Assessment of surface water quality and monitoring in southern Vietnam using multicriteria statistical approaches – Sustainable Environment Research

3.1

Evaluating surface water quality in southern Vietnam

The temperature between months of April to December ranged from 25.3 to 35.3 °C, while it fluctuated between locations from 28.6–31.4 °C, with an average of 58 sites for eight months 30.1 °C. The pH ranged over time and space between 5.2–9.0 and 5.9–7.9, with a median of 58 sites for eight months of 7.1 (Fig. 2a). The pH at some locations was slightly alkaline and exceeded the allowable limit of 8.5 [7], but it is still within the allowable limit of 9. EC and TDS fluctuated greatly (Fig. 2c and d). EC at the sampling periods and locations fluctuated from 15 to 59,200 and 38–47,400 mS cm− 1, respectively, reaching the average value of 58 sites for eight months at 5390 mS cm− 1. Moreover, TDS fluctuated over time and space were 8–34,300 and 17–26,070 mg L− 1, respectively, with the average of 58 sites for eight months at 2820 mg L− 1.

Fig. 2

Characteristics of surface water quality in southern Vietnam (Apr: April, Jun: June, Jul: July, Aug: August, Sep: September, Nov: November, Dec: December)

Full size image

DO concentration fluctuated greatly by months and sampling sites in the range of 0.2–10.8 and 0.6–7.8 mg L− 1, respectively, with an average value of 4.6 mg L− 1. DO in the study area is lower than the permissible limit of ≥6 mg L− 1 [7]. DO has great volatility in April and July (Fig. 2e). Besides that, BOD at 58 survey sites varied from 1.6 to 48.8 mg L− 1, while BOD between months of sampling ranged from 0 to 79 mg L− 1. BOD in the rainy season months (May–September) fluctuated more than that in the dry season months (Fig. 2f). BOD at most locations exceeded the allowable limit of 4 mg L− 1 [7]. Similarly, COD between months and sampling sites ranged from 8 to 91 and 3 to 139 mg L− 1, with the mean value at 17.5 mg L− 1 (Fig. 2g). COD has exceeded the allowable limit (10 mg L− 1) of QCVN 08-MT:2015/BTNMT, column A1 [7]. TSS at sites and months fluctuated from 9 to 111 and 3–495 mg L− 1, respectively, with an average of 36 mg L− 1 (Fig. 2b). TSS has seasonal variation in which the rainy season is usually higher than that in the dry season. TSS at most locations exceeded the allowable limit (20 mg L− 1).

The concentration of NH4+-N between the months of sampling fluctuated in the range of 1.0–24.9 mg L− 1, while this concentration between the sampling sites was in the range of 0.1–15.0 mg L− 1 (Fig. 2h), reaching an average value of 1.0 mg L− 1. NH4+ in the study area has significant spatiotemporal fluctuations. According to QCVN 08-MT:2015/BTNMT column A1, limit value of N-NH4+ is 0.3 mg L− 1 [7]. NO2−-N in water fluctuated in space and time with concentrations of 0–0.34 and 0–0.69 mg L− 1, respectively, with an average of 0.03 mg L− 1 (Fig. 2i). In locations with high concentration of NH4+ and low DO, the nitrite-nitrogen concentration accumulated and exceeded the allowable limit of 0.05 mg L− 1 [7]. NO3−-N between sampling months and sampling sites ranged from 0 to 3.7 and 0.1–2.0 mg L− 1, with the mean values at 0.8 mg L− 1. NO3− concenrration was highest in locations such as Cat Lai ferry terminal (Dong Nai), Ho Chi Minh City area, Tan Thanh wharf (Long An) and Vedan Port (Tay Ninh). NO3− concentration in April was significantly higher than that in other months. The fluctuation of NO3− concentration depends on the concentration of NH4+ and DO. The limit value of NO3−-N is 2 mg L− 1 according to QCVN 08-MT:2015/BTNMT, column A1 [7], so only some positions at certain times exceed the allowed regulation.

