The paper represents some aspects of the water quality in O Loan lagoon. Two surveys were performed in March and July, 2014 - dry season, and October and November, 2014- rainy season. Due to specificity of O Loan lagoon with narrow estuary, capacity of water exchange is low. A shortage of dissolved oxygen (DO) has led to the pollution of lagoon water quality ( in dry season and mg/l in rainy season). | Journal of Fisheries science and Technology Special issue - 2015 PROCESS OF ENVIRONMETAL WATER QUALITY IN O LOAN LAGOON, PHU YEN PROVINCE Nguyen Thanh Son1, Hoang Ngoc Anh1, Dao Thi Nga1, Nguyen Thi Thu Nga1, Le Nha Uyen1, Bui Vinh Dai1 ABSTRACT The paper represents some aspects of the water quality in O Loan lagoon. Two surveys were performed in March and July, 2014 - dry season, and October and November, 2014- rainy season. Due to specificity of O Loan lagoon with narrow estuary, capacity of water exchange is low. A shortage of dissolved oxygen (DO) has led to the pollution of lagoon water quality ( in dry season and mg/l in rainy season). Generally, water environmental quality in rainy season was relatively better in dry season, and concentration of environmental parameters tends to decrease. Content of pH, total suspended solids (TSS), DO, Ammonia reach the demands of NTR 10: 2008, while other parameters, including the COD, Phosphorus and total Coliform exceeded the allowed limit in both seasons. Keywords: O Loan lagoon, exchange capacity of water, environmental parameters, NTR 10:2008 I. INTRODUCTION O Loan lagoon is on the South of Chi Thanh town, next to An Cu, An Hoa, An Hai, An Hiep and An Ninh Dong commune of Tuy An district, Phu Yen province at 13013’50” to 13019’00” and from 109014’30” to 109017’30”. O Loan is a brackish lagoon with the area around ha, stretching from North to South. The lagoon entrance is narrow and water go through the sea via Tan Quy outlet. Potential aquatic life in O Loan lagoon is rather diversity, however, a large area of water surface of O Loan lagoon was invaded for aquaculture activities. It has led to the pollution and destruction of ecosystem in the lagoon. Additionally, the overfishing may lead to reduce biodiversity of aquatic life. II. MATERIAL AND METHODS (October, December 2014) to collect the water samples at 14 sites in the O Loan lagoon (figure 1). The environmental parameters were analyzed as .