A. Area strip mining
In outline, the method involves stripping away the overburden
(OB) (if present) and to recover the mineables by use of bulldozers, scrapers
or by manual operations. This obviously forms great scars on land at the site
of excavation and large piles of OB material where the waste is dumped. This results
land degradation and land pollution at the excavation site and also at the dumping
site.
OC mining needs excavation of land surface. It obviously degrades the quality
of excavated land as it looses its soil cover and gets lowered from its original
topographic height. Soil profile in the region gets disturbed and hence the soil
quality, its chemical and physical character, behaviour with water, none remains
as it was in original condition, because a huge mass of land is excavated out
from its original site and placed at a new site. The quarries generated by excavation
if left unreclaimed, that amount of land becomes useless.
The nature of degradation varies with depth of excavation. There may be cases
of OC mining which create only shallow depressions on land but no OB. Examples
of these are mainly stone quarries and clay scrapping for brick-klins. Impacts
of these on land are mostly ignored because of shallowness of the quarries; while
the fact is, these disturb the topography sufficiently to disturb the surface
water flow pattern i.e. the surface water potentialities of the region. Clay scrapping
specially causes loss of topsoil and hence greenery growing potentialities of
the region. All these add to the land degradational cycle (Fig. 3.1 & 3.2).
If the depth of excavation is such that it damages the upper part of the aquifer
underground, water flows into the excavation site continuously from the remaining
part of the aquifer. It requires continuous pumping out of water from that site
to facilitate mining. Land degradation due to this has been detailed later in
the same section under the heading "pumping out of mine water".
In some other cases where the quarry is deep enough to excavate out the total
aquifer in the region, its consequences may create damage to water table, regional
lowering of water table and hence drying-up of land and land degradation. This
excavation of aquifers generate a persistent problem. Even when the quarry is
backfilled for the purpose of physical reclamation, it is filled with a material
too loose to represent the impermeable layer that was originally existing at the
base of the aquifer. Thus the aquifer is never regenerated (Fig. 3.3). This creates
a situation which goes against sustainable greenery growth over these mining degraded
lands, even after so-called biological reclamation. The matter has been detailed
in section 3.4.4.
B. Contour strip mining
Such mining exposes fresh surfaces on sloping land and hence
makes these highly prone to rain wash, weathering and erosion, which results siltation
in the surrounding area's land and water system. Such weathering and erosion may
even cause water pollution and hence chances of land degradation. Further, if
among the minerals involved there exists pyrite, marcasite, ankerite, siderite
etc, which will produce sulphuric acid and other soluble salts such as sulphates
and oxides, all these may effect adversely on flora, fauna and chemical characteristics
of the land, hence land-use and land quality. This aggravates land degradation
by two ways:
* Siltation together with chemical pollution in water bodies adds to the cycle
of land degradation already explained in Fig. 3.1 & 3.2.
* Rolling of broken mined out (but not used) pieces of rock from hill slopes creates
land degradation just as an OB-dump detailed next.
Added to the wastes generated by mining, is the debris produced in the construction
of roads required to reach the mining site for mining related activities which
damages land just similarly.
C. UG mining
Excavation for UG mining does not create any direct impact
on land other than making dumps of materials excavated for reaching the deposit,
and the materials excavated with the deposit as gangue mineral. The matter of
subsidence is being dealt separately.
There may be different types of UG mining depending upon mode of occurrence of
the material to be exploited:
(a) If it is a bedded deposit, it is approached by shaft or incline as the case
may be and then only the deposit is mined
(b) If it is a vein deposit it is to be approached almost similarly but at the
time of exploitation the total vein is to be excavated out which requires, in
some cases (e.g. quartz-mica veins, lead-zinc veins, gold-quartz veins etc.) excavating
some unwanted (gangue) minerals together with the desired ones (ores).
(c) If the desired material occurs disseminated in pore-spaces of the country
rock (e.g., oil, water etc.) the mineral is to be gained mainly through drilling
or pumping (as the case may be), it creates minimum land degradation through excavation
other than damage by drilling and ancillary activities. However long continuation
of such action may result subsidence.