Mining Of Sea Bed Sand

Mining of Sea Bed Sand: Scientific Basis Prof. Thrivikramji, K.P (Retd.) [email protected] University of Kerala, Kariavattom 695 581 Introduction The ingenuity of man always had motivated him to explore the nature to locate usable commodities for his own sustenance and hence of Human civilization. Recent idea of gathering construction grade fine aggregate or sand from the seabed in the continental shelf is one of those. It is not at all a new idea any way. We all now know that several island nations as well as others have been gathering sand for application in the concrete or cement mortar or for recovery of valuable minerals. In this article I will examine briefly the feasibility of the proposition from a geological stand point most often ignored point. In a very short time, the public mindset in Kerala got sharply divided into one of the two camps, viz., the protagonists and the antagonists. Any construction using cement concrete and steel calls for what are called in industry circles, as coarse and fine aggregates. As far as the former is concerned, there is no great confusion as to the types or sources, by the sheer knowledge that natural processes of weathering of rocks and erosion and transport, do not produce it. Thus, obvious and uncontroversial recourse had been and still is to source it from a rock-crushing-sizing plant. Contrary to this, the fine aggregate (= commercially designated as sand) used to be sourced and gathered from the modern and ancient river channels, shallow sea bed in the inner shelf, as well as in the provinces away from the riverine or shelf sources, from the crusher tailings (after screening). With out a geo-scientific knowledge of origin of sea sand, its mining scenario may run into sentiment and emotion ridden realms and unrealities. Geological background Truly, quantities of sand sized quartz (a primary constituent of commonest surface rocks like gneisses and granite, with a share of only 30% by volume and a mineral of chemical and physical neutrality to cement) is under transport in the stream channels, from the erosion rocks and soils (disaggregated by chemical weathering). Further, this sand has many temporary storage sites like in the channels as sand bars of various types, in the flood plains, in the abandoned channels etc. Unfortunately, in Kerala, not only that we exhausted all the stream channel sand as result of the construction boom in the last decades, but ecologically and physically killed all the rivers themselves for ever (in human terms), by continuing the approval to gather river sand, to the local bodies under the three tier panchayats. For the cash strapped local administration, this policy was the Magna-carta for survival. River sources its sand immediately from the soil only; majority of us do not know that a meter of soil needs at least a million years to form. Further in an in situ soil, sand sized quartz is just about 20 to 30 percent. Hence the rate of sand production is an extremely slow geological process and a non-event in human terms. In addition, sand that reaches the sea bed is again worked by the wave and tide generated currents and gradually transported beach ward to be deposited there, sometimes permanently. Like in many beaches of Kerala, sand that accumulated in the beach face is eroded and shifted massively to the inshore to be stored in the in shore as long-shore submarine bars, and to be slowly re-transported and deposited in the beach any way.

Due to the large scale fluctuations of sea level, in the Pleistocene, shorelines of the continents including that of India, stood several 10 s meters ( for India 100 m.) below the present shoreline, forcing the rivers draining the continents to flow through longer courses (over the modern continental shelf) and to deposit the sand there. This scenario is unfortunately not in the knowledge base of lay citizens and the non-specialists - hence, all the hue and cry now foul the air. Data - State of the art The most productive shelf region of Kerala, is rather vast and stands at 22400 Km2 (length 560 Km and width of shelf ~ 40Km) or 20,000 Km2 - to strike conservative figure. As a marine geologist, I say that there exists a considerably large quantum of reliable data with the NIO, MW of GSI and some others, indicating the earlier lower stands of sea level and hence of the past shoreline elevations. Mapping of the sandy sediment in the seabed can be taken up by shallow geophysical and geological work, in order to isolate potential areas for commercial dredging. The dredged sand can be washed in seawater, and later on further washed in fresh water and not exactly drinking standard water. More over introduction of special cement that would accommodate mildly salty sand will go a long way in reducing the water use by this process. We often ignore, vastness of the resources of the continental shelf - living and non-living and renewable to varying degrees (including sea bed sediment of a decameter or two in depth) which is so large in proportion to the land based resources, due to the very size of the sea bed. We have been gathering large quantities of the living resources of the oceanic realm to complement our (sea) food needs. Large hue and cry is expressly unheard of about this issue, as it supports and contributes to the gastronomic needs and also to the GDP. On top of this we also believe in the ability of the species to regenerate. I might state that the sandy-mud or the gravelly-sandy-mud (with black minerals in certain parts off Kerala, and certain other continents) covering sea bed is a non-living resource in the conventional sense, though only forms at a very low rate today. But it was supplied continuously to the seabed or shelf by the west flowing rivers - large and small draining the land surface in the Quaternary, i.e., say during the last 1.8 million yr. BP. Therefore, it is wise to keep an inventory of all the viable sources of the raw materials like quartz sand and mineral sand, useful to mankind. This is only commonsensical any way. The question of express exploitation comes only as the close second. Numbers game With the geological background of the sea bed, rate of soil formation, erosion and transport processes, weathering of rocks, rock composition and hence nature of sediment cover, some volume estimates are essential to fully appreciate the concept underlying sea sand mining. Given a land area of 38863 Km2 for Kerala, it is possible to estimate the size of the sand content (in the sea bed sediment), for a source slab of uniform thickness of 1.0 m., provided we have data on percentage content of quartz sand (for us 30%) and Sp.Gr. of 2.65. By weathering and erosion, this slab ought to yield quartz laden sediment headed to the sea to the west toward the Laccadive Sea. Estimated sand content of this slab is (31 e9 Tons) stands at ~30 Billion Tons. By the way, the same slab also would have an ilmenite (at a Sp.Gr. of 4.5 and abundance of 0.5% in the source rock) content of (874 e6) or say 874 Million tons. There are certain limitations any to these estimated numbers. Here, I consider the land-river-sea system as a closed system (which is more or less true), basement rocks to have uniform character and expressly no rate of erosion is assumed. Then, I have not estimated the sand storage in the strand plain of Kerala. If we allow the entire Pleistocene to be the period of erosion - well over 1.8 Million years which is more than what is essential for the formation of 1.0 M of soil. This exercise is carried out only to broaden the realities of the sand content of the sea bed. I still believe that these are fair estimates as ceiling figures. Further work can refine these projections realistically.

Certainly, post depositional sediment transport by wave and tide generated currents would have effected further redistribution of this sand on the sea bed at preferred sites and forms to be discovered only by mapping by appropriate tools available to marine geo-science to day. Unfortunately, some data or facts (and not the science per se) of geo-science are very simple to be appreciated by the non-specialist and the layman alike. Most of the time such people, become highly opinionated just for the fun of being one (?). In the case of sea sand also we face such a dilemma. Obviously, these need to be truly proven to be true by sea bed mapping and exploration, and only then, one can finally offer another source for the sand starved construction industry. -----------------