Mechanism of groundwater response to recharge and ITS quantification for shallow aquifers in Bangladesh

Abstract

Groundwater is used intensively in Bangladesh part of the Bengal Basin but the processes by which groundwater recharge (i.e., replenishment) takes place under different hydrogeological conditions remain inadequately understood. In Bangladesh, municipal water supplies in major cities (e.g., Dhaka) derive primarily from deep groundwater at depths greater than 150 m below ground level (bgl) whereas supplies for dry-season irrigation mostly use shallow groundwater (< 150 m bgl). Shallow groundwater occurs in two dominant depositional environments: (i) permeable alluvial deposits of the Holocene that occupy about 80% land area of Bangladesh, and (ii) terrace deposits of the Plio-Pleistocene age that cover about 8% land area that are overlain by relatively impermeable clay and silty-clay deposits (i.e., Barind and Madhupur Clay). This study, for the first time, characterises groundwater recharge to shallow alluvial aquifers in two geologically contrasting environments in Bangladesh using high-resolution (half-hourly) records of groundwater levels and rainfall and develops a conceptual model of recharge pathways. Responses of intensive abstraction for dry-season irrigation and climate change (i.e., high intensity rainfall events) to shallow aquifers were investigated in Bhuanpur of Tangail (Holocene) and Savar of Dhaka (Plio-Pleistocene) where automatic data loggers were installed in shallow boreholes with co-located rain gauges at each site. In addition, weekly monitoring records from the Bangladesh Water Development Board were analysed to characterise long-term impacts of intensive abstraction for irrigation and of climate variabilities. These high-frequency, short-term (2009-2011) and low-frequency, long-term (1987-2014) observations reveal both high-frequency (e.g., irrigation pump on and off, earth-tide response) as well as longer-term changes (e.g., seasonality, declining trends) in hydrographs. Hydrograph analyses also reveal contrasting responses to high intensity rainfall at Bhuanpur and Savar sites. At Bhuanpur, groundwater levels rise rapidly due to high intensity rainfall indicating rain-fed, diffuse recharge. At Savar groundwater levels do not respond to high intensity rainfall and recharge is restricted to indirect (i.e. focused) recharge occurring where the incision of river channels through low-permeability clays permits flow from rivers to shallow groundwater (or alluvial aquifers). A clear lag between surface water and groundwater levels suggests that there is no discernible ‘poroelastic’ effect (i.e. groundwater level responses arising from compression) in these two sites. The compiled, evidence-based insight into recharge processes highlights the importance of indirect recharge and implies that there are areas in the Bengal Basin where the estimation of recharge cannot meaningfully be based on the assumption that disregards indirect recharge and is unreconciled surface geology. A cluster analysis of 464 BWDB borehole hydrographs (1994-2013) reveals characteristics (i.e. typologies) of groundwater regimes at the national-scale in Bangladesh and shows where ‘Bhuanpur’ and ‘Savar’ type responses of irrigation abstraction and climate change in groundwater levels can be expected. In addition, the cluster analysis has identified four dominant types of aquifer characteristics depending on their long-term groundwater level trend and seasonality. The reason behind the behaviours of the aquifer typologies were explained using three dominant factors namely, abstraction, surficial silt-clay thickness and local topography of the lands. For example, long-term hydrograph analysis reveals that shallow groundwater depletion is pronounced in areas (13% of total 484 boreholes) where low hydraulic conductivity of surficial soils (Savar-type Plio-Pleistocene environment) inhibit rainfall-fed recharge and leads to reduced seasonality in hydrographs. Here, situation of high abstraction, relatively higher lands and thick (upper) silt-clay cause unsustainable groundwater level condition. In contrast, one of the typologies of aquifer in Holocene that comprises high permeable surficial geology (20% of total 484 boreholes) at relatively low lands under moderate/high abstraction can be a potential source of increased (induced) groundwater recharge where potential recharge exceeds actual (pre-development) recharge. This concept of capturing additional recharge from rainfall, floodwater and surface water through increasing storage in shallow aquifers by pumping during the dry-season is referred to as the “abstraction-induced recharge (A-IR)”- an untested concept found in literature. Moderately high lands when come under moderate abstraction exhibit moderately declining groundwater level (both in terms of trends and fluctuations) but further increase in abstraction volumes may lead to unsustainable ‘Savar-like’ fluctuation pattern in future. The local lateral groundwater flow contribution (i.e., lateral replenishment from neighbouring higher grounds) from mixed type of local relief with low to moderate abstraction situation are mostly responsible for displaying slightly declining-type groundwater level pattern. Testing the popular assumption that increased rainfall (i.e., projections for future climate scenarios) will only favour groundwater recharge in Bangladesh, this research challenges the widely applied assumption that all recharge occurs directly through the direct infiltration of rainfall and reveals that groundwater recharge may only increase through greater rainfall in certain areas of Bangladesh under projected climate change where AI-R concept is observed to operate. From a basin-wide perspective, the research also shows clearly that there are areas in the Bengal Basin where the A-IR concept does not operate. Example of such areas include but not limited to Savar-type Plio-Pleistocene environments where increased abstraction and projected climate variabilities may further decline groundwater storage. Considering the impacts of future climate projections on low-flows in major rivers, this study proposes four potential sub-basins or areas in Bangladesh where piloting of the A-IR concept as a potential climate-adaptation strategy can be implemented. Specifically, areas along the upper River Brahmaputra and the north of the Teesta floodplain have the highest capacity to induce greater recharge. As the annual cropping intensity of the north of Bangladesh has already risen to more than 220%, it is proposed that the areas within few kilometres of the buffer zone along the River Brahmaputra can be considered as pilot projects for the tests of the A-IR concept.