# Localized recharge processes in the NE Mekong Delta and implications for groundwater quality

T.A. Tu, Sarah Tweed, N.P. Dan, M. Descloitres, K.H. Quang, J. Nemery, A.

Nguyen, Marc Leblanc, C. Baduel

► To cite this version:

T.A. Tu, Sarah Tweed, N.P. Dan, M. Descloitres, K.H. Quang, et al. Localized recharge processes in the NE Mekong Delta and implications for groundwater quality. Science of the Total Environment, 2022, 845, pp.157118. 10.1016/j.scitotenv.2022.157118. hal-03747305

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1 Localized recharge processes in the NE Mekong Delta and implications for groundwater quality

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3 Tu T.A.¹,², Tweed S. ¹,³*, Dan N.P.¹,², Descloitres M.¹,⁴, Quang K.H. ¹,², Nemery J. ¹,⁴, Nguyen A.¹,⁴,

4 Leblanc M.¹,⁵,⁶, Baduel C.¹,⁴

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6 ¹Centre Asiatique de Recherche sur l'Eau (CARE), Ho Chi Minh City University of Technology

7 (HCMUT), Ho Chi Minh City 700000, Vietnam

8 ²Vietnam National University-Ho Chi Minh City (VNU-HCM), Thu Duc City, Ho Chi Minh City, Vietnam

9 ³UMR G-eau, IRD, SupAgro, Montpellier, France

10 ⁴Université Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, Grenoble, France

11 ⁵UMR EMMAH, Hydrogeology Laboratory, University of Avignon, France

12 ⁶IWRI (International Water Research Institute), UM6P-UM5, Morocco

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14 *Corresponding author, sarah.tweed@ird.fr

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23 **Abstract**

24 Understanding recharge in the Mekong Delta is critical for the delta's groundwater resources, and 25 requires the investigation of recharge processes at the local scale. In this study of the north eastern 26 area of the Mekong Delta, time-series of environmental tracer data (δ¹⁸O, δ²H, major ions and ³H) and 27 markers of rural pollution (NH₄ and NO₃) were used to highlight localized recharge and impacts on 28 groundwater quality. Results highlighted new hydrological insights into recharge processes, including 29 that the Pleistocene aquifer receives recent recharge (< 60 years), predominantly during high rainfall 30 months (> 100 mm/month). However, due to shallow clay layers there are significant spatial variations 31 in these recharge processes, which were observed in the seasonal fluctuation of groundwater δ¹⁸O 32 values in groundwater. Wet season δ¹⁸O changes ranged from below analytical uncertainty (≤ 0.10 ‰) 33 to up to 0.56 ‰, and the calculated fraction of rainfall contribution to the aquifer is ≤ 5% to 16%. 34 Rainfall recharge via the acrisol soils results in low groundwater EC (20-55 µS/cm), acidic groundwater 35 (pH 3.6-5.6), and may also have resulted in the low groundwater NO₃ concentrations (≤ 5.3 mg NO₃/L) 36 at many sites due to adsorption, therefore delaying not reducing NO₃ contamination. Site specific 37 variations in nitrogen processes includes increased NO₃ (to 29.7 mg/L) from fertiliser transfers or 38 nitrification, and increased NH₄ (to 1.4 mg/L) likely due to the recharge of irrigation waters. Unlike 39 other recharge areas across the northern Mekong Delta, this north-eastern region provides a 40 groundwater resource unaffected by arsenic contamination. Therefore, these results should inform on 41 priority areas for protection from further contamination by rural anthropogenic activities. 42 43 44

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45 **Key words**

46 Localized recharge, groundwater, stable isotopes, tritium, nitrate, acidity.

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# 1. Introduction

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The investigation of localized recharge processes has largely been documented in semi-arid regions. Localized recharge processes have been found to result from many land surface or subsurface features, including flow via sinkholes in calcrete layers in the Stampriet Basin (Kirchner et al., 2002), via localized depressions focalising rainfall recharge in south-west Niger (e.g. Leduc et al., 2001; Seddon et al., 2021), and via preferential recharge pathways in dual porosity mediums in southeast Australia (Cartwright et al., 2007) and in southern California (Manna et al., 2017). However, in more humid climates, identifying localized recharge processes is also important for groundwater resources, both in terms of groundwater resource quantity and quality (Hartmann et al., 2021).

Regional groundwater recharge rate assessments can mask significant variations at the local scale that potentially play an important role in groundwater recharge. For example, previous work in the tropical humid Mekong Delta found that regional estimates of groundwater recharge were underestimated when localized recharge processes were not included (Minderhound et al., 2017; 2020). In addition, there are often significant temporal and spatial heterogeneities associated with localized recharge processes, which are critical when considering groundwater resource protection and contaminant transfers. For example, the groundwater recharge for Ho Chi Minh City (HCMC), which is predominantly from rainfall and river sources in the Saigon-Dong Nai river catchment basin (HCMC Geography, 2020), is significantly impacted by land use and land cover change (Adhikari et al., 2020).

More than 55 million inhabitants of Vietnam (about 60 % of the total population) are groundwater-dependant (Carrard et al., 2019). Many households that use groundwater as their primary source of drinking water, which are mostly located in rural areas, have private shallow bores rather than sourcing groundwater from government-managed plants (Carrard et al., 2019). Therefore, there remains an elevated health risk for inhabitants where shallow aquifers are exposed to contamination. A majority of the rural population in Vietnam lives in the Red River Delta and the Mekong River Delta where the use of groundwater for domestic purposes dates back at least 100 years (Anderson, 1978; Luu, 2019). In the Mekong Delta, the high groundwater extraction rates have resulted in declining groundwater levels being observed in all aquifers (Ha et al., 2019b; Duy et al., 2021). Groundwater is exploited for agricultural, industrial and domestic uses beyond the recharge capacity around HCMC. The over-exploitation has resulted in land subsidence and salinity intrusion along the coast of the Mekong River Delta (Stefan, 2014; Duong et al., 2010).
Despite a heavy reliance on groundwater as a resource in the Mekong Delta, there still remains many questions relating to localized recharge processes and implications for groundwater quality. In this study we analyse the localized recharge processes in an area where the shallow aquifer belongs to the northeastern margins of the Mekong Delta groundwater system, but the river forms part of Saigon- (Dong Nai) river catchment basin. The seasonal variations in recharge processes, origins of water recharging the aquifer, and impacts of recharge on water quality parameters are analysed using environmental tracer data. The time-series of environmental tracer data (stable isotopes of water ($\delta^{18}$O and $\delta^{2}$H), major ions and tritium ($^{3}$H)) are used to determine whether recharge processes are recent and localized, and therefore an assessment of whether the presence of suspected large shallow clay deposits could act as confining layers for the shallow aquifer and protect (or not) the groundwater resources from surface contaminants. The combined environmental tracers, pH, electrical conductivity (EC), and nitrogen concentration data are also used to determine whether recharge processes affect groundwater quality processes.

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## 2. Study Area

The study site located in a rural area of HCMC, Vietnam (Fig. 1). The area is about 31 km to the northwest of the city centre. The elevation varies from 0 to 25 m AMSL (above mean sea level). Near the Saigon River, the elevation is lowest (< 2 m AMSL), and increases with distance from the river in the north-easterly and south-westerly directions. The highest elevation in the north eastern and south eastern sides are about 30 and 25 m AMSL, respectively. The area has a monsoon wet tropical climate, with a rainy and a dry season resulting in significant seasonal changes in rainfall (e.g. Fig. 2a). Average annual rainfall is around 1900 mm and monthly temperature varies from 27.5°C (September) to 29.5°C (March) (GSO, 2016).

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HCMC is the most important area for economic development and is the most populated area in Vietnam (population up to 8.2 million people). The land use pattern in the area has rapidly changed because of the economic development and urbanization during the last two decades (Nguyen et al., 2016; Son et al., 2017). In particular, urban areas have expanded 4.8-fold from 1990 to 2012 (Kontgis et al., 2014). In 2015, the land of HCMC was used for urban (29.8%), agriculture (32.4%) and forest area (15.9%) (GSO, 2016). At the study site land use is mostly for agriculture. Soil in the study site is mostly acrisol, forming through the deep weathering and extensive leaching of soluble components due to high air temperatures and precipitation (Spaargaren and Deckers, 2005; FAO, 2015). The soil is characterized by red and yellow appearance due to high contents of Fe-/Al-oxides/hydroxides, which therefore has ferralic properties (Ha et al., 2019b). The soil quality has acidic pH (average ± standard
deviation, 4.72±0.4), low clay fraction (37±12.5%), low organic matter (<1.2) and low cation exchange capacity (CEC, 4.7±1.67 meq/100g; % of total bases/sediment exchange capacity, 15±16%; Do et al., 2002). Quartz (~12%) and kaolinite or halloysite (~92%) are the most dominant minerals, and no calcite is observed in the clay fraction of the soil (Nguyen et al., 2008).

The main river systems in the HCMC area are the Saigon and Dong Nai Rivers, which are connected to two reservoirs, the Dau Tieng and Tri An Reservoirs in the upstream area of the Saigon River and Dong Nai River respectively. The reservoirs were built to both store water for water supply and manage the river water levels and salinization in the downstream areas in HCMC. Due to the low elevation, seawater can strongly influence HCMC, causing flood and salinization in the area (Vachaud et al., 2019).

The geology is characterized by unconsolidated sediments of Miocene to Holocene overlying Mesozoic to Paleozoic bedrocks (SVGMD, 2004). The unconsolidated sediment layer is progressively thicker from north to south, and the thickness is up to 300 m in the south of HCMC. The Holocene sediment covers low land area with elevations less than 5 m AMSL. Older sediment outcrops in the area where the surface elevation is generally greater than 2 m AMSL. The outcrops of Pleistocene sediment are characterized by a highly permeable laterite layer. Therefore, the high elevation areas are recognized as a recharge zone of the aquifer systems (Ha et al., 2019b). At the study site, Holocene sediment deposits along the Saigon River overlie the upper Pleistocene sediment. The Pleistocene sediment outcrops in high elevation areas on both sides of the Saigon River. Below the Pleistocene sediment is the sediment of the middle Pliocene. The lower Pliocene sediment is the deepest layer in the area. According to the Southern Vietnam Geological Mapping Division (SVGMD, 2004), the aquifer systems in the HCMC can be divided into 7 aquifers including the Holocene (qh), upper Pleistocene (qp³), upper-middle Pleistocene (qp²⁻³), lower Pleistocene (qp¹), middle Pliocene (n₂²), lower Pliocene (n₂¹). Each system comprises two parts including low permeability silt, clay, or silty clay layers that act as aquitards, and the underlying aquifer that consists of fine to coarse sand (Fig. 1c). One of the major questions in this study is to determine whether the shallow clays acts as a continuous confining layer and therefore limits local recharge, or whether the clays are present as discontinuous lenses and so allows spatially heterogeneous recharge to occur and therefore the transfer of pollutants.

