The hydrogeology of the Oju area, Eastern Nigeria: an initial assessment

MacDonald, A.M. ORCID:; Davies, J.. 1997 The hydrogeology of the Oju area, Eastern Nigeria: an initial assessment. British Geological Survey, 52pp. (WC/97/054) (Unpublished)

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The aim of this report is to give a broad flavour of the hydrological and hydrogeological conditions within Oju. Existing information has been collated and a literature review undertaken of similar environments. Oju was visited a number of times to collect geological, geophysical and hydrochemical data (reported separately). In addition to a general review of the hydrogeology, a specific study on the potential of the river gravels has been carried out. Oju suffers from an acute seasonal water shortage. The high annual rainfall (1600 mm) occurs mainly between April and October leaving a five month dry season with virtually no rainfall. The low permeability geology results in very high runoff in the wet season, and negligible river flows in the dry season. Some of the larger rivers have pools of water throughout the dry season - commonly forming a source of guinea worm. In the wet season water is taken mainly from shallow wells and streams close to the villages. In the dry season most of the shallow wells quickly dry out. This is because many of them tap shallow water bearing zones from the laterite layers. Only wells that penetrate permeable rocks below the laterite layer, or deeper fractures are sustainable through to the dry season. In the south of Oju there are approximately 40 boreholes; most of these are drilled within the Asu River Group. Many of these boreholes have a history of breakdowns. This is often perceived to be solely a pump problem but is more likely a result of poor borehole design and siting, or low permeability geology. The majority of inhabitants take water from small ponds and seepages or travel to ponds in the larger rivers to get water in the dry season. The hydrogeology of Oju is complex. The greater part of Oju is underlain by Cretaceous sediments. These consist of interbedded shales, siltstones sandstones and limestones. Most common, however is shale. Literature from other studies suggest that only where the shale is significantly fractured will groundwater supplies be feasible, and even then it is a marginal resource. More promising sources of groundwater would be the interbedded siltstones and sandstones. Prior to drilling and testing it is difficult to assess these areas for groundwater potential. South of Oju town, however, the sediments have been affected by regional metamorphism which has made them harder and more fractured. Boreholes located in fracture zones might be the best method of groundwater abstraction in this area. Drilling and testing a series of boreholes would help to clarify the best method of abstraction. There is one good aquifer within Oju. The Makurdi Sandstone runs across the middle of the Oju area and comprises fine to medium grained sandstone and arkose. There are few streams on the outcrop area suggesting a more permeable rock unit. Wells dug into the Makurdi Sandstone have tended to collapse due to the swelling of near surface clays and collapse of highly weathered sands. Properly constructed wells which are lined and strengthened through the weathered zone should provide reliable community supplies. A study has been made of the potential of the river gravels to provide groundwater. A hand auger was used to test the thickness and nature of the gravel at several sites: > River gravel deposits in the base of the major rivers are thin and intermittent - they have very little potential for groundwater storage. > Thin gravel (-1 m) can be present in smaller rivers on Asu River Group. Although the gravel contains groundwater early in the dry season this resource soon dries out. > Seepages in the river bed are not generally fed by flow from the gravels, rather it is flow from the sides and the underlying geology that is important. A monitoring system has been setup within Oju. Rainfall data are being collected at 7 sites throughout Oju. Groundwater-levels are being monitored at 10 sites. This should give valuable data into the natural groundwater fluctuations prior to any development. However, due to the stress on water resources in the dry seasons, the boreholes used for monitoring are also pumped. It would be advisable to drill dedicated piezometers for long term monitoring.

Item Type: Publication - Report
Programmes: BGS Programmes > Groundwater Management
Scale: 1:100000
Funders/Sponsors: Department for International Development (DfID)
Additional Keywords: GroundwaterBGS, Africa, Groundwater, development, mudstones, International development
NORA Subject Terms: Earth Sciences
Date made live: 21 Feb 2013 15:09 +0 (UTC)

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