Surface Water Quality and its Impact on Water Supply Scenario in Khulna City of Bangladesh

Abstract

To get potable water is an acute problem now a days which is increasing in an alarming rate in southern region of Bangladesh. Khulna, the third largest city in Bangladesh, is located in the southwest of the country and has a population of 1.5 million. Ground water is the main source of supply water presently used by the Khulna Water Supply Authority, KWASA. Water supply for Citizens in Khulna has been suffering from limited access to water supply services. Out of approximately one and half million of population, only 17% has access to piped water supply, and the rest resorts to alternative sources, such as shared public taps and tube wells built privately. The present water supply in Khulna is mainly from ground water sources drawn from both deep and shallow tube wells. In the long term as demand increases, conjunctive use of ground water and surface water will be required, even though surface water may suffer from salinity intrusion in dry season. Due to increasing water demand in the city, the water supply system is vulnerable to supply required water to the people. Extensive abstraction of ground water from deep aquifer causes depletion of ground water level. Water in ground aquifer is not replenishing at the same rate of abstraction. In future, it may not be possible to yield sufficient quantity of ground water necessary for Khulna city. Therefore, to cope with current insufficient supply and increasing demand of potable and domestic water, it is necessary to allocate alternate source instead of groundwater. Therefore, a comparative study of water quality for different rivers in and around Khulna city is necessary. The main objective of this study is to investigate the usability of surface water to reduce the vulnerability of ground water for the water supply system of Khulna City Corporation (KCC) area. There are many rivers flow through south western zone into or nearby the Khulna city and meet with Bay of Bengal in southern part of Bangladesh. Those are the Vairab, Rupsha, Kobadak, Pashur, Mayur, Modhumoti etc. In this thesis, firstly the water quality parameters for four stations of Gorai- Modhumoti river system are presented. These stations are Station-1: Haridaspur (Kumar river), Station -2: Modhupur (Gorai-Modhumoti river), Station -3: Chapailghat (Modhumoti river) and Station -4: Mollarhat (Modhumoti river). After that the water quality parameters of Rupsha- Bhairab river system are presented. Among the water quality parameters turbidity, pH, TDS, EC, chloride, salinity etc. are tested in the laboratory. Mayur river has not been considered in this study due to its insufficient water flow and its upstream source is almost moribund. The water flow source of Mayur river is mainly KCC drainage waste water and rain water in rainy season. Recently, KWASA has selected the Mollahat Point of Modhumoti River as the intake point of raw water for the alternate water source to supply after treatment. In this study, the historical flow characteristics of Gorai-Modhumoti river is also studied to explain the sustainability of the raw water intake site for KWASA. From comparison of water quality parameters, it is found that Modhumoti River contains very low amount of chloride concentration and salinity as compare to the other rivers. The salinity is found to be increasing from February and peak in April. Chapailghat, Modhupur and Haridaspur stations are found to have almost same salinity; highest is about 500 mg/l and lowest is 90 mg/l. Mollarhat contained lowest salinity among the four stations. All of stations contain higher salinity in summer due to lower discharge of river. For low discharge the sea water enter the downstream of river easily and contain higher salinity as compare to the upstream of the river. However, the salinity level is found to be increasing with years. At Mollarhat point of Gorai-Modhumoti river, the discharge is found to be varies nearly zero at winter season to about 4000 cumec in monsoon. The maximum discharge is found to be varied from 1500 to 4000 cumec per year. Annual minimum discharge is varied from zero to 85 cumec for different years. Very low discharges are observed in the years of 2009 to 2011. Like discharge, water level is found to be varied with seasons from nearly 1 m at winter season to about 8 m in monsoon. From the linear trend line it is observed that from an average maximum water level of 8.23 m in 1995, it is decreasing by 0.056 m per year. Annual minimum water level is varied from 1.0 to 3.0 m for different years. Due to the dredging at the mouth of Gorai river in 2011, the water level is increased in 2012, which is again gradually decreased in the following years due to siltation at the mouth of Gorai. In the low water flow season, the average water flow rate in Modhumoti river was 25.5 m3/sec in 2015. The planned water intake volumes for Khulna City by KWASA are 110,000 m3/day (= 1.273 m3/ sec) in 2025 and 220,000 m3/day (= 2.546 m3/ sec) in 2030. Therefore, in low flow season the water will be drawn by a rate of 5.0% in 2025 and 10.0% in 2030. However, the feasibility study of KWASA assumed that safe amount to intake water from the river is less than 5%. Modhumoti River is mainly comes through Gorai river from Ganges. A small part comes from Arial khan River through upper Kumar river. In both the source rivers, the discharges are found in decreasing trend both in monsoon and winter. It is evident from the Gorai river discharge and dredging history at Gorai mouth that dredging effectively increases the flow in Gorai river, though huge siltation affect negatively afterwards. Therefore, to increase the discharge of Gorai river, it is necessary to keep dredging on a regular basis of Gorai River, which will maintain potential flow of Modhumoti River. In addition, Kumar river bed dredging to divert flow of Arial Khan river may help further to increase the flow in Modhumoti River. Flow Divider at Gorai Off-take can be installed to divert Ganges flow into Gorai-Modhumoti river.