23 May SAND: MINING AND ENVIRONMENTAL IMPACT
SAND AND PLACER SAND: MINING AND ENVIRONMENTAL IMPACT
(Professor Emeritus, University of Kerala), C32, Krishnavilas, Sankar Lane, Sathamangalam, Trivandrum 695 010 e-mail: email@example.com
Sand, largely made of granular unconsolidated particles of 2.00 -0.625 mm size, is a direct derivative from disintegration or chemical weathering of igneous, sedimentary and metamorphic rocks.. Water, carbonic and humic acids are the chief agents that decompose and transform the silicate minerals like feldspars and ferro-magnesians of the parent rock to clay, by slow leaching and removal of elements like Na, Ca, K, Mg, Fe, P, Si for transport as dissolved (ions) load. On the other hand, clays (size = <2.0 microns) are transported as suspended load. Durable essential minerals like quartz (chemically SiO2) and the minor but durable accessory-opaque (for e.g.,ilmenite, magnetite) and non-opaque minerals (like sillimanite, garnet, zircon, monazite and rutile) released during weathering of parent rocks dominate the sand fraction.
While the dissolved (Ld) and suspended load (Ls) travel to their final oceanic sink with same velocity of water, the durable minerals like quartz and accessories travel downstream toward the oceanic sink as bedload chiefly by rolling and sliding on the bed and/or with occasional jumps into the overlying water column. Bedload transport is an extremely slow process. Consequently, river beds are adorned with variously shaped and positioned sand bars.
In nature, sand occurs with varying ages and geometries, in that a sand body can either be modern or ancient. Sand accumulations are characteristic of modern depositional environments like river bed (as sand bars of various descriptions), shorelines (as beach sand), deserts (as sand sheets) and modern continental shelf receiving sediment from medium and large rivers. The fluvio-glacial alluvium, ancient beach deposits, and submarine sheets of sandy-clayey sediment are geologically ancient and wind and waves unceasingly continue redistributing them. The redistribution (by the storm waves of SW monsoon) of sandy-muddy shelf sediment off Quilon, supplies a phenomenal quantity of heavy minerals including ilmenite to the beaches of Chavara-Kayamkulam sector and injects mud in sizable quantities to enliven the mudbanks in Thirkunnapuzha and beyond. Even the recent Tsunami, dumped, a few hundred thousand of tons of black sand on the affected beaches.
As the supply of sand to the rivers is an extremely slow geological process, the anthropogenic removal of river sand out of river channels or ancient river alluvium, is an act several orders of magnitude larger than the natural rate of sand production, transport and deposition. As a result, the intrinsic aspects of physical, hydrological and biological systems of the river are damaged, affecting the channel geometry, groundwater regimen and biodiversity.
However, the mechanized gathering of blacksand from the beach placers of Kerala or Tamil Nadu, is very much like collection of the checked baggage from the conveyor belt by an airline passenger. These beaches are like the conveyor belt and baggage is equivalent of black sand. In respect of blacksand placers, it is delivered to the beach during the beach reconstruction phase of the SW monsoon wave regime. Further, during the Pleistocene (say 2.0 Ma ago), the modern continental shelf off Kerala, for one thing was a large sedimentary basin in which sea-level stood lower (at least by 100 m). Hence, the then rivers had run longer courses to reach the sea to deposit their load and like any modern sink zone, the rivers had constructed a large prisms of alluvial sediment only to be covered later by the rising waters and sediment of the Laccadive sea (as the ice and snow melted due to warmer global temperatures before finally vanishing by say 6000 y BP)..
This format of placer formation is the foundation encouraging the continued mining of blacksand. Moreover, the preferential landward transport of heavy minerals enhances the proportion of quartz sand and mud in the residual seabed sediment. With careful scientific planning, the latter can be dredged, washed and processed for use in the construction industry without harming the shelf environment.