THERMONIC GROUND-WATER STUDY OF AN AREA OF CRYSTALLINE ROCK IN SIERRA LEONE, WEST AFRICA
William M. Turner, Ph.D.
Open-pit mining of specular hematite enclosed within schist of Precambrian age is carried out at the Marampa mine near Lunsar, Sierra Leone, West Africa. The project area is shown in Figure 1. The specular hematite deposits are more highly resistant to erosion than adjacent rock and stand as hills above otherwise low-lying swampland.called "dambos". Concern exists that, when the open pit is deepened beneath the level of the surrounding swamp, ground water from the swamp will flow into the pit in troublesome amounts. To evaluate the problem, AGW was contracted to conduct a Thermonic study of the area to determine the paths of ground-water flow into the open pit mine.
HYDROGEOLOGIC SETTING
The project area is comprised of Precambrian crystalline rock which is overlain by a thick layer of residual soils and laterite. The soils and laterite are the product of chemical weathering of the basement rock and the leaching of the soluble chemical compounds by the more than 12 feet of annual rainfall in the area (> 2,440 mm/yr). Part of the rainfall percolates directly through the soil horizon and into the numerous fracture zones within the basement rock. The fracture zones are the major conduits through which water flows into the open-pit mine. They must be located and either grouted with cement or pumped to control the ground water in them. From the viewpoint of water supply, these are zones of optimum well-sites.
PROCEDURE
A hydrogeologic and Thermonic reconnaissance investigation was made of the Campbell Town Hill area near Lunsar. Temperature measurements were made in all accessible bore holes within the saturated and unsaturated zones. In addition, water-level measurements were made in all bore holes which contained standing water.
DISCUSSION
Ground-water-level-elevation contours show the elevation of the water table above sea level. Analysis of the ground-water-level contours show that the direction of ground-water flow is to the northeast, away from the axis of Campbell Town Hill. The highest ground-water-level elevations occur along the topographic axis of the ridge forming Campbell Town Hill. Campbell Town Hill is a zone of ground-water recharge. Ground-water recharge occurs from infiltration of rainfall. The ground water discharges to the swampy area northeast of Campbell Town Hill.
The shape of the ground-water-level contours shows that the transmissivity of the water-bearing rock in the Campbell Town Hill ridge is not uniform. The extent of transmissivity variability is not readily apparent from potentiometric data because of the sparse potentiometric data points.
Thermonic data stations are more dense and the interpretation of the data, when used in conjunction with the potentiometric data, gives a more accurate and detailed picture of the ground-water-flow system in the Campbell Town Hill area.
Thermonic data shows there are two zones of low transmissivity trending southwest to northeast across Campbell Town Hill almost perpendicular to the long axis of the hill. One zone extends northeast from the intersection of Northing 13,000 - Easting 10,000 for a distance of about 500 feet (152 m) The other zone extends northeast from the intersection of Northing 13,800 - Easting 9,500 for about 500 feet (152 m/y). Intermediate between these two zones is a zone of higher transmissivity.
Based on the ground-water-level contours, a probable rate of ground-water recharge of 5-feet per year (1,524 mm/yr), and the distribution of Thermonic contours, a rough estimate of the transmissivities can be made. The transmissivity in the zones of low transmissivity is about 41.7 Igpd/ft (0.6 m2/d). The transmissivity in the zone of higher transmissivity is about 833 Igpd/ft (12.4 m2/d). A water well drilled in the zone of high transmissivity should be capable of producing 16.6 to 25 Imperial gallons per minute (1.3 to 1.9 l/s).
The distribution of transmissivity in the crystalline rock in the Lunsar area is structurally controlled. The trend of the zones of low transmissivity is aligned with the trend of the ore bodies in the area just north of Campbell Town Hill. Surrounding the base of Campbell Town Hill is alluvium that is saturated to the land surface. In this swampy area the transmissivity is probably higher than in bedrock areas of Campbell Town Hill.
CONCLUSIONS
The study near Lunsar, Sierra Leone indicates that Thermonics is a valuable tool for mapping zones of high transmissivity within areas of crystalline rock. Estimates of transmissivity and water-well yield can also be made. The zones of high transmissivity are optimum well locations for dewatering or water supply wells.
Despite the positive results of the Thermonic survey, the greater water problem for the mine seems to be 12 feet per year (3.65 m/y) of rainfall over the mine itself.
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