Assessment of water quality in the peripheral rivers of Dhaka city

Abstract

Dhaka city, the capital of Bangladesh, is surrounded by a circular river system. The riversystem includes the Turag river, Buriganga river, Dhaleswari river, Lakhaya river, Baluriver and Tongi Khal. Sufficient quantity of water remains in the rivers during the fivemonths of monsoon season but the flows are practically nil during the dry period excepttidal backflow from the Meghna river. The river system receives solid wastes, sewage andwastewaters discharged from domestic, commercial and industrial activities both withinand outside the city. These waste materials are tremendously deteriorating the waterquality in the rivers. The contamination level.reaches at such an alarming stage during dryperiods that not only the ecosystem in the river collapses but also the water treatmentplants of the city water supply system, developed based on the river water, virtually lossestheir expected quality of production. Besides, extensive dependency on groundwater forcity supplies in the last decade has severely depleted the groundwater table. Thus,preservation of water quality in the river system is crucial for its sustainability as well asto have a probable alternative source to meet up demand of city water supplies. This studyhas put best efforts to give an overview about the present contamination scenario of theperipheral river system around the city including historical trend of the pollution.It is observed that required dissolved oxygen (DO) for sustaining aquatic lives (5 mg/l)prevails only in the Dhaleswari river and in a very short downstream reach of the.Lakhaya river throughout the year. The rest of the river reaches maintain lower DO level,usually less than I mg/l, in the dry period.In consideration of drinking standards, the river water is contaminated by severalparameters, which include organic matter, suspended solids and microorganisms.Ammonia level in different reaches of the river system is well above the permitted valuespecified in the United State Environmental Protection Agency (USEPA) guideline toavoid toxic effect on fishes. Concentrations of Nitrate (N03-),Phosphate (P043-),Zinc(Zn), Chromium (Cr), Lead (Pb) and Mercury (Hg) in the river system are well below theallowable limits specified in different Environmental Quality Standards (EQS).In general, water quality in the peripheral rivers is deteriorating day by day. TheDhaleswari river (Nabinagar to Kalagachia) and down stream of the Lakhaya river(Moshinabanda to Kalagachia) are the least polluted reaches. Minimum content ofdissolve oxygen falls below 5 mg/l in the upstream of the Lakhaya river (Demra toRupganj). although concentration of other pollutants is comparatively less. It isnoteworthy that the Dhaleswari river (Nabinagar to Kalagachia) is the only reach thatmaintains suitable dissolved oxygen for fishes throughout the year.The assessment from this study identifies that water of some least polluted river reacheslike, the upper and lower reaches of the Lakhaya river (Derma to Rupganj andSiddhirganj to Kalagachia) and the Dhaleswari river (Nabinagar to Kalagachia) could beused in city water supply providing required treatment for few contaminants such as,organic matter, ammonia, suspended solids and microorganisms.The study findings highlights that it is essential to make provisions for improving waterquality in the peripheral rivers to sustain the city water supply system, the ecosystem inthe rivers and, above all, the overall environment of the capital city.