Great Findings on the Dug Wells Water Quality in Old Dhaka

Author: Shovon Barua

Dhaka, the capital and central city of Bangladesh, with a population of over 15 million, suffers from both water scarcity and water pollution. Dhaka city is dependent mainly on groundwater resources from fluvio-deltaic Dupi Tila aquifer of Pliocene age. The water productivity is about 2180 MLD (million liter per day) against the current demand for 2250 MLD that is withdrawn by about 622 DWASA tube wells with 3040 km long pipeline network. City of Dhaka immediately requires alternative water resources. This study was conducted to investigate the water quality of existing dug wells to determine the potential of dug wells to use as alternative source of safe water.

Geology of Dhaka City

Physiographically Dhaka city is situated in the southern half of the Madhupur Tract and Floodplain area with southern river system. Regional elevation of the area gradually declines towards Buriganga River on the south and the elevation ranges from 10m to 17m but is generally around 14m above mean sea level. Geologically Dhaka city belongs to Bengal Foredeep and is situated in the Pleistocene uplifted block (Madhupur Tract) within the passive margin surrounded by subsiding floodplains bounded on the west by a series of NW-SE trending en-echelon faults including the Dhamrai, Maijail and Kaliakoir ones. Land surface is covered by gray floodplain and non-Cretaceous floodplain soils. Stratigraphically, Old Dhaka is characterized by hundreds of meters thick unconsolidated sequence of fluvio-deltaic deposits many composed of gravels, sands, silts and clays of Plio-Pleistocene age.

Procedures

There were as many as 120-150 Dug wells in Old Dhaka during the British tenure. Now-a-days the figure is reduced to 50-60. Among them thirty six dug wells water samples were collected randomly and four river water samples were collected from four different boat terminals in Buriganga River during both dry and wet periods. Analytical results of samples have been evaluated and compared with various drinking water quality parameters following guidelines of World Health Organization (WHO, 2011), Department of Environment, Bangladesh (DOE, 1997) and United States Environmental Protection Agency (USEPA, 2011).

Great Findings on the Water Quality of Dug Wells in Old Dhaka, Bangladesh Revealed. Location map showing the position of sampling points in Old Dhaka

Location map showing the position of sampling points in Old Dhaka, Bangladesh

Findings

The hydrochemical classification shows that both dug wells and river water samples are Ca2+-HCO3 type indicating that the dug wells receive water from the adjacent Buriganga River. Above 50% dug well water samples exceed the DW standard of Ca2+, K+ and Fe2+ ions and below 25% Buriganga river water samples are within the acceptable limits during both dry and wet periods. Calcium carbonate and bleaching powder are used to treat water from dug wells for domestic purposes. Plastering is employed to avoid the caving in and also to keep the dug well wall dry and algae-free. It has not been deciphered whether the dug wells are being recharged from surface water flowing directly into the well or groundwater that has recharged via riverbank filtration. It is urgent to ensure the proper maintenance of these historical dug wells by the concerned authorities as well as to mitigate water scarcity problem using dug well as alternative water resource.

See How MAR Improves Groundwater Quality in Bangladesh

Author: Md. Mahadi Hasan

Seasonal safe drinking water scarcity is very severe in the southwestern coastal belt of Bangladesh. The occurrence of naturally brackish groundwater, vulnerability to climate change and sea level rise and repeated contamination of surface water sources e.g. ponds by both natural (e.g. cyclonic storm surges) and anthropogenic activities (e.g. brackish water aquaculture) are responsible for the increasing demand of fresh water in this region.

To meet this ever increasing demand of fresh water, managed aquifer recharge (MAR) has been tested as an alternative, cost effective and disaster resilient option to supply fresh water during the dry period.

Model of Managed Aquifer Recharge implemented in Assasuni upazila, Bangladesh

Model of Managed Aquifer Recharge implemented in Assasuni upazila, Bangladesh

 

Concept of Managed Aquifer Recharge

Managed aquifer recharge, also known as artificial recharge (AR), can be defined as the infiltration or injection of water into underground formations by some methods and encompasses both aquifer recharge without abstraction and recharge in times of surplus for later abstraction to meet need in time of demand.

An aquifer is a porous and permeable geologic formation or group of formation or part of a formation that contains and yields water to wells or springs and is replenished by both naturally or by human activities. The aquifer can be considered as “warehouse” for storing water and is the attractive and technically feasible and environment friendly alternatives for storing surplus monsoon run off. Aquifer recharge through human activities can be grouped into the following categories:

Unintentional: It includes removing phreatophytes, seepage from irrigated areas, pipeline leaks and sewers.

Unmanaged: It encompasses storm water drainage wells, septic tank leach fields for the disposal of unwanted water without the purpose of reuse.

Managed: Aquifer recharge by injection or infiltration through managed way for later recovery.

Artificial recharge method has been divided into several categories by various authors but the recharge method can be broadly divided into two categories-infiltration and injection. Infiltration methods include recharge basins, surface spreading, irrigation pits, and trenches whereas injection is carried out using injection wells or series of bores. Infiltration is suitable for shallow, porous, unconfined aquifer and injection is carried out for deeper confined aquifer. See more about managed aquifer recharge here.

 

Managed Aquifer Recharge in Bangladesh

For piloting the potentiality of managed aquifer recharge, areas with no or very low densities of conventional safe water sources like STW, DTW and other sources like SST, VSST, PSF, have been identified as primary sites by GIS analysis. Different physical, physico-chemical, hydrogeological, geochemical, social and economic criteria have been investigated in the field. Aquifer characteristics has been determined by exploratory drilling and grain size analysis. Finally two test sites have been selected to test managed aquifer recharge based on the selected set of criteria.

Four to six infiltration wells of 12 to 22 inches diameter have been drilled to a depth of 60 or 75 feet applying direct circulation rotary drilling method using locally available materials and drillers. Roof top rain water has been diverted and pond water has been pumped into the infiltration wells at two test sites after primary treatment in the filtration tank fitted with sand filter to remove turbidity and providing with a total head of about 3 feet. During the study period (monsoon 2011) a total of 392m³ and 827m³ of water have been infiltrated using gravity at the two Managed Aquifer Recharge test sites in Assasuni and Batiaghata respectively.

 

Results and Findings of Managed Aquifer Recharge

Different physico-chemical parameters have been analyzed before and after the implementation of managed aquifer recharge. The initial electrical conductivity of the ambient groundwater were as high as 5.82mS/cm and 1.79mS/cm respectively while after infiltration the electrical conductivity decreases to 0.82mS/cm and 0.71mS/cm respectively at the two sites.

The concentration of major chemicals of concern such as arsenic, manganese, chloride of the ambient groundwater were above the established Bangladesh drinking water standard. After infiltration, arsenic concentration substantially decreased from 0.1 to 0.01mg/l, manganese dropped from 0.63 to 0.15mg/l and 0.2 to 0.14mg/l and chloride concentration declined from 1324 to 500mg/l and 450 to 140mg/l at Assasuni and Batiaghata sites respectively.

Though the quality and quantity of the source water is the main limiting factor, the findings of this study is very encouraging to apply managed aquifer recharge for water banking in shallow brackish aquifer to improve the groundwater quality and to ensure access to safe drinking water in the water stress coastal belt of Bangladesh particularly in the dry period.