable isotopes serve as an effective tracer to identify the origin of water and determine the extent of mixing, which is essential for water resources management and possible strategy mitigation in the Mekong Delta.

Figure 6 (a) δ¹⁸O vs Cl^-, and (b) δ²H vs Cl^- of surface water in the Mekong Delta, Vietnam.

4 DISCUSSION

4.1 Isotopic insights into hydro-meteorological dynamics

Rainwater isotopic compositions indicate that, albeit having a small evaporative influence during rainout, the coast relies mainly on oceanic sources of moisture. Lower δ¹⁸O and δ²H in inland sites, accompanied by widely variable d-excess, emphasize the role of evaporation and terrestrial water recycling, especially in reservoirs and river floodplains. The isotopic compositions of surface water from inland freshwater to coastal saline waters (Figure 3) illuminate an increasing enrichment and emphasize the importance of evaporation and tidal mixing. Freshwater river samples retain a meteoric signature, but brackish waters and seawater samples reveal isotopic enrichment and combined evidence for seawater mixing. The pronounced positive salinity-enrichment relationship also suggests that the mixture of freshwater and seawater is affected by evaporation and seawater intrusion, especially during the dry season marked by a reduction in river flow. The d-excess changes are key discriminators in evaporation strength, atmospheric moisture sources, and hydrological connectivity.

The d-excess values, a key moisture source and evaporation process indicator, averaged 7.07‰ for all rainwater samples (Table 1). This value is slightly lower than the classical oceanic value of 10‰, indicating moderate sub-cloud evaporation or recycling of terrestrial moisture into rain. However, the high-stdev value (8.29‰) for d-excess suggests that each rainfall event had been affected by a different degree of secondary evaporation and different moisture pathways. Very low d-excess values (minimum -13.25‰) suggest a compelling evaporative enrichment in some events, potentially occurring during dry periods, coupled with high atmospheric saturation deficits, and high values (maximum 26.09‰) are indicative of very fractionated rainwaters sources generated in intense convective storms or instantaneous precipitation from moisture-laden air masses. There was also much variability in precipitation amounts (P) from touching off 26.11 mm naturally; however, the balanced mean was 12.91 mm per event, and stdev was 16.17 mm, inferring the differences between small rainy events and large stormy ones. Observed rainfall ranged from a minimum of 0.07 mm (occasional light rain or mist) to a maximum of 89.60 mm (heavy tropical showers typical of southwest monsoon influence). The relation between δ¹⁸O, δ²H, d-excess, and precipitation amount emphasizes significant hydrological mechanisms. Typically, negative δ¹⁸O and δ²H values indicate greater rainfall, by the known amount-effect in tropical areas where heavier rains have more depleted isotope signatures. On the other hand, shorter precipitation events often have isotopic compositions and d-excess values enriched concerning the GMWL due to stronger atmospheric evaporation processes and less organized convective systems. In general, the isotopic and precipitation data confirm that the hydrology of the Mekong Delta at large is mainly controlled by synoptic atmospheric dynamics in these climates during the rainy season, modified by local evaporation, moisture recycling, and surface-atmosphere interactions in the dry seasons. These results demonstrate the significance of incorporating both isotope compositions and precipitation data when studying the regional water cycle and hydrological modeling in the Mekong Delta.

4.2 Implications for water resources management

Stable and non-stable isotopic compositions (δ²H, δ¹⁸O, and d-excess) of rainwater and surface water in the Mekong Delta are essential indicators of trace water sources, processes, and dynamics in the region (Figure 4 and Figure 5). The differential isotope values observed at five study sites (Ho Chi Minh City, Lich Hoi Thuong in Soc Trang province, Dau Tieng Reservoir in Tay Ninh province, Rach Gia City in Kien Giang province, and Chau Doc City in An Giang province) reflect oceanic moisture evaporation and hydrological mixing input. This knowledge is essential for improving water management, especially concerning climate change, salt intrusion, and the growing need for water in the region.

Most rainfall at coastal localities, including Lich Hoi Thuong (7.15‰), and Rach Gia (6.26‰) and Ho Chi Minh City (mean d-excess: 8.33‰) shows high d-excess values, and therefore, the moisture predominantly comes from the ocean, the South China Sea or Indian Ocean, while evaporation from the land plays but a minor role. These values are consistent with the characteristics of tropical maritime rainfall. In contrast, the d-excess values in the terrestrial collected water samples from the inland, e.g., the Dau Tieng reservoir (average d-excess: 0.19‰) and Chau Doc (3.46‰), are lower and show higher diversity, indicating a more substantial impact by the continental moisture recycling and the local evaporation. The low d-excess (-11.56 ‰) of Dau Tieng indicates strong evaporation fractionation, probably caused by the reservoir surface's exposure.

On the other hand, surface water samples generally have lower d-excess values than precipitation due to post-depositional processes such as evaporation and mixing. The
d-excess of inland surface water has a wide variation, ranging from -32.34‰ to 23.47‰. This is most likely caused by the contribution of rainfall input, the Mekong River inflows and evaporation processes. For instance, the depleted mean d-excess of -0.39‰ of surface waters in the rainy season clearly indicates the dominance of isotopic depletion from intense rainfall input and the reduced influence of evaporative enrichment during high-flow conditions (Figure 4). In contrast, an increase of d-excess in the dry season (5.51‰) indicates enhanced evaporative enrichment combined with reduced rainfall input, as well as possible contributions from recycled atmospheric moisture and longer surface water residence times.

