The hydrogeology of the Oju/Obi area, eastern Nigeria: Anyoga Eddi Adum East area data report

The groundwater potential of the Makurdi sandstone was investigated at Anyoga Eddi Adum East during February-March 1998. Fifteen hundred meters of EM34-3 surveying was undertaken, using a 20 m inter-coil spacing, as were two resistivity soundings. Three boreholes were drilled: two deep exploratory boreholes and one shallow cored observation borehole. Two boreholes (BGS 36 and BGS 37) were completed with screen and casing; BGS 38 contained little water and was therefore backfilled. BGS36 and BGS 37 were test pumped; during these exercises water samples were obtained for hydrochemical analysis. The following conclusions are made from the results obtained: The Makurdi Sandstone at Anyoga Eddi Adum East comprises interbedded sandstones and mudstones. Sandstone layers up to 4 m thick are present. The sandstones are fine to medium grained, well cemented, feldspathic and frequently bioturbated. The sandstones contain interbedded thin mudstone beds that often show load casting features. The mudstone can be soft and is often shaley and carbonaceous. Thin sandstone, siltstone and limestone layers occur interbedded within the mudstone sequence. Where sandstone occurs at shallow depths, it has been weathered. Within the top few metres, feldspar has been leached increasing the inter-granular permeability and porosity; kaolinite clay is also present. At slightly greater depths, secondary silicate deposition is present, which produces very hard and dense siliceous sandstone layers with negligible inter-granular permeability. Where predominately mudstone is present at shallow depths, it has been weathered to olive-green illite-smectite rich clay (similar to Edumoga). The aquifer properties of the Makurdi Sandstone at Anyoga are disappointing. Groundwater was only found where significant sandstone occurred within the weathered zone. The inter-granular permeability of the sandstone is low, the majority of the water found in the two successful probably supplied by slow seepage from the sandstone. The yield from neither borehole is sufficiento sustain a hand pump. A hand dug well constructed through the weathered zone to the fractures below would be the most appropriate technology for abstracting groundwater. The storage coefficient at Anyoga was typical of a confined aquifer (0.0001). The exploratory boreholes could be kept open to routinely measure water levels within the aquifer to monitor the sustainability. The electrical conductivity measured using the EM34-3 varied from 10-30 mmhos/m. The lower measurements were associated with sandstone in the weathered zone, while the higher measurements indicated more mudstone within the weathered zone. Therefore, low EM34-3 readings are the best targets for groundwater. Resistivity soundings could also distinguish the sandstone from the mudstone. It is recommended that a more detailed study is made of the weathered zone of the sandstone to discover more about how and where the hard siliceous layers are likely to occur (e.g. in valleys or on ridges). A diamond-coring bit would be required to obtain samples.