Development of adaptation strategies to groundwater level declination in Dhaka city

Abstract

Dhaka city, having a population of more than 15 million, depends on groundwater (GW) as a source of quality water. Recently, the city is encountering GW diminution and the declining scenarios are dissimilar in different parts of the city. This study was carried out to analyze the GW depletion in different parts of Dhaka city from 1970 to 2019. Groundwater level (GWL) data of different locations were collected from Bangladesh Water Development Board (BWDB) and Dhaka Water Supply and Sewerage Authority (DWASA). Trends in annual maximum, minimum and median GWLs were assessed to quantify GW depletion rates by simple linear regression method. Non-parametric tests, namely modified Man-Kendall and Sen’s slope, were used to identify and quantify the trends and their significances. Also, contour maps were generated to develop the contemporary GW level scenario of Dhaka city. Spatial analyst tool, Inverse Distance Weighted method, was used to prepare the maps in Arc-GIS. Three contour maps were generated for the trends in maximum, minimum and median GWLs. The contour map of maximum GWL trend exhibits that groundwater flows from all around to central area of Dhaka city. Maximum GWL is found to be decreasing all over Dhaka city. The contour map of minimum GWL trend exhibits that the southwestern part of the city like Khilgaon and Motijheel is facing severe depletion rate. Recharge to upper aquifer to these areas is very low. The contour map of median GWL is similar to that of minimum GWL. From GWL trend analyses and contour maps, vulnerable locations susceptible to severe GW depletion are identified as Khilgaon, Sobujbagh, Motijheel, Dhanmondi, Sutrapur areas and some parts of Cantonment and Mirpur areas. DWASA is solely responsible for supplying piped water for domestic and commercial purposes. Around 920 deep tubewells (DTWs) of DWASA and 2600 DTWs of private users now extract water from GW sources. DWASA is following some strategies to reduce the pressure on groundwater. Some strategies are focused on supply augmentation like surface water treatment plants, sustainable groundwater plant and installation of new DTWs, and other strategies are focused on demand management like reducing non-revenue water (NRW) by replacing old connection pipes, emergency water supply through water carrier, safe drinking water to consumer by water ATM booth and adopting rainwater harvesting. The effectiveness of these strategies was assessed by SWOT analysis to stand for future adaptabilities. Prior to operation of the Padma (Jashaldia) Water Treatment Plant (Phase-I), water supply ratio from GW to surface water (SW) sources was 78% to 22% in 2019 which had been reduced to 65% to 35% nowadays. DWASA is highly concerned about rapid GW depletion and therefore has prepared a water supply master plan in 2014 with a timeframe up to 2035. DWASA has undertaken some projects to augment supply from SW sources. Construction of two more water treatment plants is ongoing which will add 950 MLD of water from SW sources. A sustainable GW Plant named as Tetulzhora-Bhakurta Well Field Plant Project (Phase-I) is functioning and adding another 150 MLD to the existing water supply. Moreover, two projects focusing on surface water treatment plant and one project on GW plant with capacity of 1150 MLD are planned to be implemented in near future. The aim of DWASA is to change the water supply scenario and to reduce the GW extraction below 20%. Earlier DWASA was facing the problem of high system loss, but at present, it is reducing the system loss by replacing the old connection pipes. After establishment of all District Metered Areas, the NRW would reduce to 15% from 40% and the target is to reduce it to below 5%. Before functioning of all surface water treatment plants, to address supply shortage, the emergency water supply project will add more DTWs including rehabilitation and regeneration of existing DTWs. As demand management strategies, block tariff structure is mentioned in the master plan, though it is yet to be approved. Provision for rainwater harvesting and artificial recharge (AR) is included in the master plan and a prototype design for artificial GW recharge at household level had been done. Yet DWASA could not implement RWH and AR at household level. Dual plumbing system to incorporate RWH and AR as well as greywater reuse should be made compulsory to consumers. Implementation of managed aquifer recharge is still in planning stage. To develop new adaptation strategies, key informant and semi-structured interviews were performed with DWASA officials, GW experts, researchers and DWASA consumers. Upcoming and current strategies by DWASA were discussed with the professionals. They have emphasized some of the strategies adopted by DWASA and also suggested some new strategies to face upcoming GW depletion scenarios. They also explained how to move forward without causing more depletion. They also addressed some strategies adopted by DWASA which are not praiseworthy. Strategies those are highly emphasized in line with DWASA are adopting more surface water treatment plants, creating alternative sources, providing special attention to RWH, adopting increasing block tariff system, reducing NRW, monitoring family wise consumption by metering, introducing water saving faucets and promoting public awareness.