Assessment of Water Quality in Coastal Zone of the Red River Estuary (Nam Dinh Province, Vietnam)

Abstract

The assessment of water quality and eutrophication for seawater environment is always necessary for protection and sustainable use of water resource. This study aims to assess the surface water quality of the coastal zone of the Red River estuary (in Nam Dinh province). The sampling campaigns were organized at 10 sites along the coastal zone of Giao Thuy district, Nam Dinh province during the years in 2017 – 2018. The monitoring results showed that dissolved oxygen (DO) ranged from 5.6-6.5 mg/L, averaging 6.1 mg/L; pH ranges from 7.0-7.9, averaging 7.6; Total suspended solids (TSS) ranges from 64.6-216.8 mg/L, averaging 158 mg/L; Ammonium varied from 0.06-0.38 mgN/L, averaging 0.13 mgN/L; Nitrite ranged from 0.07-0.21 mgN/L, averaging 0.09 mgN/L; nitrate ranged from 0.05 to 0.93 mgN/L, averaging at 0.21 mgN/L; Phosphates ranged from 0.007-0.027 mgP/L, averaging 0.014 mgP/L; particulate organic carbon (POC) varied from 2.2-4.3 mg/L, averaging 3.1 mgC/L. Total Coliforms fluctuated from 1000-3275 MPN/100mL, averaging 2189 MPN/100mL. Almost physicochemical variables were within the allowable values of the Vietnam standards QCVN 10-2015/BTNMT for coastal zone, except for ammonium, TSS and TC. Nutrients which tended to be higher in rainy season may reflect an important contribution of diffuse source from the Red River basin to the coastal zone. Using different methods for classification of trophic state and basing on our monitoring results, the coastal water at this area was at Mesotrophic level, of which, higher values of trophic index were found in the rainy season than the dry season. The results also showed the obvious impact of untreated wastewater on the coastal water quality at some studied sites nearby the wastewater sewers from different coastal communes. The results suggested that controlling wastewater quality discharged into the coastal zone is needed for protecting water quality and ecosystem of the coastal zone of the Red River estuary.