Tokyo is the national capital of Japan. The city encompasses 2,820 sq km
and is about the same size as Los Angeles, with nearly three times the
population. The actual population of the Metropolitan Tokyo area is 28,
447, 000 people. The national currency is the yen. The practiced religion
is Buddhism and Shintoism. Japan is nine hours ahead of Greenwich Mean
Time. Health standards in Tokyo are equal to or surpass those found in
other highly industrialized countries. The water is safe to drink, and
the food is safe to eat. (www.city.net/countries/japan/tokyo/?page=factsheet)
According to the Koppen-Geiger system, Tokyo lies in within the Mesothermal
Climate (Cfa) classification. This humid subtropical hot summer climate's
warmest month is above 22 C (71.6 F). The Cf indicates that Tokyo is moist
all year around with the f indicating that all months receive precipitation
above 3 cm (1.2 in). This climate system is influenced during the summer
by the maritime tropical air masses that are generated over warm coastal
waters off the eastern coast. In the fall, winter and spring maritime tropical
and continental polar air masses interact, generating frontal activity
and frequent midlatitude cyclonic storms. Overall the precipitation averages
100-200 cm (40 - 80 in) per year. Tokyo's summer precipitation is quite
a bit higher than its winter precipitation because of the effects of the
Asian monsoon. (Christopherson, 1992)
Tokyo lies in the western part of the Kanto Plain, otherwise known as the
Kanto Tectonic Basin. The Kanto Mountains lie in the most western part
of Tokyo. The Musashino Terrace found in the centre of Tokyo, was developed
in the northern part of the Tama River. Shitamachi Lowland lies in the
east at zero elevation. The quaternary sediments underlying Musashino Terrace
and Shitamachi Lowlands are divided into stratigraphically into two groups;
the lower is the Kazusa Group and the upper the Tokyo Group. The Kazusa
Group consists of the Kitatama and Higashikurume Formations (FMs) and in
ascending order, the Tokyo Group consists of the Toneri, Edogawa, and Takasago
FMs. These deposits consist of unconsolidated layers of silt, sand, and
gravel, which act as confined aquifers throughout the terrace and the lowland.
The Kanto Loam FM, Terrace Sand and Gravel FM lie above these quaternary
sediments on the terrace and provide the unconfined groundwater systems
to the Tokyo metropolis. The lowland is capped with the youngest sediments
in this region, the Nanagochi and Yurakucho FMs. The Kitatama FM of the
Kazusa Group contains the oldest sediments discussed here. Found both within
the terrace and the lowland, this formation mainly consists of impermeable
massive silty layers. Formations overlying the Kitatama FM contain permeable
sand and gravel. Next in the younging succession is the Higashikurume FM,
which consists of intercalated silty beds. In the lower most parts of this
formation are the Johoku Sand and Gravel Bed stratigraphic units. This
formation acts as a confined aquifer all over the Musashino Terrace. The
Toneri, Edogawa and Takasago FMs of the Tokyo group consists of alternating
layers of silt, sand, and gravel, which are sufficient groundwater reservoirs.
The Toneri FM forms the base of the Tokyo Group and provides water to wells
in the terrace and the lowland except in the western and southern parts,
where erosion has occurred. The top layer of the Musashino Terrace is covered
by Kanto loam, which consists of volcanic ejecta originating from the volcanoes
of Fuji and Hakone. A bed of clay separates the loam from the gravel terrace
beneath. Infiltration of the groundwater is restricted by this bed, thus
causing the groundwater to be retained in the Kanto Loam. West of the terrace,
a series of continuous groundwater tables reflect a fan-type landform,
with close similarity to the surface contours of the area. In the middle
of the terrace the groundwater table shows a trend of transition from high
in the west to low in the east with multiple perched water tables and groundwater
mounds. In the east, groundwater is found at closer to the surface, at
shallower depths. The Shitamachi Lowland is not covered by the Kanto Loam
and its surface expression is represented by sediments that consist of
sandy clay. Shallow unconfined water levels are found within the sandy
clays.
