Groundwater quality assessment for drinking purposes: a case study in the Mekong Delta, Vietnam | Scientific Reports

Hydrochemical and microbiological parameters of groundwater in the Mekong Delta

The groundwater quality data in the Mekong Delta study area are presented in Table 2 and Fig. 2. The average parameter values were compared with the Vietnamese regulation on groundwater quality QCVN 09-MT:2015/BTNMT26. The pH values of the water samples varied from 6.88 to 7.75 with an average of 7.29 ± 0.22 (Fig. 2a), which were still within the Vietnamese standard (5.5–8.5). The pH data observed in this study were similar to the ranges observed in the area by previous studies. During 2009–2018, the average pH value of groundwater samples collected in An Giang province was reported in the range of 6.7–7.2 in the dry season and 6.5–6.9 in the rainy season13. pH values in Bac Lieu province were in the range of 7.16–8.209. The difference between the pH variation observed in An Giang and the one observed in Bac Lieu could be attributed to different anthropogenic pollution in both areas since the middle-upper Pleistocene aquifer was highly exploited by industrial and household uses27.

Table 2 Groundwater quality in An Giang province, Dong Thap province, and Can Tho city in 2019.

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Figure 2

Box and whisker plot of hydrochemical and microbiological parameters in groundwater samples: pH (a), total hardness (b), nitrate (c), Fe (d), Pb (e), Hg (f), As (g), and coliforms (h). The outliers of the data are presented by the dots with their sampling locations (AG An Giang province, DT Dong Thap province, CT Can Tho city).

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Total hardness in groundwater samples indicates high cation concentration, e.g., calcium and magnesium. In this study, the total hardness ranged between 23.6 and 999.7 mg/L, with an average of 231.2 ± 227.4 mg/L (Fig. 2b). Nine out of 64 collected samples (14.1%) exceeded the limit of total hardness for groundwater (500 mg/L). These sampling sites are located in An Giang province (AG1, AG3, AG4, AG7, and AG9 sites) and in Dong Thap province (DT2, DT8, DT9, and DT21 sites). Consumption of these groundwater sources without treatment can lead to health impacts. Comparing with nearby provinces observed within the close time period (2017–2020), the total hardness of groundwater observed in this study was within the range measured in Soc Trang province, which greatly fluctuated from 13 to 3080 mg/L8, and higher than the range found in Bac Lieu province, which was from 98 to 172 mg/L9.

The NO3− concentration in groundwater samples was ranging from below the detection limit (< 0.05 mg/L) to the maximum of 10.19 mg N/L with the average of 0.81 ± 1.34 mg N/L, which all were within 15 mg N/L of the Vietnamese standard (Fig. 2c). Comparable NO3− concentrations in groundwater have also been detected by the previously mentioned studies in Mekong delta with the sampling times during 2017–2020: 0.01–2.96 mg N/L in An Giang province28, 0.1–260 mg N/L in Soc Trang province8, and 0.41–1.91 mg N/L in Bac Lieu province9. Since Soc Trang province is located further in the south of this study area (adjacent to Hau Giang province in Fig. 1), and Bac Lieu province is located further in the south next to Soc Trang and Hau Giang provinces, it could be implied that the groundwater in this area of the Mekong Delta has been contaminated with mild NO3− pollution during the sampling period. An increase in NO3− concentration in groundwater could be influenced by the impacts from domestic and industrial wastewater, agricultural runoff, and excessive fertilizer application3,17,29. Previous studies showed various agricultural activities in these provinces such as rice and fruit production30,31,32; therefore, sources of nitrate pollution could be from these agricultural activities in the Mekong Delta area.