The water for irrigation in HCMC is extracted from both the river system and the Dau Tieng Reservoir, and distributed via canals. Drinking and industrial water used in the area is supplied mainly from both the Dau Tieng and Tri An reservoirs and from groundwater. About 40% of the total water supply for domestic and industrial use are extracted from the aquifer system (Ha, 2015; Ngo et al., 2015). The
aquifers that are predominantly used for groundwater resources in the study area, are the Middle-
Upper Pleistocene aquifers (qp²⁻³) and the underlying middle Pliocene aquifer (n₂²) (Ha et al., 2019b).

Climate change, sea level rise and anthropogenic activities such as urbanization and agriculture are
threatening the surface water quality in HCMC. In the city center the river water quality is generally
poor due to high concentrations of nutrients (such as NH₄ and PO₄), coliform and trace metals. In down-
gradient areas of river system, salinity becomes a greater issue and water cannot be used for drinking
(Nguyen et al., 2019). In the coastal region, groundwater resources are also affected by salinity, and
high Fe and Mn concentrations (Nguyen et al., 2014; Ngo et al., 2015; Ha et al., 2019b; Ha et al., 2022).
In comparison, in the higher-altitude area (including this study area), groundwater quality is less
impacted by salinity but more by acidity, high NO₃ and high Al concentrations (Ha et al., 2022).

## 3. Methods

In this study, the environmental tracers (δ¹⁸O, δ²H, major ions and ³H) are used to inform us on the recharge processes in the rural area north of HCMC. The hydrochemical tracers are also used to identify processes affecting pH values and NO₃ concentrations. Monthly field visits of groundwater and river water sites were conducted for hydrochemical sampling and measurements from January 2019 to May 2020. Samples of rainfall isotopes were collected from 2018 to end 2019, and rainfall volumes and river water level data were obtained for the period from January 2018 to August 2020.

### 3.1 Water levels and rainfall

Water levels of the Saigon River were monitored at the Thu Dau Mot station (located at 106.6531, 10.9494), and were accessed from the Southern Regional Hydrometeorological Center. Rainfall data was also accessed from this center, which is located approximately 13 km southeast of the study area.
The monthly rainfall values were calculated as the sum of hourly measurements.

Hydraulic head data for 5 bores in the study area (locations shown in Fig. 1b) are from the government monitored groundwater bores (Division for Water Resources Planning and Investigation for the South of Vietnam; DWRPIS, 2018). The depth of bores ranges from 21.7 to 30.5 m with screen intervals of up to 8m (all are screened within the aquifer qp2-3). Data from these bores was only available from 1994 to 2015.
## 3.2 Hydrochemistry of river water and groundwater

To analyse the origin of groundwater, and thereby determine the relative contributions of rainfall and river recharge to the shallow groundwater system, the stable isotope ($\delta^{18}$O and $\delta^{2}$H) values of the different waters were compared, and coupled with EC, major ion and $^{3}$H data. Monthly samples during one wet season (2019) and two dry seasons (2019 and 2020) were collected from three river sites (RW2, RW7 and PC) and seven groundwater sites for hydrochemical tracers (Table 1). The locations of the groundwater and river water sites are presented in figure 1b, except river sampling at site PC which is located 10km downstream from the study area: 10.980959°N, 106.643781°E. Many of the groundwater sites were selected to represent locations in proximity to the Saigon River, a potential recharge source for the shallow groundwater. Five of the groundwater sites are located between 50-400 m from the Saigon River's edge, whilst the 2 other sites (GW1 and GW6) are located 1.3 and 3 km respectively from the river. Stable isotopes of rainfall (41 samples during the wet seasons of 2018-2019) were also collected at the CARE HCMC weather station (latitude: 10.774551, longitude: 106.661356), which is located 30 km SSE of the study area. Collection of these rainfall samples also included the analysis of EC and pH (Nguyen et al 2020).

All except one site (GW8) were privately owned bores that are used for domestic (including drinking water supplies) and/or agricultural purposes. The groundwater samples from these bores were collected using the installed pumps, which pump groundwater from near the base of the bore. The site GW8 (with a screened interval of 7-11 m) is a piezometer installed during this study that is located close to the Saigon River (50 m during the dry season). The depths for the groundwater bores sampled range from 11-28 m, which corresponds with the depth of the shallow Upper-Middle Pleistocene ($qp_2$-$^{3}$) aquifer (Fig. 1c). One bore (GW5) is screened in the deeper Lower Pleistocene aquifer ($qp_1$) and has a depth of 40 m. Since private bores are screened along the length of the casing, the GW5 samples are likely a mix of the shallow and deeper aquifers.

For the hydrochemical analysis, all groundwater bores were pumped until the EC stabilised, after which the field parameters were measured and samples were collected for laboratory analysis. All river samples were collected at ~30 cm depth from the middle of the river. For the pH data from the longitudinal profile of the Saigon River, this includes data from the study of Nguyen et al 2022 (in revision).

For both river and groundwater samples the EC and temperature were measured in the field using a field-calibrated WTW 3420 meter, and the pH was measured using a WTW 3310 meter. For the concentrations of N species ($NH_4$, $NO_3$, and $NO_2$) samples were analysed upon return to the CARE
laboratories at the Ho Chi Minh City University of Technology. Samples were immediately filtered through a Whatman GF/F filter (porosity 0.7 µm) for analysis of dissolved nitrogen species. These parameters were analysed by standard colorimetric methods (American Public Health Association:APHA, 1995). Additional water samples were collected in the field monthly for stable isotopes $\delta^{18}$O and $\delta^{2}$H, major ions and tritium, with results presented in Tables 1 and 2.

The major ions and stable isotopes were analysed at the Hydrochemistry Laboratory, University of Avignon, France. Samples for ion analyses were filtered in the field (0.45 µm cellulose nitrate filter) and an aliquot was acidified to pH < 2 (ultrapure 16N HNO$_3$). Anions and cations were analysed using ion chromatography (Dionex; ICS1100) and ion analysis uncertainty is in the order of 3 %. Stable isotope values ($\delta^{2}$H and $\delta^{18}$O) were analysed using a LGR (Los Gatos Research) laser spectrometer; the uncertainty is $\delta^{18}$O = ±0.1‰ and $\delta^{2}$H = ±1‰. Samples were also analysed for $^{3}$H contents (Table 2) by low level liquid scintillation counting after electrolytic enrichment using a scintillation counter (Tri-Carb 3180 TR/SL) at the Hydrochemistry Laboratory, University of Avignon, France. The results are returned to 2-sigma and the uncertainty associated with $^{3}$H results ranges from 0.2 to 0.6 TU (Table 3).

## 3.3 Groundwater storage calculations

Water table fluctuations, when measured over a seasonal-scale in shallow aquifers, represent changes in groundwater storage ($\Delta S_{gw}$) (Healy and Cook, 2002; Koïta et al., 2018). To quantify changes in wet season groundwater storage in the study area the equation (1) (Healy and Cook, 2002) was used;

$$ \Delta S_{gw} = S_y \cdot \Delta h \qquad (1) $$

Where the difference in hydraulic head values ($\Delta h$) over a one year period is calculated as the change in hydraulic head from the peak of the wet season to the point exponentially extrapolated from the antecedent recession curve (Healy and Cook, 2002). Specific yield values ($S_y$) were estimated from the collation presented by Johnson (1967) based on the lithological logs provided by DWRPIS for each bore in the shallow $q_{p2-3}$ aquifer; medium to rough sand mixed with gravel for bores WL2 and WL3 ($S_y$ = 0.25), and fine to medium sand for bores WL1, WL4 and WL5 ($S_y$ = 0.21). These values are similar with the values calculated for the shallow $q_{p2-3}$ aquifer in the HCMC region, which averaged 0.26 (Vuong, 2010).

## 3.4 Geochemical modelling

A component mixing model was used to determine the relative recharge to the aquifer from wet season rainfall. The model used to calculate wet season rainfall recharge is based on monthly time step calculations of the equation (2) described in Clark and Fritz (1997).
$$F_r = (C_{\text{mix}} - C_{\text{gw}}) / (C_r - C_{\text{gw}}) \qquad (2)$$

where the fraction of rainfall contribution to the groundwater aquifer ($F_r$) at the site is calculated using the $\delta^{18}$O values for groundwater from the aquifer one month prior ($C_{gw}$), the $\delta^{18}$O values of groundwater after recharge ($C_{mix}$), and the weighted $\delta^{18}$O values of rain water from the previous month ($C_r$). The values used to calculate $F_r$ and the $F_r$ results for groundwater sites GW3 and GW4 are presented in Table 1.

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## 4. Results

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### 4.1 Comparison of seasonal changes in rainfall and river water

During the sampling period for this study (Jan-18 to May-20), the seasonal rainfall peaks occurred during the months of May to November in 2018 and May to October in 2019 (Fig. 2a). In comparison, seasonal river level peaks are observed during October 2018 to January 2019 for the first year, and during October 2019 to February 2020 for the second year (Fig. 2b). Therefore, there is a time lag in the peak of the river water levels of up to 5 months after the rainfall peak. The river water levels shows little correlation with rainfall due to many factors, including the regulation by the upstream Dau Tieng Dam and tidal influences (Camenen et al., 2020).

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### 4.2 Comparison of annual groundwater level fluctuations and rainfall

Hydraulic head data from the government monitored groundwater bores that are screened within the same shallow aquifer (qp2-3) and are located nearby the sites sampled for stable isotopes (Fig. 1b), highlights significant wet season fluctuations (Fig. 3a). From 1994-2015, for the five monitoring bores the groundwater fluctuations increased during the wet season on average by 2.3-4.4 m, with maximum increases of 4.9-7.7 m. The groundwater hydraulic heads mostly peaked during the month of November, and therefore at the end of the rainfall wet season, showing a time lag of 6 months from the start of the rainfall season (typically in May). This indicates that groundwater from these sites are more influenced by rainfall rather than river recharge.

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The calculated values of annual changes in groundwater storage for each year of the monitoring data are presented in figure 3b, and when plotted relative to the annual rainfall values this highlights that rainfall is not the only influencing factor on changes in groundwater storage. When the storage and rainfall data are linearly correlated the $R^2$ values range from 0.35 to 0.52. The highest correlation is at site WL1, which also shows the lowest increases in hydraulic heads during the wet season (average of 2.3 m). In comparison, sites WL2 and WL4 show the lowest correlation values between changes in
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groundwater storage and rainfall, and these sites also have the highest increases in hydraulic heads during the wet season (4.4 and 4.0 m respectively). Therefore, and especially at sites WL2 and WL4, the large seasonal fluctuations in ground storage are also potentially influenced by other factors such as declines in the water table due to dry season groundwater pumping, and/or lateral flowing groundwater that has originated from rainfall recharge in other regions. The changes in groundwater storage calculated using the water table fluctuation method over the seasonal scale represents both recharge and discharge processes, and storage values are heavily influenced by changes in pressure effects during these processes. Therefore, it is not possible to analyse seasonal recharge processes in the study area just by using this method and the seasonal hydraulic head data.

