Geophysical Mapping Of Groundwater Potential In A Rural Water Supply Project: Malawi, Africa

A total of 250 boreholes were sited in the East Mangochi District of Malawi, using a unique<br>combination of borehole geophysics, terrain conductivity, frequency domain electromagnetics, and<br>electrial resistivity tomography. The project represents one of the most intense applications of<br>geophysical surveys to rural water supply in deeply weathered crystalline basement terrains. A total of<br>388 line kilometers of EM, 22 line kilometers of ERT, and 23 borehole logs were collected in a threemonth<br>program in 1999. These data have been processed and interpreted in conjunction with previous<br>hydrogeological reports and satellite and airphoto interpretations, to produce over 480 targets for lowyield<br>village hand pumps. Such a detailed geophysical survey was considered necessary in the region<br>because of the highly irregular distribution of aquifers and the low success rate of past drilling programs.<br>The EM surveys proved to be the most efficient in terms of rapid surveying, but the ERT sections were<br>essential for resolving ambiguities in the EM response of lateral and vertical resistivity changes. Water<br>saturated deep weathering zones, and vertical fracture sets were the most commonly mapped targets<br>having strong responses in both the EM34 and the Apex MaxMin systems. Resistivity sections across<br>these features accurately mapped the depth to unweathered bedrock, the presence of a saprock layer, and<br>the thickness of the shallow laterite. Data management and interpretation was aided by the use of a<br>Geographical Information System (GIS) that allowed continual monitoring of the survey, and rapid<br>access to the results. Drilling of the boreholes commenced in September 2000 using the Global<br>Positional System (GPS) locations, village maps, and physical markers on the ground to locate the<br>targets. To date, of the 35 wells drilled, 27 have been completed as successful water wells.