A generalized regression model of arsenic variations in the shallow groundwater of Bangladesh

Abstract

Localized studies of arsenic (As) in Bangladesh have reached disparate conclusions regarding the impact of irrigation‐induced recharge on As concentrations in shallow (≤50 m below ground level) groundwater. We construct generalized regression models (GRMs) to describe observed spatial variations in As concentrations in shallow groundwater both (i) nationally, and (ii) regionally within Holocene deposits where As concentrations in groundwater are generally high (>10 μg L−1). At these scales, the GRMs reveal statistically significant inverse associations between observed As concentrations and two covariates: (1) hydraulic conductivity of the shallow aquifer and (2) net increase in mean recharge between predeveloped and developed groundwater‐fed irrigation periods. Further, the GRMs show that the spatial variation of groundwater As concentrations is well explained by not only surface geology but also statistical interactions (i.e., combined effects) between surface geology and mean groundwater recharge, thickness of surficial silt and clay, and well depth. Net increases in recharge result from intensive groundwater abstraction for irrigation, which induces additional recharge where it is enabled by a permeable surface geology. Collectively, these statistical associations indicate that irrigation‐induced recharge serves to flush mobile As from shallow groundwater.

Security of Deep Groundwater in the Coastal Bengal Basin Revealed by Tracers

“…Where higher As concentrations are found in deep pumping wells in this study (e.g., Barisal and Kuakata, Figure ) residence time indicators (Figure ) and hydrochemical evidence (Figure ) indicate that they are unlikely to be associated with the vertical migration of shallow modern water but may represent zones where there have been lower rates of groundwater flushing due to local variations in permeability (Cardenas & Jiang, ; Gassiat et al, ; Hoque et al, ). Indeed, this has been observed elsewhere in the shallow BAS (Shamsudduha et al, ).…”

“…Evidence from small‐scale studies (Harvey et al, ; Neumann et al, ) suggest that surface inputs of organic matter, induced by pumping, can lead to As mobilization in shallow aquifers (<100 m). More recent work by Shamsudduha et al () challenges this suggestion as a regional‐scale phenomenon and shows, using statistical models from national data sets, that groundwater irrigation serves to flush As from the shallow groundwater system and redistribute it to the surface. Elevated salinity and associated high Cl, B, and SO 4 (Figure ) are found at shallow (<100 m) to intermediate (100–200 m) depths at coastal sites and pose a potential threat to underlying fresh groundwater regionally in the coastal BAS.…”

Hydrogeological typologies of the Indo-Gangetic basin alluvial aquifer, South Asia

“…8). Arsenic is thought to result from a higher proportion of argillaceous material and organic matter associated with the fine-medium sands of the Holocene alluvium, which produces reducing groundwater conditions, and within which, historic flushing of groundwater has been limited (McArthur et al 2004;Shamsudduha et al 2015). Many millions of people have developed symptoms of arsenic poisoning from use of the shallow groundwater for drinking water supply since the 1980s in this region (Ravenscroft et al 2004).…”

“…In areas with extensive clay soils (e.g. central Bangladesh), studies have indicated that groundwater recharge is lower than where the soil is more permeable (Goel et al 1977;Shamsudduha et al 2011Shamsudduha et al , 2015.…”

“…Potential rain-fed recharge is high as annual rainfall is greater than 2,000 mm across the typology; actual recharge is limited by available aquifer storage and also by the presence of low-permeability soils in some places (CGWB 2007;Shamsudduha et al 2011Shamsudduha et al , 2015. Deeper groundwater, which forms a strategic resource in this typology due to As-contaminated shallow groundwater, receives little modern groundwater recharge (BGS and DPHE 2001;Shah 2008;Hoque and Burgess 2009;Fendorf et al 2010;Burgess et al 2010).…”