Estimating evaporative loss through d-excess is a valuable tool for water resource management. Because the Mekong Delta will have more frequent higher temperatures and drier weather, the low or negative d-excess values in rainwater and surface water imply an area where water vapor loss due to evaporation will be a concern. First, interventions that can be targeted, such as open water shading, reservoir storage maximization, or cover cropping, may reduce evaporative losses. On the contrary, high d-excess values in coastal rainwater indicate an important natural freshwater source, which should be considered when planning water storage and/or aquifer recharge strategies.

Isotope study can be used to improve hydrologic modeling by using independent tracers to annotate the source of runoff as precipitation-dominated runoff, from groundwater, or baseflow. The isotopic differences between upland (low δ¹⁸O and δ²H) and fringe (high δ¹⁸O and δ²H) sites across the Mekong Delta have the potential to serve as a test for models of streamflow distribution, evaporation rates, and groundwater recharge. For example, the large spread of d-excess of 26.09‰ in rainwater (Table 1) and 11.50‰ in surface water (Table 2) in the study area suggests complex evaporation-precipitation cycling /modulated by the river, with potential inclusion into dynamic water balance models for better predictions of the impact of climate change. Even though the monitoring of stable isotopes is included in the water management scheme, one can benefit from a more flexible and robust approach. In coastal estuarine and deltaic settings, such as Rach Gia city, Kien Giang province and Lich Hoi Thuong, Soc Trang province, where rainwater still resembles ocean water after the isotopic composition of ocean water, policies could target the capture and storage of monsoon precipitation to moderate saltwater intrusion. The local isotopic signature enriched by the Mekong River in Chau Doc city should be considered for a specific monitoring program to address river-atmosphere interactions and their implications on the regional water resources. Isotopic analysis finally emphasizes the importance of having water management procedures in the Mekong Delta. D-excess gradients in isotopic composition can also provide insight into proglacial meltwater sources. This perception might help develop mitigation approaches and rationalize water distribution and engineering measures to reconcile infrastructure planning with regional hydrological dynamics. Such knowledge is essential to maintaining the integrity of the Mekong Delta's water resources considering environmental and climate perturbations.

5 Conclusion

This study demonstrates that δ¹⁸O, δ²H, and d-excess are useful indicators for identifying hydrological processes and sea-fresh-water mixing in the Mekong Delta. Stable isotopes of rainwater samples varied markedly between seasons, reaching from -10.40‰ to 1.98‰ for δ¹⁸O, and from -73.15‰ to 4.58‰ for δ²H. These isotopic signatures in combination with high d-excess values (up to 26.09 ‰) point to a more convective type of precipitation from oceanic sources and occasional evaporative enrichment. Spatially, the coastal sites, e.g., Rach Gia City (Kien Giang) and Lich Hoi Thuong (Soc Trang), were more depleted in isotopic ratios with δ¹⁸O values between -6.32and -5.64‰, compared to the inland location of Dau Tieng Reservoir (Tay Ninh) with a mean δ¹⁸O of -4.35‰ and a minimum value of d-excess at 0.43‰, indicating stronger oceanic moisture influence in coastal rainfall and enhanced evaporative modification in inland surface waters.

The mean δ¹⁸O and δ²H values of freshwater river samples were -5.97‰ and -45.14‰, respectively, representing the contributions of recent rainfall. However, coastal surface water (CSWD2023) and seawater (SEAWATER2023) samples showed an enrichment in δ¹⁸O (e.g., -1.60‰ to -1.87‰) related to tidal mixing and evaporation. The variation d-excess values of surface water are between -32.34‰ and 23.47‰, indicating strong spatial and temporal heterogeneity driven by evaporation, mixing with meteoric inputs, and variable residence times of water bodies.

More importantly, it is noted that there is a strong correlation between δ¹⁸O and δ²H with the chloride contents (r = 0.60–0.74, p < 0.001), indicating that they are useful for saltwater intrusion assessment. These connections are essential implications for readjusting crops or rescheduling agricultural practices in areas with high evaporation, optimizing the use of water resources and predicting the impacts of sea-level rise and hydrological alteration in the upstream part of the Mekong Delta. These findings reveal how isotope signatures support tracking and consolidating information on salinity trends for local farmers. Therefore, a robust tracer framework should be one of the future directions. Such a framework is a foreseeable approach to interpret the dynamics of water sources and construct the adaptive guidance on water management in coastal deltas which have been increasingly exposed to climate change consequences.

Acknowledgments

The authors sincerely appreciate the support of institutions and individuals who contributed to this research. This research was supported by the Ministry of Science and Technology (MOST), Vietnam, through the project “Study on the Development of Characteristic Lines of Meteoric, Surface, and Groundwater to Determine the Contribution Ratio of Upstream Water Sources to the Mekong Delta” (Project Code: ĐTĐL.CN-54/22). The support provided by MOST was instrumental in facilitating data collection, analysis, and interpretation, contributing significantly to the completion of this study.

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