Naturally occurring arsenic in the groundwaters in the southern region of Fukuoka Prefecture, Japan

In March, 1994, arsenic over the permissible level for drinking use (0.01 mg/l) was detected in wellwaters in the southern region of Fukuoka Prefecture, Japan. The highest concentration found was 0.293 mg/l, being quite high compared to other arsenic-containing wellwaters reported in Japan. The polluted wellwaters contained NaHCO3 as the major dissolved inorganic component, which made the water weakly alkaline. The concentration of dissolved oxygen was much lower than those in unpolluted areas. The organic arsenic was not detected. These data combined with geological–geochemical inspections by boring proved that the arsenic pollution did not originate artificially but occurred naturally through an elution process long ongoing on the rocks and soils by the stagnant underground water. The mechanisms of arsenite/arsenate elution from the soil proposed were which involved (i) anion exchange with OH− and (ii) reductive labilization of arsenic through conversion of arsenate to arsenite.