Through assessment of surface water quality at Siep stream in Di An city, Binh Duong province. Siep Stream is a level 1 tributary that flows into Dong Nai River. Dong Nai River is currently being exploited and used for domestic water supply purposes. Sampling location of Siep stream at the culvert on National Highway 1K from January 2022 to November 2023, through statistical monitoring data compared with QCVN 08-2023/BTNMT (level A) and QCVN 08: 2015/BTNMT (column A2) - National technical regulation on surface water, it shows that the parameters increase or decrease without stable, most tend to increase with about 86.9% COD exceeding the standard 1.0 ÷ 7.8 times, 95.6% SS parameters exceeding the standard 1.8 ÷ 30.4 times, 95.6% N-NH4 number exceeds the standard of 0.16 ÷ 21.8 times, 52.1% of N-NO2 parameters exceed the standard by 0.52 ÷ 8.9 times, 65.2% of P-PO4 parameters exceed the standard by 0.35 ÷ 6.25 times, 91, 3% of Colifrom parameters exceeded the standard of 1.0 ÷ 3.3 times. The WQI (water quality index) results show that the surface water quality of Siep stream is mostly average, this result is not suitable for domestic water supply purposes, there needs to be solutions to improve water quality at the area Siep Stream.

Over the past few years, there are many activities had negative impacts on the surface water of the Dong Nai river such as industry, agriculture, and service development activities; therefore river water quality is significantly relieved. This study was conducted to evaluate the effect of elements on the water quality in 2022 through analyze the parameters of pH, temperature, DO, BOD5, COD, N-NH4+, P-PO43-, TSS, turbidity, total Coliform on 54 samples, compared with QCVN 2015 for river water quality and the water quality index calculation WQI. The analysis results show that the water quality of Dong Nai river, sections flowing through Bien Hoa city from Jan to Dec is not quite good, most parameters reach the limit value B1 according to QCVN 08:2015/BTNMT; however, the values of parameters DO, total coliforms, TSS exceed the limit value B1 significantly. WQI values fluctuate at a fairly high level from 17 to 68, and the average WQI of the river reaches is quite low, so the authority should have promulgated the law to protect the water surface of this river; therefore water quality assessment is significant for better control and management of the surface water environment in Vietnam.

Chau Pha Lake is one of the large reservoirs providing water for domestic, irrigation and production of people in Phu My town, Ba Ria Vung Tau province. This study was conducted to analyze the parameters of pH, temperature, DO, BOD5, COD, N-NH4+, P-PO43-, TSS, turbidity, total Coliform on 16 samples in 2020 and the water quality index calculation WQI. The analysis results show that the water quality of Chau Pha lake in the rainy season is quite good, all parameters reach the limit value A2 according to QCVN 08:2015/BTNMT; however, in the dry season, the values of parameters DO, BOD5, P-PO43, TSS exceed the limit value A2 but not significantly; WQI values fluctuate at a fairly high level from 71 to 92, which is correlated with turbidity, COD, and BOD5 parameters. Thus, the average WQI of the lake reaches 85.4 points, corresponding to level 2 (green) - water used for domestic water supply purposes but needs appropriate treatment measures.