Confined groundwater systems are being developed in the Tama District,
the western part of the terrace, and are mainly recharged with river water
form the Tama River. The unconfined groundwater system is being developed
in the surface layers such as the terrace sand and gravel formations and
Kanto Loam FM. Both the unconfined and confined water is used for household
water supplies, industrial and air- conditioning use for nearly seven decades.
Road paving and vegetation removal accompany urbanization. These and other
alterations to the landscape have caused a major change in the amount of
recharge to the unconfined aquifer. Capping a surface results in reduction
of water seepage into the soil causing an increase in runoff and erosion
rates. Sewage ducts reroute the sewage water collected and drain it into
outlying areas. This results in drastic flow reduction in the river within
a drainage area. This may also result in drying up of the affected area
of the river. There was once a well-known spring that formed such ponds
as Inogashira Zenpukuji, and Sanpoji, but they are now drying due to a
continually diminishing flow of the springs in the area. Organic chlorine
compounds such as trichloroethylene, tetrachloroethylene, and trichloroethane
were detected in 74 of 302 wells; most of these were drilled into confined
aquifers. In 21 of 74 wells, the trichloroethylene levels exceeded the
accepted standard for service water. It was suggested that these compounds
were orginially derived from the waste matter dumped by metal product manufactures
and laundries. This is a critical issue for people living in the Tama District
where a great deal of groundwater is pumped for household use.
The seasonal drop in the confined water table is and has been a result
of heavy pumping for house hold water supplies and air-conditioning use
during the summer months. These temporal variations in confined groundwater
levels in Tokyo have indicated two different regional patterns which occur
separately in the Wards District and the Tama District. The groundwater
pumped in the Wards District is very small compared to the extensive pumping
in the Tama District. Land subsidence has occurred in both districts. The
Koto Region in the Wards District has suffered from land subsidence due
to confined groundwater withdrawal for several decades and has experienced
many sever disasters from high tides and flooding. Parts of the Koto Region
subsided 4.5 m from 1918 to 1987.In the Tama District subsidence has extended
all the way up to the northeastern part of this region in the cities of
Kiyose, Hoya, and Higashikurume. Positions of well screens in confined
aquifers as legally required were at inadequate depths before 1975. In
the Tama District wells for household water supply that were constructed
before 1975 are allowed to continue to be used. In this district the groundwater
has been mostly withdrawn for household water supplies. As the population
increases more groundwater is pumped to supply household, industrial, and
air-conditioning use. Land subsidence in the lowland has been brought about
by the compaction of not only the Alluvial soft clay sediments in the Yurakucho
FM, but also the quaternary Tokyo and Kazusa Groups. The amount of compaction
of the Yurakucho FM comprised almost half of the total vertical land subsidence.
The occurrence of land subsidence is a serious problem for flood control
over the lowland. Almost all the lowland is below sea level, and now has
to be protected by extensive dikes and drainage systems that prevent disasters
relating to hightide and flooding.
In 1963 two laws were passed, the first was The Industrial Water Law and
the second was the Law Controlling Pumping of Groundwater for Use of Buildings.
As a result there was a halt in the confined aquifer decline due to the
national governments restrictions on groundwater withdrawal for industrial
purposes and for air-conditioning use in buildings. In 1975 the existing
rates of confined groundwater withdrawal were strictly revised in Koto
and surrounding regions. Well screens were deepened and the horizontal
pipe section of the pump machine was reduced. Since 1971 both public and
private facilities that were using wells for municipal offices, factories
and buildings have had to measure and report their monthly rates of groundwater
withdrawals to the Tokyo Metropolitan Government under the Tokyo Metropolitan
Environmental Pollution Control Ordinance. As a result of these increased
restriction, the quantity of confined groundwater withdrawal was decreased
and the water levels all over the Wards District have been raised since
then.
