Chapter 3 Biogeochemical Processes Controlling the Fate and Transport of Arsenic: Implications for South and Southeast Asia

Abstract

Arsenic is a ubiquitous toxin present in soils and waters resulting from both natural and anthropogenic sources. Within South and Southeast Asia, tens of millions of people are drinking groundwater having arsenic concentrations exceeding the recommended standard of the World Health Organization (10 μg L−1). Arsenic originates within minerals of the Himalaya. During weathering and erosion, arsenic is transported down the large river systems draining the mountains in the sediment load primarily as arsenic-bearing iron oxides and then deposited in the Bengal Basin, and Irrawaddy, Mekong, and Red River Deltas. The key biogeochemical step leading to human exposure of arsenic via drinking water is a result of arsenic release from soil/sediment solids into pore water. With the exception of ‘extreme’ pH values (pH 9) or high concentrations of competing anions such as phosphate, arsenic release is predicated on the aeration (or redox) status of the environment. Within aerated soils and sediments, arsenic predominates in the As(V) oxidation state and typically binds strongly to soil solids. Upon a transition to anaerobic conditions, arsenic is reduced to As(III), and while binding appreciably to Fe(III) (hydr)oxides, provided they are present, is labile and thus subject to migration and biological uptake. The transition from As(V) to As(III) is, in fact, an important transformation impacting As within reducing environments (discounting sulfogenic systems). The fate and transport of arsenic under anaerobic conditions, however, is further modified by reactions of and with Fe. Although transformation products of ferrihydrite reduction can lead to transient sequestration of arsenic, iron (hydr)oxide reductive dissolution further promotes dissolved concentration of arsenic. Within the aquifer systems of South and Southeast Asia, organic matter (co-deposited, incorporated, or dissolved) promotes anaerobic conditions in soils/sediments residing below the water table, leading to reductive release of arsenic. Arsenic release is most prominent in environments where organic matter is incorporated into anaerobic systems (such as permanently saturated wetlands), yielding the greatest magnitude of arsenic and iron reduction.