Abstract
Arsenic contamination of groundwaters in South and Southeast Asia is a major threat to public health. In order to better understand the geochemical controls on the mobility of arsenic in a heavily arsenic-affected aquifer in northern Kandal Province, Cambodia, key changes in inorganic aqueous geochemistry have been monitored at high vertical and lateral resolution along dominant groundwater flow paths along two distinct transects. The two transects are characterized by differing geochemical, hydrological and lithological conditions. Arsenic concentrations in groundwater are highly heterogenous, and are broadly positively associated with iron and negatively associated with sulfate and dissolved oxygen. The observed correlations are generally consistent with arsenic mobilization by reductive-dissolution of iron (hydr)oxides. Key redox zones, as identified using groupings of the PHREEQC model equilibrium electron activity of major redox couples (notably ammonium/nitrite; ammonium/nitrate; nitrite/nitrate; dissolved oxygen/water) have been identified and vary with depth, site and season. Mineral saturation is also characterized. Seasonal changes in groundwater chemistry were observed in areas which were (i) sandy and of high permeability; (ii) in close proximity to rivers; and/or (iii) in close proximity to ponds. Such changes are attributed to monsoonal-driven surface-groundwater interactions and are consistent with the separate provenance of recharge sources as identified using stable isotope mixing models.
Highlights
Millions of people in South and Southeast Asia are chronically exposed to dangerous concentrations of geogenic arsenic in groundwater, at levels which exceed the World Health Organization drinking water provisional guideline of 0.13 μM (10 μg·L−1) (Smedley and Kinniburgh, 2002; Charlet and Polya, 2006; World Health Organization, 2011; Ravenscroft et al, 2009)
The arsenic in these shallow aquifers is widely thought to be released into groundwaters following the reductive dissolution of arsenic-bearing Fe(III) minerals (Islam et al, 2004), a process which is driven by metal reducing bacteria and fuelled by organic matter providing an electron donor (Charlet and Polya, 2006; Islam et al, 2004; Postma et al, 2007; Bhattacharya et al, 1997; Rowland et al, 2009)
In the Mekong Delta of Cambodia, anoxic sediment conditions created by extensive monsoonal flooding cause favourable anaerobic processes such as arsenate respiration to occur (Charlet and Polya, 2006; Islam et al, 2004; Postma et al, 2007; Bhattacharya et al, 1997; Rowland et al, 2009; Ying et al, 2015)
Summary
Millions of people in South and Southeast Asia are chronically exposed to dangerous concentrations of geogenic arsenic in groundwater, at levels which exceed the World Health Organization drinking water provisional guideline of 0.13 μM (10 μg·L−1) (Smedley and Kinniburgh, 2002; Charlet and Polya, 2006; World Health Organization, 2011; Ravenscroft et al, 2009) The arsenic in these shallow aquifers is widely thought to be released into groundwaters following the reductive dissolution of arsenic-bearing Fe(III) minerals (Islam et al, 2004), a process which is driven by metal reducing bacteria and fuelled by organic matter providing an electron donor (Charlet and Polya, 2006; Islam et al, 2004; Postma et al, 2007; Bhattacharya et al, 1997; Rowland et al, 2009). Chemometrically defined redox zones, as defined by trends in various redox-sensitive solutes such as arsenic, methane, iron, manganese, sulfate, nitrate and ammonium, can overlap as result of overprinting of different redox conditions, leading to partial equilibrium conditions and complex interactions among electron acceptors (Mukherjee et al, 2008), as well as affecting kinetic limitations on redox processes and/
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