The Impact of Aquifer Flushing on Groundwater Arsenic Across a 35‐km Transect Perpendicular to the Upper Brahmaputra River in Assam, India

Abstract

Well testing in the floodplain of the Brahmaputra River in Golaghat and Jorhat districts of Assam, India, shows that groundwater arsenic (As) concentrations increase with distance from the river. To establish the origin of this pattern, an additional 900 wells <60 m deep were tested for As and 9 sites were drilled along a 35-km transect perpendicular to the river. The field data show no relation between groundwater As concentrations ranging from <1 to 660 μg/L along the transect and (a) As concentrations of <1–5 mg/kg in cuttings of aquifer sand recovered while drilling or (b) the degree of reduction of iron oxides in these cuttings. The drilling indicates, however, a marked increase in the thickness of a clay layer capping the aquifer starting from <1–5 m near the river to over 60 m at the most distant site towards the base of the Naga foothills. Organic radiocarbon ages of 18–46 kyr obtained from all but one of 13 clay samples indicate pre-Holocene deposition of the underlying sands across the entire transect. Radiocarbon ages of dissolved inorganic carbon of 0.2, 4.7, and 17.8 kyr were measured in groundwater from 3 monitoring wells installed to 30–60 m depth at distances of 10, 20, and 40 km from the river, respectively. A conceptual groundwater flow model consistent with monitored heads and groundwater ages suggests that thick clay layers capping the aquifer further from the river inhibited flushing of the aquifer and, as a result, preserved higher As levels in groundwater.

Conceptual Model of Arsenic Mobility in the Shallow Alluvial Aquifers Near Venice (Italy) Elucidated Through Machine Learning and Geochemical Modeling

“…Geogenic (i.e., naturally occurring) arsenic (As) contamination is a well‐known issue that affects unconsolidated and fractured aquifers worldwide (e.g., Choudhury et al, 2018; Desbarats et al, 2017; Erickson et al, 2018; Huang et al, 2018; Jakobsen et al, 2018; Pedretti et al, 2019), including Italian aquifers (e.g., Aiuppa et al, 2003; Carraro et al, 2013; Molinari et al, 2013; Rotiroti et al, 2014; Ungaro et al, 2008). Identifying a valid site‐specific conceptual model able to describe As mobility in groundwater is fundamental for many purposes related to As risk management.…”

Predicting the distribution of arsenic in groundwater by geospatial machine learning technique in two worst hit districts of Assam, India: a risk to public health

“…1). The pattern of this As occurrence is the opposite in comparison to the areas downstream in Bangladesh (Choudhury et al, 2018). Such an observed pattern makes this study unique and challenging in delineating the spatial extent of elevated groundwater As concentrations.…”

“…The measured As concentrations in groundwater were compiled from two sources (Mahanta et al, 2015;Choudhury et al, 2018). These data were generated through a combination of All rights reserved.…”

“…The observation of greater clay fractions and clay capping to greater aquifer depths support leaching of As to groundwater and lack of monsoonal dilution by rainwater. Such an observation increases extensive sediment-water interactions and minimal flushing (Choudhury et al, 2018). Therefore, the composition of the aquifer materials is vital to the occurrences of As in groundwater (Bindal and Singh, 2019).…”

“…Mahanta et al (2015) reported As concentrations >10 ug/L in 29% of the total tested wells based on the state-wide survey of 56,180 tube wells. Verma et al (2016) and Choudhury et al (2018) reported that the high As concentration in groundwater is associated with thick clay capping at the top of the aquifer. Choudhury et al (2018) further suggested that the thick clay layer inhibited flushing of the aquifer, which resulted in high As concentrations in groundwater.…”