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.
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.