10 Continental J. Earth Sciences 6 (2): 10 - 17, 2011 ISSN: 2141 – 4076 © Wilolud Journals, 2011 http://www.wiloludjournal.com ` Printed in Nigeria MINE DEWATERING: A VERITABLE COMPONENT IN MINE COST ANALYSIS IN THE DEVELOPMENT AND MINING OF THE LAFIA-OBI COAL, NIGERIA Stephen J. Mallo Dept. of Geology and Mining University of Jos, Nigeria ABSTRACT The impact of water in surface and underground mining activities is of great concern to Mining Engineers and Geoscientists to the effect that its accumulation in excess quantity can render mining of minerals ineffective. Surface and underground mines must therefore constantly be dewatered in order to forestall over-flooding and its attended effects on mining machinery, infrastructures industrial health, and productivity. In mine planning and design the cost of dewatering of mines is often captured as capital costs at the initial stage of mine development and afterwards in the cause of mining as operating costs both playing significant roles in profit or loss of Mining companies. Mines are often excavated below the water table where mine voids serve as low-pressure sinks inducing groundwater to move to the openings from the surrounding saturated rock. The result is the dewatering of nearby rock units via drainage of fractures and water-bearing strata in contact with the mine workings. There is also the potential for impacts to more remote water-bearing units and surface water bodies depending on the degree of hydrologic communication. The extent and severity of the impact on the local surface water and groundwater systems depends on the depth of the mine, the topographic and hydro-geologic setting, and the hydrologic characteristics of adjacent strata. The dewatering of mines and its cost implications starts from mineral exploration and mine development from where the rate of water flow is approximated thereby providing the initial clue to the choice of dewatering pumps and other drainage infrastructures. The paper dwells on the sources of water and its menace in surface and sub-surface mines, its control, and effects on the cost analysis of mineral investments. KEY WORDS; Acid Mine Drainage, Underground Water, Dewatering, Room-and –Pillar, Cost Centers Brief Geology The study area is bounded by latitudes 8 0 25 1 43 11 N and 8 0 20 1 40 11 N on its northern and southern boundaries, and by Longitudes 8 0 48 1 80 11 E and 8 0 55 1 55 11 E on its eastern and western boundaries. It covers an area of 48Km 2 and is located between Obi and Agwatashi Village located some 40 Km southwest of Lafia and 80 Km north of Makurdi by the River Benue. The geology of the area has been studied by Falconer(1911) and Bain (1942) who in their regional studies provided information on the occurrences of brown coal, Farington (1956), Shell-BP (1957), Crachley and Jones (1965), the Mines Development Syndicate (1948, 1949), Offodile (1973, 1974, and 1976) and the Coal Group of N.S.D.A (1978). The various reports on the area included data and interpretation from an excess of 130 sunken boreholes and of two exploration pilot shafts 1 & 2 located at Agwatashi and ---respectively. The stratigraphic sequence developed from previous studies on the Obi coal area provides the four distinct formations namely, Agwu, Keana, Ezeaku and Arufu Formations in deepening order respectively( Table 1). While these formations are overlain by about 400m of sediments, the Agwu formation which predominantly harbours the upper and lower coal measures has a total thickness of about 595m. The Keana sandstones and Arufu formation are underlain by the Precambrian basement at a depth beyond 995m. The 900m of dominantly shale, with subordinate limestone, sandstone and several coal seams constitute the main stratigraphic unit of the study area. This Formation ( Agwu) is characterized by quick succession of shale and sandstone, limestone and coal seams indicating rapid change in positional depositional environment[ 11]. The upper coal measure is 355m thick it includes marine and non-marine sediments it consist of rapidly alternation of shale, sandstones, siltstone, and limestone with over 16 non-commercial seams in carbonaceous shale layers. The lower coal measures on the other hand, is 230m thick with the top 80m consisting of abundant coal horizons with shale intervals inter-bedded with siltstone, sandstone and occasionally limestone beds.