4.7. Implications of the Expected Nature of Arsenic in Groundwater on Treatment

     The graphs of arsenic concentration versus oxidation-reduction potential and arsenic concentration versus pH are shown in Figures 14 and 15, respectively.

Figure 14
Figure 15
Figure 14. Graph of arsenic concentration (mg/L) versus oxidation-reduction potential in water from all tubewells regardless of depth.
Figure 15. Graph of arsenic concentration (mg/L) versus pH in water from all tubewells regardless of depth.

The increase in arsenic concentration at relatively low oxidation-reduction potentials and moderate pH values suggests that soluble As(III), rather than difficultly soluble and potentially colloidal As(V), was the dominate form in the most highly affected tubewells (19). If the arsenic is in the soluble As(III) oxidation state, then oxidation to difficultly soluble As(V) followed by coagulation, filtration, or sorption is required for effective treatment (16).