Availability of Groundwater for Drinking Purpose in Khulna City

Abstract

Khulna, the industrial and port city, is the third largest city of Bangladesh which is situated in the south west part of Bangladesh and lies in the delta of the Ganges. The city itself stretches 15 km along the River Bhairab and Rupsha, covering the area of approximate 46 sq. km. having population around 1 million. The scarcity of water has been augmented gradually owing to increasing resettlement from the surrounding districts for rapid urbanisation and industrialisation. On the other hand the water supply situation has been aggravated due to lack of necessary parallel growth in the water supply infrastructure. This study focused on identification of the number of aquifers and potentiality the aquifers by investigating the available geological, hydro-chemical, and hydrodynamic parameter of the aquifers. Comprehensive geological cross-sections and a conceptual model of the deep aquifer have been developed using lithologs of previous LGED study (MSP 2005) and DPHE (Rus, 1985) investigation in Khulna area. The estimation of annual ground water recharges in the most potential aquifer has been performed to identify the exploitable (usable) locations for drinking purposes. Data on lithologs, static water level, electrical conductivity, temperature, pH, arsenic, and pumping tests in different aquifers have been collected and analyzed in order to fulfill the study objectives. The data of two special ground water monitoring sessions in 2009 has been collected in order to assess the present situation, comparison to the situation in 2005, for the deep aquifer in Khulna. Three different aquifers have been identified such as upper shallow, shallow and deep aquifer. It seems that, in some areas the upper shallow aquifer is connected with shallow aquifer. The observed fluctuating water heights for deep, shallow and upper aquifers are 2.13 m, 2.26 m and 1.97 m, respectively. It has been observed that the transmissivity values of deep aquifer varies from l.3x10·3 m2/sec to 7.lxlff2 m2/sec and the storage coefficient varies from 9.7x10-4 to 4.7x10·3.

It is also observed that the average temperature of KWASA production wells in the central part of the city is much higher than those in other part of the study area. There is no indication of increase of average temperature neither in KWASA production well nor in MSP monitoring well since 2005.There is no increase in the average EC value for the MSP monitoring wells since 2005. The average SWL in the project area was - 1.42 m in 2005 and - 2.06 m in 2009 it from mean sea level (MSL), thus the net average declination of SWL in last four years is 0~63 m. Most important observation is that, this mass of declination of the SWL has affected neither EC nor Temperature. Note that the average temperature of water in the central area is higher than the normal range. For highest static water level in the month of October, usable recharge has been obtained as about 40,470 m3/day which is slightly more than the average abstraction rate of 40,000m3/day while for lowest static water level in the month of April, usable recharge has been obtained as about 36,070 m3/day. It is true that the complete dependency on ground water for Khulna city water supply, with the present aquifer system, may not sustainable for long term abstraction planning. Since the fresh water resource of the deep aquifer is limited and cannot sustain substantial increase in abstraction rates, an additional volume of drinking water may be derived from blending of water of different sources. The Electrical Conductivity of the deep aquifer near and south of the city centre is very low. There might have few solutions to mitigate the water crisis of the city; i) small quantity of moderately mineralised water from the shallow aquifer can be mixed with water from deep aquifer within the acceptable limit this will increase substantial amount of overall water production. It can be reminded that the water quality of the shallow aquifer is good (and arsenic free) except for the high concentration of saline and with some iron. Iron of the shallow aquifer might be removed by simple aeration and filtration technique. ii) use of surface water for the period of low salinity in the surface water (7-8 months' time in the Bhairab- Rupsha River) to preserve and restrict the abstraction from deep aquifer for those period and supply the ground water for the period of high saline in the surface water (February to June). iii) under WASA Act: 1996, KWASA can impose control over the industrial abstraction from the deep aquifer. In order to restrict abstraction from deep aquifer, for industrial use, KWASA can provide them alternate water through the surface water sources throughout the year.