To ensure the quality of drinking water in private wells in the Tama District, Tokyo, the Tokyo Metropolitan Institute of Public Health has surveyed water quality for many years. The primary contaminants in 698 wells within the District were monitored for ten years (1997-2006). In 1997, 51% of the wells exceeded drinking water quality standards. The following items exceeded standard values: total coliforms (34.3%), standard plate count bacteria (11.2%), tetrachloroethylene (PCE, 11.2%), nitrate-nitrogen (nitrate-N, 8.5%), and trichloroethylene (TCE, 2.1%). Escherichia coli was used to replaced total coliform as a standard to increase the accuracy of fecal matter determination and, as a result, the percentage of wells exceeding standards decreased to 30% in 2004. It was rare for the concentration of nitrate-N, PCE, or TCE to decrease below standard values over time. These contaminants were particularly common in wells in northeastern Tama. Thus, it is necessary to continue monitoring wells where contaminants exceed standard values.
