What is the biochemical explanation for the observed toxic effects of arsenic contaminated well water in Bangladesh?

It is believed that the high levels of Arsenic in the groundwater was because of abstraction of water from alluvial / deltaic aquifers. The cycling of toxic arsenic could be because of cyclic chemical and biochemical reactions like oxidation, reduction, volatalization, absorption, methylation, and precipitation.

Also the oxidation of arsenic pyrites and oxy-hydroxide reduction of arsenic can also be the results of arsenic contamination in water.

These processes might have expended the arsenic concentration in groundwater in bangladesh.