The maximum Fe concentration at the locations was 4.4 mg L− 1 and at the time of sampling was 16.1 mg L− 1, averaged at 1.8 mg L− 1. All samples having Fe concentration exceeded the allowable limit of 0.5 mg L− 1 [7]. Fe in the rainy season tended to be higher than in the dry season (Fig. 2o). High Cl− concentration is concentrated in the area near the sea of Ho Chi Minh City (HCM1, HCM10–12) with concentrations exceeding the allowable limit of 250 mg L− 1 [7]. The remaining positions all have Cl− concentrations within the allowable limits. Cl− has a marked seasonal variation in which the dry season is higher than that in the rainy season. The trend of Cl− fluctuations is similar to that of EC and TDS. High Cl− concentrations lead to high EC and TDS (Fig. 2c and d). Pb in the study areas was within the allowable limit of 0.02 mg L− 1 [7], except for some locations such as Ong Buong bridge (HCM5 in May), lower Tri An dam (BD6 in June) and Be River estuary (BP4 in June) exceed the allowable limit (Fig. 2m). Cd and As at all locations through the sampling sessions were within the allowable limits of 0.005 and 0.01 mg L− 1, respectively (Fig. 2l and n) [7].

3.2

WQI

The calculation results of WQI in the study area are presented in Fig. 3. WQI in Binh Duong (95–100) and Binh Phuoc (91–99), An Giang (94–100), Tien Giang (92), Ben Tre (92), Dong Thap (95) indicated that the water quality in these provinces was classified as very good. Water quality in Tay Ninh is classified as good, with WQI ranging from 76 to 96. The water quality in Dong Nai was varied greatly and graded from moderate to very good (WQI = 66–100). Locations with medium water quality included DN11, DN12, DN14 and DN15 (WQI = 66–73). Water quality in the Ho Chi Minh City area was divided into three classes, as follows: class with bad water quality (WQIHCM1-HCM3, HCM5 –HCM7 = 42–48), medium (WQIHCM4, HCM8-HCM11 = 52–74), and good (WQIHCM12 = 81). Meanwhile, water quality in Long An was classified bad (WQILA1-LA2 = 45–48), moderate (WQILA3 = 58) and good (WQILA4 = 76).

Fig. 3

Distribution of WQI at the monitoring locations

Full size image

3.3

Evaluating the sampling sites and frequencies of the surface water quality monitoring

Water quality in the South of Vietnam varies greatly. Based on the Euclidean distance, the water quality at 58 sampling sites was classified into 15 clusters (Fig. 4). Water quality similarity of sites in each cluster can be suggested to reduce the number of monitoring sites in the future. Specifically, the water quality characteristics in the identified clusters are presented in Table 3. Cluster 1 has 3 positions (BD1–2, BP1) located in Binh Duong and Binh Phuoc provinces. In cluster 1, TSS, NH4+-N, COD, Fe exceeded the permissible limits of 20, 0.3, 10 and 0.5 mg L− 1, respectively. Cluster 2 has 6 positions (VT1, DN3–6); cluster 3 has 6 positions (DN7–8, TG1–2, DT1, BT1). Clusters 2 and 3 have parameters TSS, DO, COD, Fe that do not meet the requirements of the allowable limits. These clusters mainly belong to Dong Nai, Tien Giang, Dong Thap, Ben Tre, Ba Ria-Vung Tau provinces. Likewise, Cluster 4 has 3 positions (BD4, BD6, BP4); cluster 5 has 10 positions (BD5, BP2–3, VT2–3, DN1–2, DN9–11); cluster 6 has 5 positions (AG1–5). TSS and Fe indicators in cluster 4 (located in Binh Duong and Binh Phuoc provinces) exceeded the allowable limits; while BOD, COD, N-NH4+, Fe in cluster 5 (including locations in Binh Duong, Binh Phuoc, Dong Nai, Ba Ria-Vung Tau) exceeded the allowable limits. Cluster 6 (mainly An Giang) has only TSS and Fe criteria that do not meet the permissible limits of 20 and 0.5 mg L− 1, respectively.

