Abstract
Although the importance of potable groundwater has been declining in recent years in Okinawa, the groundwater is still indispensable for supporting daily lives. A continuous monitoring of groundwater is carried out in accordance with the provisions of the Water Pollution Control Law. This monitoring consists of an overview and a continuous monitoring survey. On the basis of the results of the Okinawa Prefectural Government survey in FY2010, the amount of arsenic exceeded the environmental standard at four out of seven sampling sites. The total mercury level was below the detection limit at three sampling sites. The amounts of volatile organic compounds (VOCs) were either not detected or below the environmental standards at four sampling sites. The nitrate- and nitrite-nitrogen level was below the environmental standard at one site. The contamination by VOCs and nitrate- and nitrite-nitrogen is due to human activities. However, in Okinawa, on the basis of the results of the field survey, arsenic and total mercury are considered to be of natural origin. Arsenic in groundwater can be derived via two pathways: (1) in its reduced condition, arsenic adsorbed to iron is eluted to groundwater when iron in soil is eluted to groundwater and (2) arsenic adsorbed to iron in minerals is eluted when minerals come to contact with slightly alkaline groundwater. The source of total mercury contamination is assumed to be mercury vapor which rises from deep underground along faults. The findings of the survey from FY1989 to FY2010 will serve as important reference data when considering the quality of groundwater in Okinawa in the future.
