Relationship of arsenic concentration with ammonium–nitrogen concentration, oxidation reduction potential and pH of groundwater in arsenic-contaminated areas in Asia

• Reduced ORP and neutral to slightly basic pH were observed in the groundwater.
• The dominant As species in groundwater with high As concentration was hydrogen arsenate.
• We identified a positive effect of NH4–N on As in groundwater with high As concentrations.

This study examines how arsenic (As) concentration is related to ammonium–nitrogen (NH4–N) concentration, oxidation reduction potential (ORP) and pH in groundwater to determine if they have common characteristics in groundwater with high As concentrations in the As-contaminated areas of Bangladesh, Nepal, Cambodia and Vietnam. For the groundwater samples having the three highest As concentrations (top three groundwaters) selected as representatives at each location, As and NH4–N concentrations varied, but ORP and pH did not vary by locations. The ORP value of 30–110 mV, indicating the reducing condition, and the neutral to slightly alkaline pH (pH 7.0–7.6) were presumed to be the key conditions for high As concentrations in groundwater of the study areas. According to the ORP (Eh)–pH diagram, the dominant As species in the top three groundwaters from each location was hydrogen arsenate (HAsO42-). Out of the correlations between the four elements for the top three groundwaters in the areas, only the correlation between As and NH4–N concentrations was positive and significant. Thus, it was identified that NH4–N had an effect on increasing As concentration in groundwater.