Environmental Engineering Reference
In-Depth Information
In. general,. available. Selenium. (Se). is. relatively. low. in. paddy. soils,. for.
example,. as. found.in.the.Taihu.Lake.region,.where.an.average.of.1025.soil.
samples.showed.only.0.010-0.030.mg/kg.belonging.to.Se-deicient.soil.type.
(Cao.and.Lin.2006)..Under.anaerobic.conditions.the.availability.of.Se.exhibits.
decreasing. trends. due. to. the. reduction. of. Se
6+
. to. Se
4+
. and. Se
0
. to. Se
2−
. (Cao.
et. al.. 2001,. John. et. al.. 1991,. Liu. et. al.. 2004,. Mikelsen. et. al.. 1989);. hence,. the.
remediation.of.Se.in.polluted.soils.was.proposed.(Kharakal.et.al..2008).
Other Benefits for the Environmental Protection of Urbanized Regions
As.a.man-made.wetland.system,.paddy.ields.can.provide.potential.advan-
tages. in. addressing. other. environmental. degradation. issues,. which. have.
resulted. from. the. urbanization. of. the. surrounding. region.. These. beneits.
include.the.reduction.of.the.urban.heat.island.effect,.supplementation.of.the.
groundwater. table. in. minimizing. land. subsidence,. maintaining. the. biodi-
versity. of. the. suburban. area,. and. cleansing. the. air. to. prevent. the. dangers.
that.may.be.encountered.if.dust.or.sand.storms.were.to.arise.in.urbanized.
areas.
Groundwater Supplementation in Preventing Land Subsidence
In. 2007,. CCTV. news. reported. that. land. subsidence. was. very. severe. in. the.
center. of. the. Yangtze. River. Delta,. including. Shanghai. City,. Hangzhou-
Jiaxing-Huzhou. plain,. and. Suzhou-Wuxi-Changzhou. belt. (CCTV. 2007)..
The.most.severely.reported.subsidence.was.located.along.the.Huangpu.River.
of.Shanghai,.from.1921.up.to.the.ground.surface.subsidence.for.2004.of.about.
2.63.m..For.the.past.80.years,.the.average.land.subsidence.was.1.69.m.for.the.
whole.of.Shanghai.City,.whereas.Huangpu.River.is.considered.to.be.a.“hang-
ing.river”.during.the.looding.period.of.the.monsoon.season..In.Lujiazhui,.
the. annual. land. subsidence. for. the. concentrated. area. of. skyscrapers. is.
reported. to.be.in.the. range.of. 12-15.mm.. In. the.Suzhou-Wuxi-Changzhou.
belt,. which. is. a. fast-growing. economic. development. zone. of. Jiangsu. prov-
ince,. the. land. subsidence. level. during. the. past. 20. years. was. over. 20.cm. in.
an. area. totaling. 6000.km
2
.. A. groundwater. funnel. of. around. 45.m. in. depth.
was. also. discovered. surrounding. Jiaxing. city,. which. also. happened. to. be.
the.land.subsidence.center.of.Hangzhou-Jiaxing-Huzhou.plain.in.Zhejiang.
Province..The.factors.that.may.have.induced.such.rapid.and.severe.land.sub-
sidence.are.excessive.groundwater.extraction.and.the.construction.of.many.
high-rise.buildings.and.skyscrapers..However,.half.of.the.rice.ields.lost.in.
the.region.over.the.past.30.years.may.also.be.regarded.as.another.causative.
factor,. ending. with. a. swift. deterioration. in. land. subsidence. and. ground-
water. levels.. Consequently,. around. 56.25.×.109.m
3
. of. water. may. have. been.
wasted.and. could.not.be. used.as.groundwater.anymore..It.is.estimated.by.
the.Chinese.Bureau.of.Geological.Survey.that.the.total.inancial.losses.sus-
tained.by.land.subsidence.in.the.Yangtze.River.Delta,.over.the.past.20.years,.
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