A huge volume of groundwater was being pumped out for factories and to serve a growing population when ground subsidence was first detected in Tokyo in the years after 1910. Over the ensuing decades the water table dropped, falling to as low as 58 meters below sea level in 1965. The volume pumped out continued to grow until 1970, when it peaked at close to 1.5 million cubic meters per day (m3/d). The depth of subsidence increased over the decades and the area affected continued to expand. At some places the ground surface was dropping over 10 centimeters per year (cm/yr), peaking at about 24 cm/yr in 1968. Meanwhile, the Tokyo Metropolitan Government (TMG) introduced pumping regulations for the thousands of wells in the region in order to slow and reverse the pace of land subsidence. Pumpage declined and the rate of subsidence slowed dramatically. The water table began to rise again in the early 1970s and is now at 6-10 meters (m) below sea level. Even in the areas that were most affected, the rate of subsidence has slowed to about 1 cm/yr in the past five years. Up to 550,000 m3 of groundwater was still being pumped up daily for public water supply and other uses in 2003. With over 80 percent of the ground surface in the wards of Tokyo covered by buildings and pavement, and farmland area in the Tama region shrinking, however, only a fraction of rainwater percolates into the soil to recharge groundwater. It is therefore important to increase the infiltration of rainwater by conserving green areas and farmland. To this end, the government has issued guidelines and requested that parties who install pumping facilities also include rainwater infiltration facilities. It also requires building owners to submit plans that contain environmental considerations and include the use of reclaimed wastewater or rainwater infiltration, and encourages residents to use water more efficiently. As of the end of March 2003, the TMG's waterworks system in the Tama area was operating up to 290 wells, most of them at a depth of 100-350 m, and treating the groundwater at 50 water purification plants, each using a varying combination of chlorination, iron and manganese removal, aeration, and microfiltration membranes to treat groundwater in some wells that has been contaminated with various pollutants, such as cryptosporidium, nitrate nitrogen, nitrite nitrogen, hexavalent chromium, cis-1,2-dichloroethylene, and 1,4-dioxane, among others.
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