HYDROGEOLOGICAL IMPLICATIONS OF GULLY EROSION IN ORLU AND ENVIRONS IMO STATE, NIGERIA

Author: Iheme Obinna Kenneth
Department: Geological Sciences
Affiliation: Nnamdi Azikiwe University Awka

Field geological mapping, hydrogeological and physiographic studies were done in the gullyerosion prone areas of Orlu and environs in Imo State, Nigeria. Three geologic formations were observed, namely the Ameki, Ogwashi-Asaba, and Benin Formations. About 27% of the gullies observed occurs in Ameki Formation comprising moderately to poorly sorted sand with hydraulic conductivity values ranging from 1 x 10-3cm/s to 5.4 x 10-2 cm/s and transmissivity values ranging from 1.21 x 10-2cm2/s to 5.61 x10-1cm2/s,while 73% occurs in Benin Formation comprising sands with hydraulic conductivity values ranging from 1.5 x 10-2cm/s to 6.6 x 10-2cm/s and transmissivity values of 1.11 x 10-1cm2/s to 6.50 x 10 1cm2/s. No gully was observed in Ogwashi-Asaba Formation, where more plastic soils exist. The grain size distribution curves indicate that about 80% (by weight) of the sands is medium to coarse sand, while 18% and 2% are fine sand and coarse silt respectively. The plasticity indices of the formations range from 24.83% to 48.42%. Factors responsible for the development and growth of the gullies were observed to include topography, rainfall, geology, hydrogeology and anthropogenic activities. Hydrogeochemical assessment of some water samples from public water supply sources indicate objectionable concentration of heavy metals such as Pb, Hg and Cd in groundwater, and Cu, Cd, Pb and Hg in surface water. The predominant water types include calcium-sulphate waters for groundwater, and sodium-chloride-bicarbonate waters for surface water. The observed consequences of gully erosion in the study area include loss of about 22km of arable lands and two major and six minor roads, loss of about nine residential houses, and pollution of public water sources. Possible remedial measures include incipient control, afforestation, multi-channelling of floodwater, use of catch pits, slope stabilization, promulgation of enforceable environmental policies, enforcement of existing ones and public awareness programmes.

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