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## 4.3 Stable isotopes

### 4.3.1 Rainfall, river and groundwater

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The stable isotope values of the rainfall samples at HCMC show a large range in values for both δ¹⁸O (-12.07 to -0.98 ‰) and δ²H (-84.8 to 7.6 ‰), and the weighted annual averages for δ¹⁸O and δ²H values are lower in 2018 (-6.96 and -49.0 ‰) compared with 2019 (-5.89 and -36 ‰; Figure 4a). The lowest values are observed during the heavier rainfall events, and towards the end of the rainfall season (Fig. 2a). During 2018 and 2019, lowest isotope values were observed in November (weighted monthly average δ¹⁸O = -12.07 and δ²H = -84.8 ‰) and September (weighted monthly average δ¹⁸O = -9.01 and δ²H = -64.0 ‰) respectively. Lower isotope values are typical of intense wet season rainfall events for this region of SE Asia (Aggaarwal et al., 2004).

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River isotope values are plotted relative to the local (HCMC) meteoric water line (LMWL) calculated from the 41 HCMC rain samples collected during this study (Fig. 4a). Most of the water sampled from the shallow profile of the river indicate evaporation is occurring, which results in a large range in river δ¹⁸O (-7.74 to -2.22 ‰) and δ²H (-52.3 to -27.3 ‰) values (Fig. 4a). The evaporation is occurring mostly during the dry season months, resulting in increasing values during the months January to April (Fig. 2b). The Saigon River depth ranges between 5-18 m (Nguyen et al., 2019), and in 2017 samples for stable isotopes in the river were collected at the surface (within 30 cm) and at the bottom of the river along a longitudinal profile of 18 m. Results showed an isotopic stratification where the river water was evaporated in the upper profile of the water column (-6.54 to -7.02, and -50.72 to -52.09 ‰ for δ¹⁸O and δ²H respectively), and the water at the bottom of the water column was not evaporated (-8.19 to -8.24, and -55.25 to -55.29 ‰ for δ¹⁸O and δ²H respectively).

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Groundwater isotope values are also plotted relative to the LMWL, and the weighted average values for different rainfall event intensities (Fig. 4a). The results highlight that the groundwater values for both δ¹⁸O (-7.00 to -6.45 ‰) and δ²H (-45.3 to -41.9 ‰) are closest to values of medium rainfall (100-150 mm), and groundwater values also range in between the weighted average rainfall values for 2018 and 2019 (Fig. 4a). The groundwater isotope values close to the MWL also indicate that there is no significant evaporation or mixing with evaporated waters.

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### 4.3.2 Seasonal variations in groundwater δ¹⁸O

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The variability in isotope values from groundwater sampled during the wet season (May to October 2019; rainfall levels 204-514 mm/month) compared with the dry season (November 2019 to March 2020; rainfall levels < 100 mm/month) is analysed for each site (Fig. 4b). The results highlight that groundwater from site GW3 has the highest difference in wet-dry season variations, with maximum changes in wet season values up to 0.56 ‰ compared with maximum dry season changes of 0.08 ‰. This is the only site where the differences in changes during the wet and dry season are above the uncertainty associated with the δ¹⁸O values (0.10 ‰; Fig. 4b).

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Wet season variability of groundwater δ¹⁸O values is relatively high at both sites GW3 and GW4 (maximum wet seasons changes are 0.56 and 0.31 ‰ respectively) compared with all other sites (≤ 0.17), which is possibly due to differences in unsaturated and saturated zone matrix properties. Since rainfall δ¹⁸O values have a high variability (for example σ = 1.93 ‰ during the 2019 wet season), these results may indicate that at sites GW3 and GW4 there is more rapid wet season rainfall recharge compared with the other sites. Relatively rapid unsaturated and saturated zone vertical flow allows us to capture the variability of the rainfall δ¹⁸O values. In comparison, seasonal δ¹⁸O groundwater values are likely more homogenous where there is slower (or lower) vertical flow, and therefore time for mixing, and a groundwater system that is buffered from the event- or even seasonal-scale variations in rainfall isotopes.

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Since groundwater at sites GW3 and GW4 showed highest δ¹⁸O variability during the wet season, these sites were selected for isotope modelling. The results of modelling the fraction of rainfall recharge contribution to the aquifer at sites GW3 and GW4 during the wet season are presented in figure 4c. At site GW3 the results highlight that the estimated fraction of rain (F_r) in the groundwater varied from 16 % in July to 2 % in October. In comparison, at site GW4, the monthly rainfall recharge contributions were only up to 5 % in October. Based in these F_r results and the lower wet season changes in δ¹⁸O

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values at GW4 compared with GW3 (Fig. 4b), there may be relatively lower vertical flow rates at GW4 compared with GW3.

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## 4.4 Chemistry of waters

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When stable isotope values of groundwater are compared with EC values, the results show a relatively high variation in EC values of groundwater that can be divided into two main groups (Fig. 5a). Group one includes groundwater sites (GW2, GW3, GW4) where the EC values are relatively low (20-55 µS/cm; average of 39 µS/cm) and therefore closer to rainfall values (9-30 µS/cm). In comparison, Group two includes groundwater sites (GW1, GW5, GW6, GW8) with relatively high ranges in EC values (75-388 µS/cm; average of 167 µS/cm), which are closer to the ranges observed for river water (all Saigon River samples: 76-280 µS/cm, and where river samples are not evaporated EC < 130 µS/cm).

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The relatively low EC values of Group one highlights a rainfall source for groundwater recharge. This Group one includes groundwater from site GW3, where the δ¹⁸O values indicate high wet season changes due to rainfall recharge (Fig. 5b). The two other sites in this group, GW2 and GW4, are therefore recharged by rainfall but at slower rates compared with GW3. Groundwater from site GW4 shows greater wet season changes of the δ¹⁸O values compared to GW2 (Fig. 5b). These results may therefore indicate that the order from most rapid to slowest wet season rainfall recharge is GW3, GW4 and GW2.

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The relatively high EC values of Group two (GW1, GW5, GW6 and GW8) highlight either (i) a rainfall source for groundwater recharge with subsequent water-rock interaction processes increasing EC values, or (ii) recharge of a surface water source (non-evaporated), since the groundwater EC are similar to unevaporated Saigon River values during the months when river water levels are highest (Fig. 5c).

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Apart from EC values, a major distinction between the Group one and two are the SiO₂/Cl ratios, with Group one having higher ratios (5.8-9.5) compared with Group two (0.3-2.4; Fig. 6a). These differences in ratios are driven by a change in Cl rather than SiO₂, where Group two groundwater has higher Cl concentrations compared with Group one (Table 3). Relatively high Cl ratios in Group two groundwater compared with Group one may result from agricultural (fertiliser and manure) and domestic (septic leakage) contamination of groundwater. There are also relatively high nitrogen concentrations (as NO₃⁻) associated with high Cl concentrations at some of the sites (GW1 and GW8: NO₃⁻ up to 29.7 mg/L; Fig. 6b), which appear to confirm this contamination. High NO₃⁻ concentrations are not the result of river

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390 recharge to groundwater because the river NO₃ concentrations are relatively low (≤ 1.8 mg/L; Table 2).

391 However, many sites in Group two also show relatively low NO₃ concentrations (0.9-5.3 mg/L) within
392 a similar range to groundwater from Group one (NO₃ mg/L = 1.3-4.0; Fig. 6b).

393

394 Groundwater from both Group one and two have relatively high SiO₂ concentrations (10.5-13.8 mg

395 SiO₂/L, except for site GW6 where SiO₂ < 7mg/L; Table 3). This is particularly the case for Group one

396 groundwater (GW2, GW3 and GW4) where the SiO₂ concentrations are 74-80% of the TDS values. High

397 SiO₂ concentrations are indicative of mineral dissolution reactions occurring during infiltration of

398 rainfall via the heavily leached acrisol soils. Infiltration via these soils can result in relatively high Si

399 concentrations relative to contributions from other ions, and low groundwater pH values (Fig. 6a).

400

401 **4.5 pH of waters**

402 The groundwater pH values range from 3.6-5.6 (average of 4.2; Table 1), with the lowest pH values

403 observed at site GW5, and the highest at site GW6. The Saigon River has higher pH values (4.8-6.1; Fig.

404 6b) compared with groundwater, with the exception of groundwater at site GW6 that has similar

405 ranges in pH (4.6-5.6) to the river water (possibly due to recharge of diverted river water via the

406 irrigation canal). Average rainfall pH values (6.7 at HCMC) are also higher than groundwater values.

407 The influence of the relatively low groundwater pH values in this study area are observed in the

408 upstream longitudinal Saigon River pH values (Fig. 7a); which, as discussed below, highlights a localized

409 phenomenon driving low groundwater pH values.

410

411 The low groundwater pH values shows little correlation with dry or wet season variability or between

412 the two groups (Fig. 7b). At the site GW3, where stable isotopes indicate there is relatively greater

413 rainfall recharge, the standard deviation of the pH during the wet season is low (0.07) compared with

414 values at sites GW4 (0.17) and GW2 (0.29). Therefore, this indicates a relatively stable groundwater

415 pH environment during periods when rainfall recharge is highest.

416

417 **4.6 Tritium of waters**

418 The ³H values of rainfall and river water analysed in this study ranges from 1.3-1.6 and 0.8-3.5 TU

419 respectively (Table 3). These values are consistent with recent studies that have reported rainfall ³H

420 values in Vietnam ranging on average from 2-3.4 TU (Hanoi; Anh et al., 2018) and < 2 TU (Mekong

421 Delta; Nguyen et al., 2007). Therefore, today's atmospheric ³H values reflect natural conditions that

422 are no longer affected by the 1960s bomb pulse.

423
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The tritium ($^{3}$H) results for groundwater are relatively low compared with local rainfall and river water sampled in this study, with values from 0.5 to 1.7 TU (Table 3). These values suggest that groundwater in the semi-confined or confined aquifers in the area is a mixture of recent water ($<$ 60 years old) with older water ($>$70 years old or prior to 1950). There are no significant differences between wet and dry season averages detected at each groundwater site (Fig. 8). Similarly, the difference in average $^{3}$H values between the groundwater sites is insignificant.

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## 5. Discussion

433

434

### 5.1 Recharge processes

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Shallow and fresh groundwater resources in the Mekong Delta are an important resource for inhabitants. Low groundwater TDS can indicate areas of the Delta that receives greater volumes of recent recharge and no seawater intrusion compared with the more saline (TDS > 1g/L) areas of the southern Mekong Delta (Fig. 9). Across much of the Mekong Delta the Upper-Middle Pleistocene aquifer system is semi-confined by the shallow clay-rich Holocene alluvial deposits, which increase in thickness (and therefore effectiveness as a confining unit) from north to south. Therefore, the northern Mekong Delta and surrounding edges are important areas for groundwater recharge (e.g. Minderhound et al., 2000).

