Surface water quality assessment in the Bach Dang river basin, Vietnam: using water quality index and geographical information system methods

The study evaluates the surface water quality of the Bach Dang river basin. During the rainy season, the temperature, pH, EC, BOD 5 , COD, coliforms, ammonium and phosphate values were 27 °C–37 °C, 6.6–8.9, 300–2200 μ S cm −1 , 5–25 mg l −1 , 5–26 mg l −1 , 0–3000 MPN/100 mL, below 0.01 mg l −1 , and 0.02 mg l −1 , respective. During the rainy and dry seasons, Zn, Pb, Cr, As, Fe and Mn levels were below drinking water limits. In the rainy season, the water quality index varied from 76 to 91, signifying medium to excellent water quality. The most outstanding water quality sites were the QN12, QN18, and QN22. During the dry season, most areas had moderate to excellent (WQI values between 61 and 93), except for QN01, QN03, QN04, QN05, QN06, QN07, QN11, QN16, QN17, QN19, and QN23, where it dropped to moderate (61–73). The temperatures were greater and higher EC values in the southeast. The DO concentrations were higher in the northwest and southeast in both seasons. The southern and southeastern regions exhibited more excellent COD and BOD 5 values than the western, southwest, north, and northeast regions. The ammonium concentrations were higher in the southeast. During the dry season, the phosphate levels exceeded the permissible limit in the west, southeast, east, and northeast. Coliforms were below acceptable in both seasons, with greater levels in the northwest and northeast during the dry season. In addition, the Zn and Pb are favorably associated with the As, Mn, ammonium and phosphate in the rainy season. The COD positively correlated with the coliforms and BOD 5 . In the dry season, the pH inversely correlated with the Pb and Fe positively. The ammonium correlated negatively with the DO and positively with the phosphates. The pH negatively connected with the alkalinity, while the As positively correlated with the BOD 5 and COD.