4.1. Evaluating surface water quality in Southern Vietnam
Temperature fluctuations are not large because water has the function of temperature regulation [1,25]. Temperature is in the range suitable for aquatic organisms . The pH in the study areas is affected by wastewater and hydrological regime. pH in major rivers in the Mekong Delta is usually in the neutral range except for some coastal provinces where it is tested [1,19,25,27,28]. In addition, this has resulted that EC and TDS are high in estuary areas where brackish, saline water intrusion is possible. In addition, TDS and EC are also affected by inorganic substances present in wastewater or effluents discharged into receiving waters [1,4,22].
At many locations DO is very low due to the impact of wastewater containing organic compounds and N-NH4+ [22,29]. The fluctuations of DO are due to the presence of organic matters, the presence of algae, air diffusion and pH [4,26]. The results show that low DO while high BOD, COD, TSS revealed that surface water quality in the South of Vietnam has organic pollution, especially in Dong Nai area, Ho Chi Minh city and Long An where there are crowd industrial parks, and busy transport and fishing ports. Previous studies in the Mekong Delta also showed that the water was organically polluted and had seasonal fluctuations [1,19,27,28,30,31]. N-NH4+ is present in wastewater, domestic waste, agriculture, industry, and landfills [4,17,22,32]. High N-NH4+ is found in Dong Nai, Ho Chi Minh City and Long An areas where there are many industrial zones, and fishing ports. The findings show that water bodies are contaminated with nutrients, which can potentially lead to eutrophication of water environment . Nutrient pollution is a water quality issue of concern in the Mekong Delta waters, mainly caused by wastewater from treatment systems and from agricultural production [4, 30,31].
Iron is present in the aquatic environment in the study area was predicted to be fromfrom natural sources (acid sulfate soils) or from industrial wastewater [4, 9,17,22,34]. The presence of Fe in surface waters is a common problem in the Mekong Delta [1,4,31]. And other heavy metals are often derived from agricultural and industrial wastewater [17,22,35,36].
The results show that water quality varies greatly according to sampling location, the most polluted locations are in Dong Nai, Ho Chi Minh City and Long An where there are many navigation activities, industrial zones, and fishing ports. DO, TSS, BOD, COD, N-NH4+, Fe, EC, TDS, Cl- were seasonal fluctuations. The water quality in the southern part of Vietnam is contaminated with organic (low DO while TSS, BOD, and COD are high), nutrients (mainly N-NH4+) and Fe. Pb in some locations exceeded the permissible limit. Heavy metals such as Cd and As were within the allowable limits of QCVN 08-MT:2015/BTNMT, column A1 . This result is also consistent with previous studies on water quality in the Mekong Delta that have contaminated organic, Fe and nutrient, even microorganisms [1,4,9,19,25,27,30,31,37].
On the other hand, through the results of the WQI can be seen that the water quality from Dong Nai, Ho Chi Minh and Long An areas is worse than that in other areas. The reason may be that this area receives wastes from socio-economic development activities with inadequate and ineffective wastewater treatment systems [4,17,22,35]. Previous studies have shown that water quality in Dong Thap province was classified as bad to moderate; In places where water quality is affected by agricultural production, landfills, the water quality is bad, while in places with little impact, the water quality is good . Water quality in large rivers is better than in tributaries [28,31,39,40].
4.2 Evaluating the sampling sites and frequencies of the surface water quality monitoring
The rersults of cluster analysis is very consistent with the calculation results of the water quality index. This has also been reported in a previous study [28,38]. Clusters 1 to 6 have very good water quality, while clusters 7 to 14 are characterized by poor to moderate overall water quality, cluster 15 is characterized by good overall water quality. The results show that water quality varies greatly by sampling sites, mainly due to industrial production, agriculture, transportation, and marine economic activities. Previous studies have shown that locations with the same water quality, in the same water body, can be considered for reduction to save on monitoring costs [5,10,11,21]. This study proposes that monitoring sites from clusters 1 to 6 can be considered to reduce sampling sites because of similar water quality. This study proposes to reduce the total number of monitoring points of cluster 1–6 by one third from 33 locations to 22 locations, reducing monitoring costs by about 19%. Or, these reduced 11 locations can be arranged in other areas to promptly detect water quality problems from which to have timely solutions.
For the cluster according to the sampling frequency, previous studies also found similar results that water quality was clustered by seasons [10,13,14,28,40]. The order of importance of the indicators changes with the seasons, showing that these indicators are affected by the observational periods. Clusters with similar water quality may consider reducing the sampling frequency. The results show that the sampling frequency could be reduced one for each Cluster II, Cluster III and Cluster V. Thus, this study recommends monitoring frequency from 8 to 5 times/year (Apr, Jun, Aug, Sept, Nov) depending on funding and human resources. This reduces monitoring costs by nearly 37.5%. Chounlamany et al. (2017)  also proposed to reduce the frequency of sampling for monitoring surface water quality based on the results of temporal cluster analysis.
4.3. Identifying key water parameters influencing water quality
In this study, the factor leading to the pH change may be due to the characteristics of wastewater and seawater [4,22]. EC, TDS, Cl- are mainly affected by sources from saline intrusion, dissolved ions in wastewater [4,17,22]; organic substances from the discharge processes ; total suspended solids from riverbank erosion, plankton [1,4,25]; nutrients from hydrological factors, wastewater from domestic, agriculture and industry [17,22,35,36]; Fe was derived from natural and industrial wastewater [17,22,28,31] while other heavy metals (Pb, As, Cd) were derived from industrial waste [4,17]. The findings show that the parameters of temperature, pH, TDS, EC, DO, BOD, COD, N-NH4+, N-NO2-, Fe, Cl-, Pb need to be monitored while Hg, As, Cd may not need or reduce the frequency of monitoring because its concentrations is always at below the allowable level or below the detection limit. N-NO3- may also not need to be monitored because it can be estimated from N-NH4+, N-NO2- and DO. In addition, N-NO3- is less harmful to the environment than that of N-NH4+, N-NO2- in the study areas. EC and TDS have a close relationship with each other, so only one of these two indicators is selected for monitoring. In this study, the important water quality indicators such as P-PO43-, TP, TN, and coliforms have not been observed, and need to be added to the future monitoring program for a more comprehensive assessment of surface water quality in the study area.