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Transient dating tracers, such as $^{3}$H, highlight mixing between recent water ($<$ 60 years old) with older water ($>$ 70 years old), and are therefore useful to identify regions where there is recent recharge. Across the Mekong Delta, the $^{3}$H data for groundwater from the low salinity areas of the Pleistocene aquifer is collated in Figure 9. Where the mean residence times of groundwater in the Pleistocene aquifer indicates recent recharge and where the TDS is relatively low, this indicates important areas of local recharge for the Delta. In the study area, located in the northeast of the Mekong Delta, the shallow clay layers are discontinuous, which results in recent recharge. This is consistent with a more regional analysis of the $^{3}$H values of groundwater undertaken by Vinh et al. (2017; Fig. 9).

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Recent recharge was also observed for groundwater sampled from the underlying Lower Pleistocene aquifer. Dung et al. (1992) also reported that the lowest groundwater radiocarbon mean residence times in the Pliocene aquifer were observed in the northeast of the Mekong Delta (corresponding with this study area). Recharge of this semi-confined deeper aquifer, may be due either to the discontinuity of the clay-rich layer that normally confines the aquifer, or is the result of the installation of boreholes,
458

which has been found to induce vertical leakage between aquifers in the Mekong Delta (Ha et al., 2019a).

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461

Rainfall recharge of both the Upper-Middle and Lower Pleistocene aquifers imply that this is an important recharge area for aquifer systems of the Mekong Delta. In comparison, studies in the northern margin of the Cambodian Mekong Delta shows varied results. In a study by Tweed et al. (2020), the apparent ³H concentrations were below detection limits in groundwater from the Upper-Middle Pleistocene aquifer in the Cambodian section of the Delta (Fig. 9). Whereas, a separate study in the northern area of the Cambodian Mekong Delta highlighted that where the shallow clay layers are incised by rivers or wetlands, the groundwater ages of the Pleistocene aquifer can be relatively recent (e.g. < 60 years, Lawson et al., 2013; Richards et al., 2019; Fig. 9).

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Along the Mekong groundwater basin margins, the relatively younger groundwater areas also correspond to where there are fewer or thinner layers of Holocene (clay-rich) deposits, and increased outcrops of the Upper-Middle Pleistocene aquifer (e.g. Ha et al., 2019b). In deltas where surface sediments are dominated by clay-rich Holocene deposits, regional recharge rates are often low, and therefore localized recharge processes take on an important role for groundwater resources. At the regional-scale, spatial variabilities in the recharge rates of the Mekong Delta have been associated with the location of rivers (e.g. Duy et al., 2021) and the thickness of the Holocene sediments (e.g. Minderhound et al., 2020). In the study area, average daily water levels of the Saigon River from the Thu Dau Mot monitoring station are up to 0.91 m (Fig. 2b). Also data from a previous study by Ha (2015) showed that at other stations along the Saigon River the water levels are also generally less than 2 m AMSL. In comparison the monitored groundwater levels from WL1-WL4 are all > 2 m AMSL (boreholes are located 1.3-3.8 km from the river; Fig. 3a). Therefore, there is a hydraulic gradient that indicates flow from the shallow aquifer at sites WL1-WL4 towards the river. The δ¹⁸O, δ²H and EC results in this study indicate that for the groundwater sites sampled in this study, there is no recharge by evaporated river water, but in close proximity to the river (e.g. site GW8 ≤ 60 m from the river) there is potentially seasonal recharge by unevaporated river water to the aquifer during high river flows. In addition, in close proximity to irrigation channels (e.g. site GW6 ≤ 20 m from the channel) there is potential recharge by irrigation water. Further work is required to analyse spatial-temporal dynamics of near river bank recharge to the aquifer and the extent of recharge from local irrigation practices.

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Stable isotopes are commonly used in catchment-scale recharge studies because they integrate small-scale variability (González-Trinidad et al., 2017). In this study, the stable isotopes helped to identify the

492
spatial heterogeneity in seasonal recharge processes. For example, the calculations of the fraction of recent rain in groundwater from the δ¹⁸O modelling highlighted an increased fraction of rainfall at site GW3 (12-16 %) compared with site GW4 (≤ 5%). These values are similar or lower to vertical infiltration rates (mean values 12-29% monthly rainfall) calculated from iodine-131 tracing tests in the unsaturated zone, however this was undertaken in an area where the Middle Pleistocene aquifer outcrops at the surface (Nguyen et al. 2013). The isotope modelled recharge values are also lower in comparison to the calculations of changes in groundwater storage using the water table fluctuation method in this study. For example, the annual changes in groundwater fluctuations at site WL1 showed the greatest correlations with rainfall compared with the other sites WL2-WL5 (Fig. 4b), and the changes in groundwater storage at site WL1 was on average 23% of rainfall (with a range in values from 9-45 % over the 10-year monitoring period). These results are relatively high since the water table fluctuations are also affected by dry season increases in pumping, the values are therefore an over-estimation of net recharge to the aquifer.

506

## 5.2 Recharge and groundwater quality

Groundwater salinity and high arsenic concentrations are the most commonly discussed contamination risks for the shallow groundwater aquifers of the Mekong Delta (e.g. Buschmann et al., 2008; Richards et al., 2017). In the NE region of the Delta, in the study area, the groundwater system has both low groundwater salinity and low arsenic concentrations (< 50 µg/L; Fig. 9). However, there remain other groundwater quality issues such as the (i) low groundwater pH values, which has resulted in the mobilisation of trace metals, is corrosive to infrastructure and is a health risk for dependent ecosystems, and (ii) additional contaminants from the land surface including sources from domestic and agricultural practices (e.g. Ha et al., 2022).

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The low groundwater pH results observed in this study are consistent with previous work by Nguyen et al. (2014) who reported pH values ranging from 3 to 5.5 from groundwater of the Upper-Middle Pleistocene aquifer located on the opposite side of the Saigon River, and by Strady et al. (2017) who also observed decreases in the upper Saigon River pH values compared with downstream values. Others studies by Ha (2019) and Ha et al. (2022) also found that groundwater pH < 5 (as low as 3.2) is commonly observed in the high altitude area (elevation > 5 m AMSL) of southern Vietnam. In this study area the infiltration of rainfall via the acrisol soils naturally results in the low groundwater EC, which is a common occurrence in many tropical humid recharge areas where the soil and rocks resist weathering (e.g. Knutsson, 1994). The acrisol soils of the study area are characterised as being rich in stable clays (e.g. kaolinite, halloysite), quartz and Fe (hydr)oxides (e.g. goethite), poor in organic matter contents, low in cations, and low in pH (Ha et al., 2019b). Due to the low contributions of cations and
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alkalinity to groundwater in this environment, the groundwater is sensitive to acidification. A previous study in the area by Ha (2019) found that only minor amounts of nitrification (ie. where NO₃⁻ concentrations remain < 5 mg/L) in this low alkaline groundwater results in the acidification of groundwater, and subsequently mobilises trace metals, including aluminium and lead. This is distinct from the processes controlling groundwater acidification along the coastline of the Saigon River Basin and throughout large areas of the Mekong Delta, where pyrite oxidation plays a major role in controlling the pH of groundwater (Ha et al., 2019a, b). This is also distinct from many studies worldwide, where acidic groundwater has been attributed to atmospheric depositions (e.g. Hicks et al. 2008). The results of this study highlighted that during high local rainfall recharge of the Upper-Middle Pleistocene aquifer, and therefore high vertical inflows, the pH of groundwater remains relatively low and stable (site GW3), compared with other sites where there is less rainfall recharge and slower vertical flows (e.g. sites GW2 and GW4; Fig. 7b). Therefore, the nitrification reactions resulting in low groundwater pH values are occurring during the infiltration of recharging rainfall, and following this infiltration the pH values remain relatively stable.

Results from this study highlight that both the shallow (Upper-Middle Pleistocene) and underlying (Lower Pleistocene) aquifers may not be protected from surface contaminant transfers by continuous extended shallow clay layers. These clay layers do not act as effective generalized barriers of rainfall infiltration, even if in some sites much slower vertical recharge is noticed. Therefore, the spatio-temporal variations in NO₃⁻ concentrations observed in groundwater in this study area are likely related to the origins of the contaminants. The rural study area provides many opportunities for nitrate contamination sources, including agricultural practices (fertilisers, animal manure) and domestic wastewater (e.g. Nguyen et al. 2013; Nguyen et al 2019). At site GW8, the high NO₃⁻ concentrations (20.4-29.7 mg/L) are not the result of river recharge, but instead are likely the result of rainfall recharge transfers of agricultural fertiliser, or are the result of nitrification processes (since NH₄⁺ concentrations are low; 0.3-0.5 mg/L). In comparison, at site GW6, the relatively high NH₄⁺ concentrations (0.7-1.4 mg/L) compared will all other sites (< 0.7 mg/L), may indicate contamination via wastewater or animal manure. EC, pH and SiO₂/Cl data indicate that this site is potentially recharged from irrigation water, and so the contamination may be a result of natural fertiliser applications for the irrigation crop. Several previous studies have also suggested that the high NO₃⁻ concentrations in shallow groundwaters (< 50 m depth from land's surface) of southern Vietnam is caused by anthropogenic activities, such as wastewater leakage and fertilizer application (Nguyen et al., 2013; Ha, 2019; Ha et al., 2022).
However, at most groundwater sites the NO₃ concentrations remain low, for example five of the seven groundwater sites have NO₃ concentrations that remain ≤ 5.3 mg NO₃/L. The low NO₃ groundwater concentrations are not due to changes in N speciation, since values for NO₂ and NH₄ also remain low (Table 1). Neither are low NO₃ concentrations due to denitrification, since they are not associated with declines in the dissolved oxygen of groundwater (Table 1), and there may be an inhibition of denitrification in these groundwaters where pH is ≤ 5.6 (e.g. Huno et al., 2018). Previous studies have shown that the presence of acrisol soils have resulted in decreases in NO₃ concentrations in soil water and groundwater due to NO₃ adsorption (e.g. Harmand et al., 2007). Within the pH ranges for these groundwaters, adsorption of NO₃ may be occurring with the positive surface charges of goethite minerals, since the maximum surface complexation occurs at pH < 6.0 (e.g. Drever, 1997). Adsorption of NO₃ to mineral sites may therefore result in a delay, and not a reduction, in the transfers of NO₃ to the groundwater system.