Heavy metals in groundwater samples ranged from below the detection limits (< 0.1 mg/L of Fe, < 1 \(\times\) 10–3 mg/L of As and Pb, and < 3 \(\times\) 10–4 mg/L of Hg) to 35.5 mg/L of Fe, 6.50 \(\times\) 10–3 mg/L of Pb, 4.40 \(\times\) 10–4 of Hg, and 1.98 \(\times\) 10–2 of As (Fig. 2d–g). The concentrations of these metals detected in groundwater were within the permissible limits except for Fe. Three samples, including AG1, AG11, and CT9, exceeded the limit of Fe in groundwater (5 mg/L) with concentrations of 5.55, 7.63, and 35.46 mg/L, respectively (Fig. 2d). The Fe concentrations observed in An Giang province in this study were much higher than the value observed in Bac Lieu province (1 mg/L), which is located further to the south of Can Tho city9. Although the As concentration in this study is within the standard, it was higher than that observed in the nearby Bac Lieu province. Previous studies have reported serious As contamination in An Giang province for over a decade12,15,33. With the development of hydropower plant construction in the upper Mekong River and the scarcity of surface water, the increasing use of As-contaminating groundwater is unavoidable13,34,35. High concentrations of heavy metal ions in groundwater are normally associated with both natural processes (e.g., water–rock interaction) and human activities (e.g., improper-treated industrial wastewater). The detection of heavy metals in groundwater in this study could also be from the excessive groundwater extraction to serve domestic and irrigation activities, which led to lower water levels and stronger reduction condition that triggers the release of heavy metals into aquifers36.

High coliform density was observed in the groundwater samples, which was highest at 761.5 MPN/100 mL with an average value of 33.93 ± 102.53 MPN/100 mL (Fig. 2h). There were 52 groundwater samples (81.3% of total samples) exceeding the standard limit of coliforms (3 MPN/100 mL). It is considered that fecal contamination in groundwater is ubiquitous in the Mekong Delta due to leaking fecal matter from pit latrines, livestock wastewater, and wild animal droppings via improperly protected wells, in which the coliform density can greatly vary from 9 – 9,300 MPN/100 mL13,28,37.

Cluster analysis and principal component analysis of groundwater samples

According to CA, 64 groundwater sampling sites were grouped into four clusters based on hydrochemical and microbiological parameters (Euclid distance = 20)24 as illustrated in Fig. 3. The groundwater quality characteristics of each cluster are summarized in Table 3. The cluster results also revealed that the groundwater quality in the study area depends on geological locations. Cluster I includes 15 samples (AG1-4, AG6-13, DT2, DT8, DT9), which was highly polluted with coliforms. Groundwater samples in this cluster also showed slight total hardness contamination. However, the hardness of cluster I in Table 3 is a lot higher than other clusters. Most of the samples in Cluster I are located in An Giang province and some in Dong Thap province. CT9 is the only sample classified into Cluster II, which was more polluted with Fe than other clusters. Cluster III includes most of the groundwater samples from Can Tho city from the total of 27 samples (AG5, CT1-8, CT10-27). Lastly, the remaining groundwater samples collected in Dong Thap province (DT1, DT3-7, DT10-24) are grouped into Cluster IV. As same as Cluster I, Cluster II to IV were detected with coliform contamination in groundwater.

Figure 3

Dendrogram grouping groundwater samples based on hydrochemical and microbiological parameters. Each sample is described on x-axis and Euclid distance is represented on y-axis.

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Table 3 Water quality characteristics of each cluster obtained from hierarchical cluster analysis.

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The principal components responsible for the groundwater quality variation in the study area were extracted from the PCA results (Table 4). Three principal components (PC1-3) were extracted based on their eigenvalues higher than 1.0, which could explain 77.2% of the cumulative variation. PC1 accounted for 36.9% of the total variation and was recognized by moderate negative correlation with Pb, Hg, and As. This suggests the pollution from water–rock interactions and anthropogenic sources like industrial wastewater and landfill leachate, which leads to the dissolution of metallic compounds into groundwater9,13. Can Tho city is the largest city in the Mekong Delta and is well-known for its commercial, cultural, and industrialized activities while small-scale industries such as craft production can be found in Dong Thap province, leading to the contamination of heavy metals in the surface water and soil that can be leached to groundwater38,39. PC2 was associated with a moderate positive correlation with NO3− and Fe, accounting for 23.0% of the total variation. Anthropogenic sources such as excessive fertilizer use, industrial and domestic wastewater, agricultural runoff, and aquacultural wastewater are the sources of NO3− pollution3,17,29. Rice production has been reported as the dominant agricultural activity in An Giang province30 while rice starch production, fish pond, and livestock farms could be found in Dong Thap province39. These activities could then contribute to the presence of NO3− in groundwater of the study area. Since Fe can act as an electron donor in the denitrification process40, high Fe concentration in the groundwater samples can possibly reduce NO3− concentration. The third principal component, PC3, could explain 17.3% of the total variation and was characterized by a moderate positive correlation with total hardness and coliforms. Industrial wastes and natural hydrogeochemical activities are primary sources of calcium and magnesium cations41, which significantly contribute to the total hardness of groundwater. Moreover, according to the analysis, the groundwater in the study area is also susceptible to pathogenic contamination. High coliform density is associated with leaking contaminants from fecal sources such as pit latrines, sewage pipes, or livestock wastes from the farms in the study area13,28,37,39.

