Présentations Colloques

Session 8.03: Geometrical structure and hydrogeological properties of Hard-Rock aquifers.
Edet (orateur)
Hydrogeologic and hydrochemical framework of the shallow groundwater in fractured shale of Ukelle area, Lower Benue trough, SE Nigeria
**Ukelle area (SE Nigeria) is characterized by acute water shorage. This is partly due to the underlying low permeability fractured shale rock and existence of salt ponds. As a result of this, the daily water needs of the communities are being met mainly from polluted surface ponds especially during the dry season. This has resulted in many water-related illnesses, the most endemic being guinea worm infection. In order to alleviate this problem, government decided to provide clean water through the provision of shallow hand-pump fitted boreholes. The present study is therefore aimed at using surface electrical resistivity method supported by geologic data to delineate fractured water bearing layers for the siting of productive boreholes. The study also includes the determination of aquifer parameters from resistivity and pump test data as well as different water types. Lineament mapping indicate trends in NE-SW and NW-SE directions. Forty four percent of these lineaments vary between < 2 and > 10 km in length. The lineaments in the area are linked to tectonic activity and associated magmatism which resulted in the injection of intrusive rocks. This gave rise to baking and fracturing of the shales in the area as observed in the field and borehole lithologic logs. Aquifer resistivities range between 4.8 and 180.0 ohm m, while thicknesses vary from 2.3 to 209.5 m. Aquifer parameters were evaluated from geoelectrical and pumping test data. Transmissivity values range from 1.97 to 13.44 m2 day and are greater than transmissivity estimated from surface resistivity data. Chemical data show that the water types are Na-HCO3 (43%), Ca-HCO3 (30%) and Mg-HCO3 (27%). The groundwater chemistry is controlled by silicate and carbonate weathering and reverse ion exchange. The water quality meets national and international standards for drinking and domestic use. However, the use for agriculture is highly variable based on some indices.**