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## 576 Conclusions

In the northeastern region of the Mekong Delta the shallow aquifers are predominantly recharged by heavy (> 100 mm/month) wet season rainfall events. Both the shallow (Upper-Middle Pleistocene) and underlying (Lower Pleistocene) aquifers have detectable ³H throughout both wet and dry seasons, therefore they both have received recent recharge. However, the variations in seasonal fluctuations of stable isotopes highlighted significant local variations in rainfall recharge rates that reflect the heterogeneous distribution of sands and clays. The infiltration of rainfall recharge via the acrisol soils results in low EC and alkalinity groundwater, and therefore low pH values due to nitrification processes. Although there is a hydraulic connection with the Saigon River, recharge from the river to groundwater may only seasonally influence shallow groundwater that is located in proximity to the river bank (< 60m), and via irrigation diversions.

587

Identifying such localized processes is important for improved comprehension and representation of groundwater recharge processes. This is particularly important for the protection of recharge areas from contamination, which is the major issue facing groundwater resources in wet tropical regions. For the Mekong Delta, this north eastern area, where there is no salinity or arsenic issues, is a potentially critical zone for groundwater resources in the Basin, and so should be protected particularly in areas identified with relatively rapid recharge rates.
594 Acknowledgements

595 This research was conducted thanks to the financial, technical and human support of the CARE-RESCIF
596 initiative (http://carerescif.hcmut.edu.vn/) within the International Joint Laboratory LECZ-CARE
597 created between IRD and HCMUT. It is also financially supported by HCMC University of Technology
598 (HCMUT), VNU-HCM grant Tc-YIPP, and the IAEA Coordinated Research Project F33024.

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601
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<figure><img src="image_2.png" /><figcaption>Figure 1. (a) Location of the shallow (Holocene) aquifer in the Mekong Delta (after Buschmann et al., 2008). (b) Location of the sampling sites in this study for groundwater and Saigon River water, and the location of the groundwater bores that were monitored for hydraulic head data by the DWRPIS between 1993-2015. Image is from Google Earth Pro (5/11/2019). (c) Schematic section of the different aquifer depths in the study area; data from the lithological section of a bore (TU5C) next to the Saigon River (DWRPIS).</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2. (a) Monthly Rainfall levels and volume-weighted monthly average δ¹⁸O values of rain, (b) average daily water levels of the Saigon River from the Thu Dau Mot monitoring station (data from the Southern Regional Hydrometeorological Center, Vietnam) and river δ¹⁸O values, and (c) groundwater δ¹⁸O values. River stable isotope samples were collected at the sites PC (January 2018 to 1-July 2019; located 10km downstream from the study area: 10.980959°N, 106.643781°E) and RW2 (8-July 2019 to May 2020; location shown in Fig. 1b).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3. (a) Groundwater elevations from national groundwater monitoring bores in the study area from 1994 to 2015 (data from DWRPIS, 2018). (b) The annual rainfall compared with annual change in groundwater storage calculated using the hydraulic head data of the national groundwater monitoring bores.</figcaption></figure>
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

Figure 4. (a) δ²H and δ¹⁸O of groundwater, river water and rainwater (monthly weighted averages for the wet season in 2019) collected in the study area and relative to the meteoric water lines for Ho Chi Minh City (HCMC) and the global MWL (Craig, 1961). (b) Maximum changes in monthly δ¹⁸O values during the wet season (May to October 2019) and dry season (November 2019 to March 2020) at each groundwater site. Error bars represent the 0.10 ‰ analytical uncertainty. (c) Results of stable modelling using δ¹⁸O data for groundwater at sites GW3 and GW4.
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

Figure 5. (a) Rain and river average $δ^{18}O$ and EC values (open circles; with minimum and maximum values as error bars) compared with groundwater sampled from the different sites. (b) Maximum wet season changes in the $δ^{18}O$ values at each site compared with average EC values. (c) Changes in EC in groundwater over the sampling period, compared with EC of unevaporated Saigon River water and black box highlighting the months when the Saigon River water levels are at a maximum.
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>Figure 6. (a) SiO₂/Cl ratios and pH values, and (b) Cl and NO₃ concentrations for Group one and two groundwater samples.</figcaption></figure>
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>Figure 7. (a) Longitudinal Saigon River values of pH during the dry and wet season of 2019 (data from Nguyen et al., in revision), where 0 km is the start of the river profile located 15km upstream of the study area. (b) Box and whisker plot for pH using groundwater data during the wet season (May to October 2019), and the dry season (November 2019 to March 2020).</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Figure 8. Average values of ³H for the wet (Jul-Oct 2019) and dry (Nov 2019-Feb 2020) seasons at each groundwater site. The analytical error for each average value are also presented as the error bars.</figcaption></figure>
<figure><img src="image_16.png" /></figure>

