Seventy two water samples were collected in the lower Hau River to evaluate the fluctuation, stratification, and partial partitioning of water quality during 9 months. Samples were collected monthly at 4 sites that belong to Soc Trang province including Dai Ngai, Long Phu, Kinh Ba, and Tran De. At each site, water samples were also collected in the surface and bottom of the river. The parameters were monitored for temperature, pH, dissolved oxygen (DO), salinity, total suspended solids (TSS), chemical oxygen demand (COD), total ammonia nitrogen (TAN), nitrite (N-NO₂^-), nitrate (N-NO₃^-), total nitrogen (TN) and total phosphorus (TP).

The results indicated that temperature ranged from 26.5~30.5 ^oC; pH ranged from 7.3~8.8; salinity fluctuated from 0~11 ?; DO levels were high with a range from 4,5 to 7,6 mg/L; TSS were high fluctuation and reached 900 mg/L; COD ranged from 1.6 to 13.8 mg/L. TAN, N-NO₂^-, N-NO₃^-, TN and TP ranged from 0.013~0.207; 0.008~0.103; 0.005~0.599; 0.936~2.537; and 0.147~6.033 mg/L, respectively. In the surface water, no significant difference in water quality parameters was found between dry season and rain season (p> 0.05). However, in Kinh Ba and Tran De, salinity and total phosphorus concentrations in dry season were significantly higher than that in rain season (p< 0.05). Salinity, TSS, COD, TAN, TN, and TP in bottom water were higher than that in surface water. In rain season, partial partitioning of water quality among sites was low, and Dai Ngai, Long Phu, and Kinh Ba showed high similarity in water quality. In contrast, in dry season, water quality in lower Hau River was partitioned into two areas Dai Ngai-Long Phu and Kinh Ba-Tran De (Fig 3).