Fig. 4

Clustering surface water quality by sampling locations

Full size image

Table 3 Water quality in the identified clusters

Full size table

Cluster 7 (DN12–15, HCM8–9); cluster 8 has 1 position (LA3); cluster 9 has 2 positions (HCM10–11); cluster 10 has 4 positions (HCM1–4); cluster 11 has 1 position (LA2); cluster 12 has 1 position (HCM5); cluster 13 has 3 positions (LA1, HCM6–7); cluster 14 has 1 position (HCM12) and cluster 15 has 6 positions (LA4, TN1-TN5). Clusters 7–13 (including Long An, Dong Nai and Ho Chi Minh City) have TSS, DO, BOD, COD, NH4+-N, Fe exceeding the allowable limits. Particularly for clusters 12–13, organic and nutrient pollution are very serious, possibly because these places received more pollutants from wastewater and waste than the other locations. In addition, lead in cluster 12 was detected at concentrations reaching the limit of 0.02 mg L− 1 [7]. TSS, DO, BOD, COD, Fe, Cl− of cluster 14 (1 location in Ho Chi Minh City) have exceeded the allowable limits. Meanwhile, DO, COD, N-NO2− characterize water quality in cluster 15 (locations in Tay Ninh province, only one location in Long An province).

Based on the Euclidean distance, water quality according to sampling periods was classified into 5 clusters (Fig. 5). Cluster I includes April (end of the dry season); cluster II, cluster III and cluster IV include May and June, July and August, September, respectively; which all months belong to the rainy season. While cluster V was formed by two months at the beginning of the dry season (November and December). The results show that the water quality fluctuates greatly over time in which the water quality in the dry season months tended to be separate from the rainy season months. Pollution characteristics according to identified clusters are presented in Table 4. Besides that, entropy weight of parameter represents the effect of that parameter on water quality of cluster. Cluster I was characterized by indicators in descending order NH4+-N > EC = TDS > Fe > BOD; cluster II by TDS > EC > NH4+-N > NO2−-N > TSS = Pb. Cluster III was characterized by EC = TDS > NH4+-N > Cl−. Cluster IV was characterized by EC = TDS > Cl− > NH4+-N while cluster V was characterized by EC = TDS > NH4+-N > Cl−.

Fig. 5

CA results according to the sampling frequency

Full size image

Table 4 Weights of water quality parameters in the identified clusters

Full size table

3.4

Identifying key water parameters influencing water quality

The PCA results show that water quality in the southern region of Vietnam is affected by many sources of pollution. In which, PC1–5 is considered the main source because it has an Eigenvalue > 1, explaining 87.3% of the variation in water quality. Meanwhile, the sources from PC6–13 as secondary explained only 12.7% of the variation (Table 5). In addition, Table 5 also showed that the loading coefficient of the main PCs had weak correlations with the parameters of temperature, pH, TDS, EC, DO, BOD, COD, NH4+, NO2−, Fe; had moderate correlations with pH, NH4+, NO3−, and a good correlation with Cl−. Secondary PCs had moderate to good correlations with all parameters except Cl− (Table 5).

Table 5 Potential polluting sources and key water parameters influencing surface water

Full size table

The water quality indicators in the southern region are affected by many pollution sources. For example, PC2, PC5 and PC6 indicated only weakly positive values for temperature and moderately by PC7; this suggested that there were about four potential sources of temperature variation. Similarly, pH was affected by 3 factors at weak level and 1 factor at medium level; EC and TDS were weakly affected by 2 factors and moderately affected by 1 factor. DO was weakly affected by PC1, 8, 9, and moderately by PC10. BOD was affected weakly by PC1 and strongly by PC12 while COD was weakly affected by PC1, PC12 but moderately affected by PC11. TSS was weakly affected by 1 factor, moderately affected by 3 factors (Table 5). NH4+ was moderately affected by PC11 but weakly affected by PC1, PC3, and PC12. NO3− is moderately affected by PC3 but weakly affected by PC7 and PC9. NO2− was weakly affected by 4 factors, moderately affected by 1 factor. Fe was weakly affected by PC4, PC9 and moderately affected by PC7. Cl− was strongly affected by PC7 and weakly affected by PC7.