Figure 9. Areas of the shallow aquifers (groundwater sample depth ≤ 100 m) in the Mekong Delta where groundwater has TDS values < 1 and > 1 g/L (after Buschmann et al., 2008), and areas where the arsenic (As) in groundwater from aquifers is > 50 µg/L in the Cambodian (after Buschmann et al., 2008) and Vietnamese (after Erban et al., 2013) parts of the Delta. Also shown in areas of low TDS are where the groundwater ³H mean residences times are greater and less than 60 years (data from 1: Lawson et al., 2013 and Richards et al., 2017; 2: Tweed et al., 2020; 3: this study; 4: Vinh et al., 2017; and 5: Doan et al., 2018).
<table><caption>Table 1. Results of EC, pH, N species concentrations and stable isotopes of groundwater sampled from Jan-2019 to May-2020, and parameters and results of the isotope modelling for two sites.</caption><thead><tr><th>Sample ID</th><th>Date</th><th>EC</th><th>pH</th><th>DO</th><th>NH<sub>4</sub></th><th>NO<sub>3</sub></th><th>NO<sub>2</sub></th><th>δ<sup>11</sup>O %</th><th>δ<sup>15</sup>H %</th><th>δ<sup>18</sup>O % values for F<sub>r</sub>, calculations</th><th>C<sub>min</sub></th><th>C<sub>max</sub></th><th>C<sub>r</sub></th><th>F<sub>r</sub></th></tr><tr><th></th><th></th><th>μS/cm</th><th></th><th>mg/L</th><th>mg/L</th><th>mg/L</th><th>mg/L</th><th>VSMOW</th><th>VSMOW</th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td rowspan="15">GW1</td><td>24-Jan-19</td><td>282</td><td>4.30</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.60</td><td>-43.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-19</td><td>278</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.67</td><td>-43.2</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Mar-19</td><td>280</td><td>4.20</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-5.93</td><td>-41.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Apr-19</td><td>276</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.76</td><td>-43.3</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>26-May-19</td><td>304</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.85</td><td>-44.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>23-Jun-19</td><td>278</td><td>4.00</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.87</td><td>-43.8</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-Jul-19</td><td>290</td><td>4.00</td><td>n.a.</td><td>0.15</td><td>7.97</td><td>0.00</td><td>-6.72</td><td>-42.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>30-Aug-19</td><td>292</td><td>3.97</td><td>n.a.</td><td>0.09</td><td>5.76</td><td>0.00</td><td>-6.88</td><td>-44.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>02-Oct-19</td><td>291</td><td>3.94</td><td>n.a.</td><td>0.18</td><td>10.19</td><td>0.01</td><td>-6.81</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Nov-19</td><td>327</td><td>4.00</td><td>2.0</td><td>0.39</td><td>11.51</td><td>0.00</td><td>-6.80</td><td>-43.7</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Dec-19</td><td>320</td><td>4.13</td><td>2.6</td><td>0.28</td><td>12.84</td><td>0.01</td><td>-6.80</td><td>-43.8</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Jan-20</td><td>315</td><td>4.08</td><td>1.7</td><td>0.19</td><td>10.19</td><td>0.01</td><td>-6.80</td><td>-43.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-20</td><td>322</td><td>5.30</td><td>2.3</td><td>0.39</td><td>10.63</td><td>0.01</td><td>-6.74</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-May-20</td><td>304</td><td>4.65</td><td>0.1</td><td>0.19</td><td>11.07</td><td>0.01</td><td>-6.81</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="15">GW2</td><td>24-Jan-19</td><td>34</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.77</td><td>-43.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-19</td><td>40</td><td>4.00</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.63</td><td>-43.2</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Mar-19</td><td>44</td><td>4.10</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.45</td><td>-42.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Apr-19</td><td>48</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.81</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>26-May-19</td><td>40</td><td>4.10</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.92</td><td>-43.8</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>23-Jun-19</td><td>34</td><td>4.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.92</td><td>-43.7</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-Jul-19</td><td>38</td><td>4.07</td><td>n.a.</td><td>0.04</td><td>1.33</td><td>0.00</td><td>-6.96</td><td>-44.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>30-Aug-19</td><td>36</td><td>4.04</td><td>n.a.</td><td>0.12</td><td>3.10</td><td>0.01</td><td>-6.90</td><td>-44.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>02-Oct-19</td><td>39</td><td>3.91</td><td>n.a.</td><td>0.04</td><td>2.21</td><td>0.00</td><td>-6.79</td><td>-43.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Nov-19</td><td>41</td><td>4.17</td><td>2.2</td><td>0.19</td><td>2.66</td><td>0.01</td><td>-6.78</td><td>-43.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Dec-19</td><td>40</td><td>4.19</td><td>2.5</td><td>0.04</td><td>2.21</td><td>0.00</td><td>-6.64</td><td>-43.2</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Jan-20</td><td>39</td><td>3.78</td><td>3.3</td><td>0.03</td><td>2.21</td><td>0.00</td><td>-6.93</td><td>-44.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-20</td><td>41</td><td>4.11</td><td>2.2</td><td>0.08</td><td>1.77</td><td>0.00</td><td>-6.83</td><td>-43.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-May-20</td><td>40</td><td>4.88</td><td>1.8</td><td>0.08</td><td>1.77</td><td>0.01</td><td>-6.96</td><td>-44.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="15">GW3</td><td>24-Jan-19</td><td>52</td><td>4.00</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.63</td><td>-42.2</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-19</td><td>48</td><td>4.00</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.26</td><td>-41.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Mar-19</td><td>50</td><td>3.90</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.65</td><td>-42.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Apr-19</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.52</td><td>-42.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>26-May-19</td><td>54</td><td>4.10</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.67</td><td>-42.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>23-Jun-19</td><td>52</td><td>3.90</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.91</td><td>-43.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-Jul-19</td><td>55</td><td>4.08</td><td>n.a.</td><td>0.04</td><td>1.77</td><td>0.00</td><td>-6.65</td><td>-42.6</td><td>-6.65</td><td>-6.91</td><td>-5.24</td><td>0.16</td><td></td></tr><tr><td>30-Aug-19</td><td>54</td><td>4.05</td><td>n.a.</td><td>0.09</td><td>3.99</td><td>0.00</td><td>-6.35</td><td>-42.0</td><td>-6.35</td><td>-6.65</td><td>-4.49</td><td>0.14</td><td></td></tr><tr><td>02-Oct-19</td><td>54</td><td>4.06</td><td>n.a.</td><td>0.03</td><td>3.10</td><td>0.00</td><td>-6.66</td><td>-42.3</td><td>-6.70</td><td>-6.40</td><td>-9.01</td><td>0.12</td><td></td></tr><tr><td>15-Nov-19</td><td>55</td><td>4.01</td><td>3.6</td><td>0.21</td><td>3.10</td><td>0.00</td><td>-6.68</td><td>-42.7</td><td>-6.66</td><td>-6.70</td><td>-4.18</td><td>0.02</td><td></td></tr><tr><td>27-Dec-19</td><td>52</td><td>4.06</td><td>3.4</td><td>0.03</td><td>3.99</td><td>0.00</td><td>-6.63</td><td>-42.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Jan-20</td><td>51</td><td>4.10</td><td>3.2</td><td>0.03</td><td>3.54</td><td>0.00</td><td>-6.72</td><td>-42.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-20</td><td>53</td><td>4.23</td><td>2.2</td><td>0.09</td><td>3.54</td><td>0.01</td><td>-6.66</td><td>-42.2</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-May-20</td><td>53</td><td>4.78</td><td>2.1</td><td>0.06</td><td>3.10</td><td>0.00</td><td>-6.67</td><td>-42.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="15">GW4</td><td>24-Jan-19</td><td>22</td><td>4.40</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.68</td><td>-43.3</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-19</td><td>22</td><td>4.50</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.79</td><td>-43.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Mar-19</td><td>24</td><td>4.40</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.85</td><td>-43.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Apr-19</td><td>28</td><td>4.30</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.90</td><td>-44.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>26-May-19</td><td>36</td><td>4.50</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.04</td><td>-41.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>23-Jun-19</td><td>20</td><td>4.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.87</td><td>-44.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-Jul-19</td><td>28</td><td>4.44</td><td>n.a.</td><td>0.03</td><td>3.10</td><td>0.01</td><td>-6.93</td><td>-44.3</td><td>-6.93</td><td>-6.87</td><td>-5.2</td><td>0.00</td><td></td></tr><tr><td>30-Aug-19</td><td>25</td><td>4.35</td><td>n.a.</td><td>0.64</td><td>1.77</td><td>0.00</td><td>-6.87</td><td>-44.2</td><td>-6.87</td><td>-6.93</td><td>-4.5</td><td>0.02</td><td></td></tr><tr><td>02-Oct-19</td><td>26</td><td>4.42</td><td>n.a.</td><td>0.05</td><td>1.33</td><td>0.00</td><td>-6.62</td><td>-44.0</td><td>-6.75</td><td>-6.87</td><td>-9.0</td><td>0.00</td><td></td></tr><tr><td>15-Nov-19</td><td>24</td><td>4.37</td><td>2.5</td><td>0.22</td><td>1.77</td><td>0.01</td><td>-6.85</td><td>-44.0</td><td>-6.62</td><td>-6.75</td><td>-4.2</td><td>0.05</td><td></td></tr><tr><td>27-Dec-19</td><td>26</td><td>4.21</td><td>2.5</td><td>0.01</td><td>1.33</td><td>0.00</td><td>-6.86</td><td>-44.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Jan-20</td><td>27</td><td>4.83</td><td>1.0</td><td>0.01</td><td>1.77</td><td>0.00</td><td>-6.86</td><td>-44.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-20</td><td>26</td><td>3.94</td><td>1.2</td><td>0.12</td><td>2.21</td><td>0.00</td><td>-6.73</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-May-20</td><td>25</td><td>5.01</td><td>1.5</td><td>0.04</td><td>1.33</td><td>0.00</td><td>-6.90</td><td>-44.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="15">GW5</td><td>24-Jan-19</td><td>80</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.67</td><td>-43.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-19</td><td>80</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.79</td><td>-43.3</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Mar-19</td><td>88</td><td>3.70</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.81</td><td>-43.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Apr-19</td><td>84</td><td>3.70</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.81</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>26-May-19</td><td>84</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.81</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>23-Jun-19</td><td>88</td><td>3.80</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.85</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-Jul-19</td><td>94</td><td>3.82</td><td>n.a.</td><td>0.03</td><td>0.89</td><td>0.00</td><td>-6.88</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>30-Aug-19</td><td>91</td><td>3.86</td><td>n.a.</td><td>0.12</td><td>4.43</td><td>0.01</td><td>-6.82</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>02-Oct-19</td><td>90</td><td>3.94</td><td>n.a.</td><td>0.03</td><td>3.54</td><td>0.00</td><td>-6.87</td><td>-43.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Nov-19</td><td>94</td><td>4.01</td><td>1.8</td><td>0.10</td><td>3.54</td><td>0.00</td><td>-6.69</td><td>-43.3</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Dec-19</td><td>97</td><td>3.90</td><td>1.6</td><td>0.03</td><td>4.87</td><td>0.01</td><td>-6.74</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Jan-20</td><td>98</td><td>4.27</td><td>1.7</td><td>0.04</td><td>5.31</td><td>0.00</td><td>-5.67</td><td>-40.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-20</td><td>102</td><td>4.11</td><td>1.8</td><td>0.06</td><td>4.87</td><td>0.01</td><td>-6.90</td><td>-43.8</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-May-20</td><td>102</td><td>4.57</td><td>1.8</td><td>0.06</td><td>5.31</td><td>0.01</td><td>-6.98</td><td>-44.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="15">GW6</td><td>28-Feb-19</td><td>108</td><td>4.90</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.95</td><td>-44.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Mar-19</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.77</td><td>-44.3</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>26-May-19</td><td>116</td><td>4.70</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.78</td><td>-43.3</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>23-Jun-19</td><td>106</td><td>4.90</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.81</td><td>-44.0</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-Jul-19</td><td>114</td><td>4.65</td><td>n.a.</td><td>1.36</td><td>2.66</td><td>0.00</td><td>-6.90</td><td>-43.8</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>30-Aug-19</td><td>98</td><td>4.69</td><td>n.a.</td><td>1.34</td><td>3.10</td><td>0.01</td><td>-6.78</td><td>-44.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>02-Oct-19</td><td>89</td><td>4.80</td><td>n.a.</td><td>1.21</td><td>2.21</td><td>0.00</td><td>-6.79</td><td>-44.2</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Nov-19</td><td>80</td><td>4.77</td><td>2.2</td><td>1.16</td><td>3.54</td><td>0.00</td><td>-6.66</td><td>-43.4</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>27-Dec-19</td><td>75</td><td>4.65</td><td>0.1</td><td>0.73</td><td>3.10</td><td>0.00</td><td>-6.66</td><td>-43.6</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Jan-20</td><td>76</td><td>4.93</td><td>0.7</td><td>1.12</td><td>2.21</td><td>0.01</td><td>-5.83</td><td>-40.8</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-20</td><td>79</td><td>5.11</td><td>2.5</td><td>1.32</td><td>2.66</td><td>0.01</td><td>-6.76</td><td>-43.3</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-May-20</td><td>97</td><td>5.57</td><td>0.1</td><td>1.41</td><td>2.66</td><td>0.01</td><td>-7.00</td><td>-45.3</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="6">GW8</td><td>08-Jul-19</td><td>97</td><td>3.93</td><td>n.a.</td><td>0.01</td><td>3.10</td><td>0.02</td><td>-6.70</td><td>-43.1</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>02-Oct-19</td><td>388</td><td>3.86</td><td>n.a.</td><td>0.26</td><td>20.37</td><td>0.01</td><td>-6.80</td><td>-43.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Nov-19</td><td>267</td><td>4.10</td><td>0.5</td><td>0.49</td><td>29.67</td><td>0.01</td><td>-6.65</td><td>-42.8</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>15-Jan-20</td><td>135</td><td>3.62</td><td>3.5</td><td>0.27</td><td>2.66</td><td>0.00</td><td>-6.48</td><td>-41.9</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>28-Feb-20</td><td>106</td><td>4.80</td><td>3.2</td><td>0.21</td><td>4.43</td><td>0.01</td><td>-6.69</td><td>-42.5</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>08-May-20</td><td>102</td><td>4.57</td><td>0.5</td><td>0.06</td><td>5.31</td><td>0.01</td><td>-6.62</td><td>-42.2</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

