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In addition, mining degrades the value of the land for agriculture or reforestation. Mct
mining areas were originalla agricultural land or natural forests. Tq soils are Femoved during
the mining process. Once the operation oeases, a large pit perhaps 15 m deep is usually left'
with big fiies of stones and gravel scattered around on an uneven surface covered with fine
and coarse sand. Very few plants can grow and the area is subiect to severe erosion during
the rainy season.
Kitching (f9S2) estimated the area covered by tailings and the annual addition to these
areas in Malaysia, Indonesia and Thailand as follows:
Coastal dredoino
Malaysia
Indonesia
Thailand
Tailings area
(ha)
208,m0
82,m0
5l,mo
Annual increrrent
(ha)
2,000
400
100
In Tfrailand and Indonesia, offshore and coastal mining has inereased in recent years. In
1980 over 50% of tin production in Thailand came from coastal waters. Production from
coastal waters is increasing as mshore deposits become depleted.
Coastel mining csuses two major types of environmental damage. It results in the
depreciation of eoaitline aesthetic values through turbid water, mud deposition on the
boltom, and changing the monphology of the shoreline if dredging is done close to shore. It
also destroys importlnt shallow water benthic habitats sch as coral reefs, sea grass beds,
mangrove swamps and other benthic communities by direct destnuction or by disturbence from
sedimentation. Tfese coastal habitats are known to be highly productive and are important
breeding and mtrsery gnounds for commercially valuable marine orqanisms.
Mct mininq effects are from physical distuiance, mainly from siltation and direct
destruction. There are no reports of chemical hazards from tin mining, a potentially toxic
heavy metals are seldom found. Although lead and arsenic are associated with cessiterite in
some localities, they generally occur in small amounts in insoluble forms.
Dredging is the method used to remove ore deposits from swamp areas and shallow
waters down to about J0 metres. The nethods were adapted from canal or harbour dredging
and were first applied to tin mining in Phuket Bay, Thailand in 1907. Tte sediment is brought
up either by bucket ladder or suction. Tin ore is separated by gravity as in land based mines.
Tailings are discharged directly from the rear of the dnedge. Tlc heavy fractions immediately
sink to the botLom wnile fine particles form a turbid plume and are distributed in the vicinity
of the dredge depending upon the current regime before settling out.
Tfe types of dredge widely accepted as efficient in retrieving tin ore are the ladder
type chain hrcket dredqe and the suction'dredge. Tte former was lhe first kind of dredqe
used in shoreline and offshore mining. This type of bucket dredge generates sediment plunes
from all parts of its operation: dredging sediment from the bottom, transporting it up the
ladder and discharging taitings. Such a dredge.can generate quite substantiel amounts of
suspended sediment. If can remove up to IrI40 m-/hr of sediment and operate 24 hours a day.
In protected areas, it can operate up to 300 days a year. The suction type dredges are of
srnaller size and involve less capital investment. The suspended sediment plurne is generally
created by releasing the mine tailings back into the surface waters. A thltd type of dredge
eommonly found in Thailand is the diver guided zuction dredge. These were. modified from
fishing boats when near-shore dredging began l0 years ago. These small dredges are
concentrated in shallow water up to I8 metres deep along the coast of Phang-nga province in
the concession area of the provincial government. The method employed uses the same
principle as the larger zuction dredge. With a diver guiding the suction headr they can
selectively coltect the'rich deposits. Hor,rrever, the small size of their sluice box makes them
inefficienl in separating ore from tailings. In l98l there were some 5,(I)0 boatg in qeration.
This large number of Loats can cause rather serious destruction and sedimentation in the
aree. Hor^rever, no informetion is available m the extent of environnental damage created by
these srnall boats.