Table 4 Loading values of water quality parameters in each principal component.

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Integrated-weight groundwater quality index for groundwater classification

The results of IWQI calculation and groundwater quality classification for drinking purposes are presented in Table 5. The IWQI values varied from 4 to 2761 with a mean of 139 ± 370. Out of 64 groundwater samples, over half (53.1%) were classified as excellent for human consumption. There were 16 (25%), 6 (9.4%), and 3 (4.7%) samples categorized as good, poor, and very poor water quality for drinking purposes, respectively. In addition, 5 samples including AG2, AG4, AG12, DT9, and DT15 had the IWQI values over 300, which were considered unsuitable for any drinking purposes. One of the reasons for groundwater having inadequate quality for human consumption is high coliform density. The previous CA results showed that all clusters were contaminated with coliforms. This was also proved by the IWQI results, which showed a high positive correlation coefficient (0.99) with coliform (Table 6). Even though As, Hg, and Pb were highly correlated with each other, these parameters were within the allowable limits. Besides, the remaining parameters have no correlation or very weak correlation with each other. Therefore, it might be deduced that changes in groundwater quality in the study area were significantly attributable to coliform density.

Table 5 IWQI values of sample sites and corresponding groundwater categories (Rank I: excellent, Rank II: good, Rank III: poor, Rank IV: very poor, and Rank V: unsuitable for drinking).

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Table 6 Correlation matrix between water quality parameters and overall IWQI values.

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Based on the interpolation of IWQI values with inverse distance weighting, the distribution map of groundwater quality categories for drinking purposes were constructed (Fig. 4). It can be seen that most of the samples classified unsuitable for drinking (AG2, DT15, AG4, DT9) are located near large rivers. This could imply that the groundwater quality in these areas depends on the surface water quality because contaminants can directly infiltrate into the aquifers. Therefore, agricultural runoff and the discharge of untreated or improperly treated industrial, domestic, and aquacultural wastewater into surface water could consequently contaminate the groundwater in the region. It was found that surface water and groundwater in An Giang province was heavily polluted with Escherichia coli and coliforms due to the emergence of burial swine pits in 201928; thus, the leachate containing pathogens from these sites could leak into surrounding water bodies. In this study, the majority of groundwater samples with lower IWQI values were recorded in the east and southeast parts of the study area where these sampling points are located far from the burial pits. It has been shown that the fine-grained sediments separating the water table from the vadose zone can reduce the transport of microorganisms into groundwater42, which could prevent the transfer of E. coli and coliforms from the burial pits to the sampling locations. However, since microorganisms may not be degraded by this process, the continuous flow of water in the subsurface will eventually transport them to the area43. Therefore, lower IWQI values observed in An Giang area could be the result of slow transport of coliforms. From the findings, it can be implied that groundwater quality in this area was continuously affected by human activities, and it cannot be used as a safe freshwater resource for human consumption.

Figure 4

Spatial distribution map of groundwater classification for drinking purposes based on the IWQI values. The map was generated by the software QGIS version 3.14 (https://qgis.org/en/site/forusers/download.html) licensed under GNU General Public License (CC BY-SA 3.0).

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