n.a.: not analysed
<table><caption>Table 2. Results of EC, pH, N species concentrations and stable isotopes of Saigon River water.</caption><thead><tr><th>Sample ID</th><th>Date</th><th>EC µS/cm</th><th>pH</th><th>NH<sub>4</sub> mg/L</th><th>NO<sub>3</sub> mg/L</th><th>NO<sub>2</sub> mg/L</th><th>δ<sup>18</sup>O %c VSMOW</th><th>δ<sup>2</sup>H %c VSMOW</th></tr></thead><tbody><tr><td rowspan="6">RW2</td><td>08-Jul-19</td><td>108</td><td>6.30</td><td>0.24</td><td>0.10</td><td>0.00</td><td>-4.44</td><td>-29.5</td></tr><tr><td>02-Oct-19</td><td>88</td><td>5.95</td><td>0.16</td><td>0.30</td><td>0.00</td><td>-7.30</td><td>-50.4</td></tr><tr><td>15-Nov-19</td><td>118</td><td>6.08</td><td>0.32</td><td>0.50</td><td>0.00</td><td>-5.58</td><td>-35.9</td></tr><tr><td>27-Dec-19</td><td>172</td><td>6.20</td><td>0.21</td><td>0.70</td><td>0.00</td><td>-5.39</td><td>-36.8</td></tr><tr><td>15-Jan-20</td><td>182</td><td>6.21</td><td>0.05</td><td>1.80</td><td>0.00</td><td>-5.28</td><td>-36.3</td></tr><tr><td>28-Feb-20</td><td>122</td><td>6.42</td><td>0.10</td><td>0.50</td><td>0.00</td><td>-4.61</td><td>-33.1</td></tr><tr><td>08-May-20</td><td>114</td><td>6.66</td><td>0.10</td><td>0.50</td><td>0.00</td><td>-3.93</td><td>-27.8</td></tr><tr><td rowspan="5">RW7</td><td>02-Oct-19</td><td>75</td><td>5.76</td><td>0.11</td><td>0.40</td><td>0.00</td><td>-7.06</td><td>-48.6</td></tr><tr><td>15-Nov-19</td><td>82</td><td>5.84</td><td>0.24</td><td>0.50</td><td>0.00</td><td>-4.81</td><td>-32.4</td></tr><tr><td>15-Jan-20</td><td>166</td><td>6.51</td><td>0.21</td><td>0.70</td><td>0.00</td><td>-5.17</td><td>-35.8</td></tr><tr><td>28-Feb-20</td><td>108</td><td>7.20</td><td>0.15</td><td>0.50</td><td>0.00</td><td>-4.76</td><td>-33.4</td></tr><tr><td>08-May-20</td><td>123</td><td>6.75</td><td>0.07</td><td>0.30</td><td>0.00</td><td>-4.17</td><td>-28.2</td></tr><tr><td rowspan="20">PC</td><td>02-janv-18</td><td>113</td><td>6.37</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.00</td><td>-35.9</td></tr><tr><td>15-janv-18</td><td>123</td><td>6.42</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-2.71</td><td>-32.3</td></tr><tr><td>30-janv-18</td><td>106</td><td>6.44</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.54</td><td>-33.7</td></tr><tr><td>27-févr-18</td><td>79</td><td>6.09</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-2.70</td><td>-31.9</td></tr><tr><td>12-mars-18</td><td>109</td><td>6.21</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.18</td><td>-33.0</td></tr><tr><td>27-mars-18</td><td>114</td><td>6.33</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-2.22</td><td>-30.0</td></tr><tr><td>11-avr-18</td><td>125</td><td>6.20</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.56</td><td>-32.5</td></tr><tr><td>26-avr-18</td><td>128</td><td>6.48</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.24</td><td>-34.4</td></tr><tr><td>10-mai-18</td><td>112</td><td>6.08</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.02</td><td>-29.7</td></tr><tr><td>24-mai-18</td><td>93</td><td>6.37</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.04</td><td>-27.8</td></tr><tr><td>11-juin-18</td><td>111</td><td>6.31</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.68</td><td>-31.9</td></tr><tr><td>25-juin-18</td><td>76</td><td>6.15</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.56</td><td>-31.2</td></tr><tr><td>10-juil-18</td><td>88</td><td>6.39</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.40</td><td>-30.3</td></tr><tr><td>24-juil-18</td><td>99</td><td>6.30</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.26</td><td>-29.0</td></tr><tr><td>08-août-18</td><td>160</td><td>6.68</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-2.88</td><td>-27.3</td></tr><tr><td>21-août-18</td><td>177</td><td>6.83</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.09</td><td>-27.7</td></tr><tr><td>06-sept-18</td><td>178</td><td>6.43</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.49</td><td>-29.5</td></tr><tr><td>20-sept-18</td><td>132</td><td>6.36</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.12</td><td>-31.7</td></tr><tr><td>04-oct-18</td><td>93</td><td>6.22</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-5.80</td><td>-41.9</td></tr><tr><td>19-oct-18</td><td>109</td><td>6.22</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.71</td><td>-36.1</td></tr><tr><td>05-nov-18</td><td>114</td><td>6.68</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.46</td><td>-33.6</td></tr><tr><td>19-nov-18</td><td>113</td><td>6.85</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.26</td><td>-28.3</td></tr><tr><td>03-déc-18</td><td>88</td><td>6.79</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-7.74</td><td>-52.3</td></tr><tr><td>17-déc-18</td><td>128</td><td>6.75</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-7.09</td><td>-48.2</td></tr><tr><td>08-janv-19</td><td>116</td><td>6.84</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-6.11</td><td>-41.8</td></tr><tr><td>21-janv-19</td><td>142</td><td>6.79</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-5.56</td><td>-38.3</td></tr><tr><td>18-févr-19</td><td>106</td><td>6.84</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-5.01</td><td>-35.3</td></tr><tr><td>06-mars-19</td><td>280</td><td>7.10</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.96</td><td>-34.3</td></tr><tr><td>19-mars-19</td><td>191</td><td>7.17</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.96</td><td>-34.1</td></tr><tr><td>02-avr-19</td><td>193</td><td>7.03</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.69</td><td>-32.9</td></tr><tr><td>16-Apr-19</td><td>240</td><td>6.35</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-3.70</td><td>-28.8</td></tr><tr><td>02-mai-19</td><td>160</td><td>6.38</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.58</td><td>-31.7</td></tr><tr><td>14-May-19</td><td>164</td><td>6.33</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.38</td><td>-29.9</td></tr><tr><td>30-May-19</td><td>161</td><td>6.30</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-5.78</td><td>-38.9</td></tr><tr><td>13-Jun-19</td><td>159</td><td>6.24</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.84</td><td>-33.6</td></tr><tr><td>01-Jul-19</td><td>169</td><td>6.42</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>-4.03</td><td>-27.8</td></tr></tbody></table>

