Groundwater recharge processes in an Asian mega-delta: hydrometric evidence from Bangladesh

Sara Nowreen,R. G. Taylor,K. M. Ahmed,A. Zahid,M. Salehin,M. Shamsudduha

doi:10.1007/s10040-020-02238-3

Abstract

Groundwater is used intensively in Asian mega-deltas yet the processes by which groundwater is replenished in these deltaic systems remain inadequately understood. Drawing insight from hourly monitoring of groundwater levels and rainfall in two contrasting settings, comprising permeable surficial deposits of Holocene age and Plio-Pleistocene terrace deposits, together with longer-term, lower-frequency records of groundwater levels, river stage, and rainfall from the Bengal Basin, conceptual models of recharge processes in these two depositional environments are developed. The representivity of these conceptual models across the Bengal Basin in Bangladesh is explored by way of statistical cluster analysis of groundwater-level time series data. Observational records reveal that both diffuse and focused recharge processes occur in Holocene deposits, whereas recharge in Plio-Pleistocene deposits is dominated by indirect leakage from river channels where incision has enabled a direct hydraulic connection between river channels and the Plio-Pleistocene aquifer underlying surficial clays. Seasonal cycles of recharge and discharge including the onset of dry-season groundwater-fed irrigation are well characterised by compiled observational records. Groundwater depletion, evident from declining groundwater levels with a diminished seasonality, is pronounced in Plio-Pleistocene environments where direct recharge is inhibited by the surficial clays. In contrast, intensive shallow groundwater abstraction in Holocene environments can enhance direct and indirect recharge via a more permeable surface geology. The vital contributions of indirect recharge of shallow groundwater identified in both depositional settings in the Bengal Basin highlight the critical limitation of using models that exclude this process in the estimation of groundwater recharge in Asian mega-deltas.

Highlights

Thick sequences of unconsolidated sediments occur in ‘megadeltas’ throughout Asia and comprise the termini of majorElectronic supplementary material The online version of this article contains supplementary material, which is available to authorized users.Engineering and Technology, Dhaka, Bangladesh 3 Department of Geography, University of Sussex, Brighton, UK 4 Bangladesh Water Development Board, Green Road, Dhaka, Bangladesh 5 Department of Geology, University of Dhaka, Dhaka, Bangladesh drainage basins that include the rivers Ganges-BrahmaputraMeghna (GBM), Indus, Irrawaddy, Chao Phraya, Mekong, Red (Song Hong), Pearl (Zhujiang), Yangtze (Chiangjiang) and Yellow (Huanghe; Fig. 1)
This paper explores fundamental questions about the nature of monsoonal recharge in a mega-delta environment: What are the dominant processes by which recharge occurs? How do these processes vary under different depositional environments and soil lithologies? How has intensive groundwater withdrawals influenced recharge dynamics? This exploratory research exploits hydrometric evidence as the Bengal Megadelta of Bangladesh (~100,000 km2) that features networks of dedicated monitoring stations for daily rainfall (~300), weekly groundwater-levels (~1,200), and subdaily to daily river stage (~300); groundwater-level records, managed by the government’s agency, Bangladesh Water Development Board (BWDB), have been collected since early 1970s
Time-series (1987–2014) records illustrate long-term patterns in BWDB groundwater-level (GWL) fluctuations at Bhuapur and Savar (Fig. 3); details of these BWDB monitoring boreholes are provided in Table 1 and Fig. S1 of the electronic supplementary material (ESM)

Summary

Introduction

The sustainability of groundwater withdrawals and the processes by which groundwater is replenished in Asian mega-deltas, remain inadequately resolved. This knowledge gap constrains the modelling of groundwater recharge in response to global change and our understanding of the vulnerability of shallow groundwater resources to contamination (e.g., Fendorf et al 2010; Burgess et al 2011; van Geen et al 2013) and depletion (e.g., Shamsudduha et al 2009, 2012; Bui et al 2012)

Results

Discussion

Conclusion