Recharge estimation in and around Khulna city by alternative methods

Abstract

Khulna city and its surroundings are selected as the study area, which is located at the south western part of the country and approximately one hundred km north of Bay of Bengal. To meet the water demand of the city, water supply from surface water source was first started in 1921. Later due to salinity intrusion in the rivers, Directorate of Public Health Engineering (DPHE) started supplying drinking water since 1960 in the city using its groundwater source. To respond to increasing water demands in the city, several investigations had been carried out to evaluate the groundwater resources of the region. In a significant investigation, it was found that Khulna City has two aquifers, shallow and deep, and opined that deep aquifer of Khulna is in mining state. Recent study that has been conducted by Local Government Engineering Department (LGED) to find out the status of the groundwater potentials in the city concluded that Khulna has three distinct aquifers. These aquifers of Khulna are continuously recharging. The present study focuses on estimation of annual groundwater recharges in different aquifers, to identify exploitable (usable) locations for drinking purposes, to estimate usable recharges and finally to compare the results with LGED investigation. 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. In order to investigate the independence of the three prevailing aquifers (upper, shallow and deep) - analysis has been done from geological, hydro-geological and quality point of views. Three geological cross-sections (one in N-S and two in E-W direction) have been constructed. Presence of separate confining layers, individual static water levels and sensibly dissimilar water qualities in different depths validate the existence of those three aquifers. In order to estimate fluctuating water heights from the monitoring wells of deep, shallow and upper aquifers in the study area, weighted average method has been applied using Thiessen polygons. It has been observed that all the three aquifers have fluctuating water levels and the average water heights for deep, shallow and upper aquifers are 2.13 m, 2.26 m and 1.97 m, respectively. Water quality has been assessed using electrical conductivity (EC), temperature, pH and arsenic parameters, in order to find out the usable water areas in different seasons and different aquifers. From water quality point of view it is seen that only the deep aquifer has potential for abstraction. All those parameters show that only 59% of the study area is usable for potable water supply during dry season, while 74% area is found usable during monsoon season. As a result, estimation of usable recharge volume in deep aquifer has ~- taken into consideration. Transmissivity and storativity have been evaluated in the study area from the collected pumping test data. It has been observed that the transmissivity values vary between 1.3x 10-3 m2/sec and 7.1 xl 0-2m2/sec and the storage coefficient varying between 9.7x 10-4 and 4.7x 10-3 . The corresponding averages of transmissivity and storativity are 3Ax 10-2 m2/s (2940 m2/day) and 2.2xl0-3 , respectively. -.t. Iso-piezometric contour lines and flow lines have been constructed for highest static water level and lowest static water level in the months of October and April, respectively in order to estimate the usable recharge volume within its closed contour water levels. 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 40,000 m3/day for the KCC water supply and industrial abstractions for Khulna city and its sl~rroundings. While for lowest static water level in the month of April, usable recharge has been obtained as about 36,070 m3/day. This amount is much less than the KCC average abstraction. As such, under existing rate of abstraction the lowest static water levels are further declining every year. This observation is found valid comparing with the last three years (2003-2005) monitoring data also. Comparison with the modeling study done by LGED also shows that the future recharges would be much less than the future scenario abstractions.