n.a.: not analysed
<table><caption>Table 3. Major ions and tritium analysed monthly from July 2019 to February 2020 for surface water, rain water and groundwater samples.</caption><thead><tr><th>Sample ID</th><th>Date</th><th>T°C</th><th>pH</th><th>EC<br>μS/cm</th><th>F<br>mg/L</th><th>Cl<br>mg/L</th><th>SO<sub>4</sub><br>mg/L</th><th>Na<br>mg/L</th><th>K<br>mg/L</th><th>Mg<br>mg/L</th><th>Ca<br>mg/L</th><th>SiO<sub>2</sub><br>mg/L</th><th><sup>3</sup>H<br>TU</th><th>±</th></tr></thead><tbody><tr><td rowspan="7">GW1</td><td>Jul-19</td><td>29.1</td><td>4.0</td><td>290</td><td>0.01</td><td>42.8</td><td>&lt;0.02</td><td>38.8</td><td>2.3</td><td>1.0</td><td>2.2</td><td>n.a.</td><td>0.7</td><td>0.2</td></tr><tr><td>Aug-19</td><td>29.2</td><td>3.7</td><td>292</td><td>&lt;0.02</td><td>46.4</td><td>&lt;0.02</td><td>39.7</td><td>2.9</td><td>1.0</td><td>2.2</td><td>12.0</td><td>1</td><td>0.4</td></tr><tr><td>Oct-19</td><td>29.3</td><td>4.0</td><td>291</td><td>&lt;0.02</td><td>43.9</td><td>0.24</td><td>40.8</td><td>2.2</td><td>1.0</td><td>2.0</td><td>12.3</td><td>0.8</td><td>0.2</td></tr><tr><td>Nov-19</td><td>29.3</td><td>4.0</td><td>327</td><td>&lt;0.02</td><td>49.6</td><td>&lt;0.02</td><td>45.4</td><td>2.1</td><td>1.0</td><td>2.3</td><td>12.2</td><td>0.6</td><td>0.2</td></tr><tr><td>Dec-19</td><td>28.3</td><td>3.9</td><td>320</td><td>&lt;0.02</td><td>48.8</td><td>&lt;0.02</td><td>44.2</td><td>2.3</td><td>1.0</td><td>2.3</td><td>12.0</td><td>1</td><td>0.5</td></tr><tr><td>Jan-20</td><td>29.3</td><td>4.0</td><td>320</td><td>0.04</td><td>49.6</td><td>&lt;0.02</td><td>41.8</td><td>2.2</td><td>1.0</td><td>2.2</td><td>11.6</td><td>1.4</td><td>0.4</td></tr><tr><td>feb-20</td><td>29.0</td><td>4.0</td><td>320</td><td>0.65</td><td>50.1</td><td>&lt;0.02</td><td>37.8</td><td>2.9</td><td>0.9</td><td>2.3</td><td>12.1</td><td>1.1</td><td>0.3</td></tr><tr><td rowspan="7">GW2</td><td>Jul-19</td><td>28.4</td><td>4.1</td><td>38</td><td>0.01</td><td>1.4</td><td>&lt;0.02</td><td>1.1</td><td>0.2</td><td>0.3</td><td>0.2</td><td>n.a.</td><td>0.9</td><td>0.2</td></tr><tr><td>Aug-19</td><td>28.4</td><td>3.9</td><td>36</td><td>&lt;0.02</td><td>1.5</td><td>0.04</td><td>1.1</td><td>0.4</td><td>0.3</td><td>0.3</td><td>12.0</td><td>0.9</td><td>0.4</td></tr><tr><td>Oct-19</td><td>28.5</td><td>3.9</td><td>39</td><td>&lt;0.02</td><td>1.4</td><td>&lt;0.02</td><td>1.2</td><td>0.2</td><td>0.3</td><td>0.2</td><td>12.2</td><td>0.9</td><td>0.3</td></tr><tr><td>Nov-19</td><td>28.4</td><td>4.2</td><td>41</td><td>&lt;0.02</td><td>2.2</td><td>&lt;0.02</td><td>1.4</td><td>0.2</td><td>0.3</td><td>0.3</td><td>12.3</td><td>0.6</td><td>0.3</td></tr><tr><td>Dec-19</td><td>28.4</td><td>3.8</td><td>40</td><td>&lt;0.02</td><td>1.6</td><td>&lt;0.02</td><td>1.3</td><td>0.2</td><td>0.3</td><td>0.2</td><td>11.7</td><td>1.1</td><td>0.4</td></tr><tr><td>Jan-20</td><td>28.4</td><td>4.0</td><td>40</td><td>&lt;0.02</td><td>1.6</td><td>&lt;0.02</td><td>1.3</td><td>0.2</td><td>0.2</td><td>0.3</td><td>11.6</td><td>1.0</td><td>0.2</td></tr><tr><td>feb-20</td><td>28.4</td><td>4.0</td><td>40</td><td>&lt;0.02</td><td>1.5</td><td>&lt;0.02</td><td>1.3</td><td>0.2</td><td>0.3</td><td>0.3</td><td>11.6</td><td>0.8</td><td>0.2</td></tr><tr><td rowspan="7">GW3</td><td>Jul-19</td><td>28.5</td><td>4.1</td><td>55</td><td>0.02</td><td>1.8</td><td>&lt;0.02</td><td>2.7</td><td>0.1</td><td>0.3</td><td>0.3</td><td>n.a.</td><td>0.8</td><td>0.3</td></tr><tr><td>Aug-19</td><td>28.6</td><td>3.7</td><td>54</td><td>&lt;0.02</td><td>2.1</td><td>&lt;0.02</td><td>2.7</td><td>0.1</td><td>0.3</td><td>0.2</td><td>11.5</td><td>0.8</td><td>0.3</td></tr><tr><td>Oct-19</td><td>28.7</td><td>3.7</td><td>54</td><td>&lt;0.02</td><td>1.9</td><td>&lt;0.02</td><td>2.2</td><td>0.1</td><td>0.3</td><td>0.3</td><td>11.1</td><td>0.7</td><td>0.3</td></tr><tr><td>Nov-19</td><td>28.6</td><td>4.0</td><td>55</td><td>&lt;0.02</td><td>2.1</td><td>&lt;0.02</td><td>2.2</td><td>0.1</td><td>0.3</td><td>0.8</td><td>11.7</td><td>≤0.5</td><td>-</td></tr><tr><td>Dec-19</td><td>28.7</td><td>3.7</td><td>52</td><td>&lt;0.02</td><td>2.0</td><td>&lt;0.02</td><td>2.2</td><td>0.1</td><td>0.3</td><td>0.4</td><td>11.5</td><td>0.6</td><td>0.5</td></tr><tr><td>Jan-20</td><td>28.7</td><td>3.8</td><td>52</td><td>0.03</td><td>1.7</td><td>&lt;0.02</td><td>2.3</td><td>0.2</td><td>0.2</td><td>0.3</td><td>11.3</td><td>0.5</td><td>0.3</td></tr><tr><td>feb-20</td><td>28.7</td><td>3.8</td><td>52</td><td>&lt;0.02</td><td>1.7</td><td>&lt;0.02</td><td>2.6</td><td>0.1</td><td>0.2</td><td>0.3</td><td>11.0</td><td>0.5</td><td>0.4</td></tr><tr><td rowspan="7">GW4</td><td>Jul-19</td><td>28.9</td><td>4.4</td><td>28</td><td>0.01</td><td>1.9</td><td>&lt;0.02</td><td>2.2</td><td>0.1</td><td>0.1</td><td>0.2</td><td>n.a.</td><td>0.6</td><td>0.2</td></tr><tr><td>Aug-19</td><td>28.9</td><td>4.4</td><td>25</td><td>&lt;0.02</td><td>1.8</td><td>&lt;0.02</td><td>1.8</td><td>0.1</td><td>0.1</td><td>0.4</td><td>12.8</td><td>1</td><td>0.4</td></tr><tr><td>Oct-19</td><td>29.0</td><td>4.2</td><td>26</td><td>&lt;0.02</td><td>1.5</td><td>&lt;0.02</td><td>1.7</td><td>0.2</td><td>0.1</td><td>0.2</td><td>12.7</td><td>0.5</td><td>0.6</td></tr><tr><td>Nov-19</td><td>28.8</td><td>4.4</td><td>24</td><td>&lt;0.02</td><td>1.5</td><td>&lt;0.02</td><td>1.1</td><td>0.1</td><td>0.2</td><td>0.7</td><td>13.3</td><td>0.7</td><td>0.3</td></tr><tr><td>Dec-19</td><td>28.8</td><td>4.0</td><td>26</td><td>&lt;0.02</td><td>1.9</td><td>&lt;0.02</td><td>1.2</td><td>0.1</td><td>0.1</td><td>0.2</td><td>12.9</td><td>1.2</td><td>0.2</td></tr><tr><td>Jan-20</td><td>28.9</td><td>4.8</td><td>26</td><td>&lt;0.02</td><td>1.7</td><td>&lt;0.02</td><td>1.2</td><td>0.1</td><td>0.1</td><td>0.1</td><td>12.5</td><td>1.1</td><td>0.4</td></tr><tr><td>feb-20</td><td>28.9</td><td>4.6</td><td>26</td><td>0.06</td><td>1.6</td><td>&lt;0.02</td><td>1.1</td><td>0.2</td><td>0.1</td><td>0.2</td><td>12.9</td><td>1.0</td><td>0.3</td></tr><tr><td rowspan="7">GW5</td><td>Jul-19</td><td>28.9</td><td>3.8</td><td>94</td><td>0.01</td><td>7.1</td><td>&lt;0.02</td><td>3.7</td><td>0.2</td><td>0.6</td><td>0.6</td><td>n.a.</td><td>≤0.9</td><td>-</td></tr><tr><td>Aug-19</td><td>28.9</td><td>3.5</td><td>91</td><td>&lt;0.02</td><td>7.3</td><td>&lt;0.02</td><td>3.5</td><td>0.2</td><td>0.6</td><td>0.6</td><td>12.9</td><td>1.4</td><td>0.5</td></tr><tr><td>Oct-19</td><td>29.0</td><td>3.4</td><td>90</td><td>&lt;0.02</td><td>7.4</td><td>&lt;0.02</td><td>3.6</td><td>0.2</td><td>0.6</td><td>0.5</td><td>12.9</td><td>1.7</td><td>0.3</td></tr><tr><td>Nov-19</td><td>30.0</td><td>4.0</td><td>94</td><td>&lt;0.02</td><td>6.3</td><td>&lt;0.02</td><td>4.4</td><td>0.3</td><td>0.7</td><td>0.5</td><td>13.8</td><td>≤0.8</td><td>-</td></tr><tr><td>Dec-19</td><td>28.8</td><td>3.5</td><td>97</td><td>&lt;0.02</td><td>7.5</td><td>&lt;0.02</td><td>5.2</td><td>0.3</td><td>0.6</td><td>0.6</td><td>13.3</td><td>0.5</td><td>0.4</td></tr><tr><td>Jan-20</td><td>29.0</td><td>3.5</td><td>97</td><td>&lt;0.02</td><td>7.3</td><td>&lt;0.02</td><td>5.4</td><td>0.3</td><td>0.5</td><td>0.5</td><td>12.9</td><td>0.5</td><td>0.4</td></tr><tr><td>feb-20</td><td>29.0</td><td>3.6</td><td>97</td><td>&lt;0.02</td><td>7.9</td><td>&lt;0.02</td><td>6.4</td><td>0.3</td><td>0.5</td><td>0.6</td><td>13.3</td><td>1.0</td><td>0.5</td></tr><tr><td rowspan="7">GW6</td><td>Jul-19</td><td>28.3</td><td>4.6</td><td>114</td><td>0.02</td><td>12.4</td><td>6.5</td><td>9.2</td><td>7.5</td><td>0.1</td><td>0.3</td><td>n.a.</td><td>0.7</td><td>0.4</td></tr><tr><td>Aug-19</td><td>28.4</td><td>4.9</td><td>98</td><td>&lt;0.02</td><td>10.5</td><td>10.2</td><td>7.5</td><td>6.1</td><td>0.1</td><td>0.1</td><td>6.9</td><td>1.1</td><td>0.2</td></tr><tr><td>Oct-19</td><td>28.6</td><td>4.9</td><td>89</td><td>&lt;0.02</td><td>8.8</td><td>11.1</td><td>6.4</td><td>5.7</td><td>0.1</td><td>0.1</td><td>6.3</td><td>0.9</td><td>0.3</td></tr><tr><td>Nov-19</td><td>28.4</td><td>4.8</td><td>80</td><td>&lt;0.02</td><td>6.2</td><td>6.5</td><td>5.7</td><td>5.5</td><td>0.1</td><td>0.1</td><td>6.5</td><td>0.9</td><td>0.3</td></tr><tr><td>Dec-19</td><td>28.3</td><td>5.0</td><td>75</td><td>&lt;0.02</td><td>6.1</td><td>9.4</td><td>4.4</td><td>4.5</td><td>0.1</td><td>0.2</td><td>6.5</td><td>1.4</td><td>0.5</td></tr><tr><td>Jan-20</td><td>28.3</td><td>5.2</td><td>75</td><td>&lt;0.02</td><td>7.0</td><td>9.7</td><td>5.0</td><td>5.3</td><td>0.1</td><td>0.0</td><td>6.1</td><td>0.8</td><td>0.3</td></tr><tr><td>feb-20</td><td>28.3</td><td>6.0</td><td>75</td><td>0.03</td><td>7.5</td><td>6.5</td><td>5.5</td><td>5.5</td><td>0.1</td><td>0.1</td><td>6.4</td><td>1.2</td><td>0.3</td></tr><tr><td rowspan="7">GW8</td><td>Jul-19</td><td>28.3</td><td>3.9</td><td>97</td><td>0.05</td><td>7.4</td><td>0.5</td><td>4.8</td><td>1.8</td><td>0.9</td><td>1.5</td><td>n.a.</td><td>1.2</td><td>0.2</td></tr><tr><td>Oct-19</td><td>28.3</td><td>3.3</td><td>388</td><td>0.10</td><td>30.4</td><td>0.1</td><td>18.2</td><td>12.8</td><td>4.7</td><td>8.1</td><td>12.1</td><td>0.8</td><td>0.2</td></tr><tr><td>Nov-19</td><td>28.5</td><td>4.1</td><td>267</td><td>&lt;0.02</td><td>20.3</td><td>0.3</td><td>14.6</td><td>10.7</td><td>3.8</td><td>7.1</td><td>11.7</td><td>0.8</td><td>0.5</td></tr><tr><td>Dec-19</td><td>n.a.</td><td>3.6</td><td>95</td><td>0.05</td><td>7.9</td><td>0.5</td><td>3.2</td><td>2.2</td><td>0.8</td><td>1.5</td><td>10.5</td><td>n.a.</td><td>n.a.</td></tr><tr><td>Jan-20</td><td>28.4</td><td>3.4</td><td>131</td><td>&lt;0.02</td><td>10.1</td><td>0.4</td><td>3.9</td><td>3.8</td><td>1.4</td><td>2.6</td><td>10.9</td><td>1.1</td><td>0.4</td></tr><tr><td>feb-20</td><td>28.0</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>1.1</td><td>0.2</td></tr><tr><td rowspan="7">RW2</td><td>Jul-19</td><td>29.9</td><td>6.3</td><td>108</td><td>0.04</td><td>8.1</td><td>16.6</td><td>8.4</td><td>3.7</td><td>1.6</td><td>4.6</td><td>n.a.</td><td>0.8</td><td>0.2</td></tr><tr><td>Oct-19</td><td>30.7</td><td>4.8</td><td>88</td><td>&lt;0.02</td><td>7.4</td><td>14.3</td><td>5.2</td><td>2.6</td><td>1.3</td><td>3.8</td><td>6.8</td><td>1.4</td><td>0.3</td></tr><tr><td>Nov-19</td><td>28.7</td><td>6.1</td><td>118</td><td>&lt;0.02</td><td>10.0</td><td>15.7</td><td>9.2</td><td>2.6</td><td>1.0</td><td>3.5</td><td>10.9</td><td>1.9</td><td>0.5</td></tr><tr><td>Dec-19</td><td>28.5</td><td>5.4</td><td>172</td><td>&lt;0.02</td><td>15.5</td><td>35.8</td><td>18.3</td><td>4.2</td><td>1.9</td><td>5.9</td><td>10.7</td><td>n.a.</td><td>n.a.</td></tr><tr><td>Jan-20</td><td>28.7</td><td>5.4</td><td>172</td><td>&lt;0.02</td><td>17.3</td><td>37.7</td><td>19.7</td><td>4.5</td><td>2.0</td><td>6.0</td><td>5.6</td><td>3.5</td><td>0.3</td></tr><tr><td>feb-20</td><td>28.9</td><td>5.4</td><td>172</td><td>&lt;0.02</td><td>15.5</td><td>27.7</td><td>11.2</td><td>3.3</td><td>1.7</td><td>4.6</td><td>7.5</td><td>n.a.</td><td>n.a.</td></tr><tr><td rowspan="2">Rain</td><td>Aug-19</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>1.3</td><td>0.5</td></tr><tr><td>Nov-19</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>1.6</td><td>0.3</td></tr></tbody></table>

n.a.: not analysed
<figure><img src="image_17.png" /></figure>

Clay-rich units: non-uniform and non-continuous (Q₄)

Fine to coarse sand (upper-middle Pleistocene: qp²⁻³, lower Pleistocene: and qp¹)