The hydrogeology of Northern Agago County in Pader District, Uganda

This report was produced to assist GOAL, an Irish NGO working in Uganda, in the provision of water supplies for displaced persons in Agago County, part of Pader District in the north of Uganda. The work contained within the report has been carried out on a voluntary basis, although considerable support has been provided by GOAL in the provision of travel and subsistence costs within Uganda, spanning the period from the 22nd September to the 12th October 2007. Funding for preparatory work and for the writing of this report was provided by the British Geological Survey (BGS). The report describes the hydrogeology of five sub-counties (Lapono, Lukole, Paimol, Parabongo and Wol) within Agago County, which cover GOAL’s area of operation for water and sanitation in Pader district. The area currently benefits from a large number of deep boreholes, although many of these have proved to be dry at the time of drilling. GOAL aims to meet the requirement for further water supplies largely through the provision of shallow, hand-dug wells. There has been no systematic geological or hydrogeological survey of the area to date and geological maps are only available at scales of 1:1,000,000 and 1:1,250,000. Further data collection during this visit has been restricted by limited exposures of rock, a lack of drill cuttings and logistical difficulties arising from poor transport and continuing security concerns. In general, however, the study area can be divided into areas of deeply weathered crystalline basement and large inselbergs, exposing a range of high-grade metamorphic rocks. Given the current emphasis on shallow well construction in the area, GIS layers of water strike data and predicted depth to water have been created. The depth to the first recorded water strikes appears to be largely controlled by the local topography and a map based on land surface curvature (rate of change in slope) has been used to highlight topographic depressions where water strikes are likely to be shallowest (negative curvature) and areas of raised ground where water strikes may be more than 50 m below the surface (positive curvature). Where the land surface curvature is below -0.001, recorded water strikes are uniformly within 40 m of the surface. Where the land surface curvature is greater than this, however, the depth to water strike is highly variable and recorded values range from 15 m to nearly 100 m below the surface. These results suggest that deep boreholes remain the most viable option for water supplies in the majority of cases. While it is possible to highlight parts of the study area where water strikes are likely to be shallowest, groundwater in these areas may still be more than 20 m below the surface. As a result, shallow wells constructed in these areas would be unlikely to succeed. Overall aquifer productivity has also been assessed and the study area has been divided into four domains with distinct hydrogeological characteristics. Transmissivity data from nearly 50 locations have been used in the delineation of these domains, along with topographic considerations. The low-lying, flat land in the southwest is shown to contain the most productive aquifers, while the areas close to the inselbergs in the north and east of the study area are shown to have aquifers with low productivity. The available data show no correlation between aquifer productivity and the depth of weathering. This has implications for the continued use of resistivity surveys in identifying suitable drilling sites, as much of the value of this technique lies in identifying the depth of weathering.