An assessment of water quality along the rivers loading to the Manado Bay, North Sulawesi, Indonesia

In the city of Manado, rivers are used for water drainage and sewer system of various wastewater discharges, including from toilets ; these rivers discharge their load to Manado Bay. Therefore it is extremely important to know the water quality of these rivers. In this study three selected rivers, Sungai Bailang (SB), S. Maasing (SM), and S. Tondano (ST), were assessed in regard to 5-days Biochemical Oxygen Demand (BOD_5), nitrate (NO_3^-), ortho phosphate (PO^_4), total coliform (TC), Escherichia coli (EC), and total mercury (Hg-tot) to indicate loads of organic matter, inorganic nutrients, bacteria, and metals, respectively. Existing concentrations, variations with seasons (dry and wet), and pollution status of the rivers, are presented and discussed in this paper. Except the levels of TC and EC, which were high in all rivers and seasons, these parameters varied according to space (within the river and among the rivers) and the seasons. Average levels loaded to the bay for (1) organic were 15.30 mg/l of BOD_5 in the dry season and 7.52 mg/l of BOD_5 in wet season ; (2) inorganic of NO^-_3 and PO^_4 were 2.01 mg/l of NO^-_3 and 1.71 mg/l of PO^_4 in the dry season, and 1.44 mg/l of NO^-_3 and 2.20 mg/l of PO^_4 in the wet season ; (3) Hg-tot in the water and the surface sediment of ST was 0.024 mg/l and 0.133 mg/kg in the dry season, and 0.081 mg/l and 0.130 mg/kg in the wet season. The level of Hg-tot accumulated in the marine bivalve, Soletellina sp., ranged between 0.012 and 0.124 mg/kg. Based on these results, the water quality of the rivers was assessed as polluted. This was perhaps attributable to the presence of input of wastewater discharging from residential sources of the city and from the hinterland agricultural area of Minahasa Regency (MR). This condition may further threaten the water quality of Manado Bay ; therefore, management measures are also identified and discussed