To clarify the effect of peat and groundwater quality parameters on groundwater arsenic (As) contamination, As concentrations and the source of nitrogen (N) in groundwater and peat, and oxidation-reduction potential (ORP) and ammonium-N concentration of groundwater were compared between As-contaminated and uncontaminated areas in Bangladesh. Groundwater in the contaminated area had a reducing ORP with a high ammonium-N concentration, whereas groundwater in the uncontaminated area had a non-reducing ORP with a low ammonium-N concentration. Peat was present only in the contaminated area, whose As concentration was very high, indicating the critical role of peat in groundwater As contamination. The source of N in peat and groundwater was identified as chemical N fertilizer. N present in peat and groundwater served as a nutrient, enhancing microbial activity. In the reducing ORP condition, As was thought to be released intensively to groundwater by the mechanism of reductive release.