Appraisal of protectivity and corrosivity of surficial hydrogeological units via geo-sounding measurements.

The surface resistivity method was used to appraise the protectivity of hydrogeological units and corrosivity of the top soil in Obot Akara County, southern Nigeria. A total of 28 vertical electrical sounding (VES) was undertaken in the area using the Schlumberger electrode configuration. The results of the VES data interpretation reveal 3 to 4 geoelectric layers in the study area. The resistivity of the first layer interpreted as the Motley top soil ranges from 34.7 to 929.7 Ωm with a mean value of 381.1 Ωm. The third layer, with a resistivity range of 99.4 to 2716.7 Ωm, constitutes the aquifer unit in most communities in the area, with an average thickness of 58.3 m, while the fourth layer penetrated in most communities has a resistivity range of 216.1 to 1475.7 Ωm with a mean value of 657.5 Ωm. The longitudinal conductance and resistivity reflection coefficient of the aquifer protective layers vary from 0.04 to 0.76 mhos and - 0.74 to 0.93, respectively. Analysis of these results shows that 89.3% of the hydrogeological units in the area is weakly/poorly protected, 10.7% has moderate to good protection, while 85.7% of the top soil at the sounding stations is noncorrosive and 14.3% is slightly to moderately corrosive. The implication of these results is that most of the hydrogeological units in the area are likely prone to contamination in particular by some ferrugenized materials from the overlying layers. Also, underground metal storage tanks and galvanized and steel pipes can be buried in the topmost layer in most communities in the area without any risk of failure. Although these findings are very promising especially in groundwater management and exploitation in the area, hydrogeochemical and microbiological analyses of groundwater samples from available boreholes are recommended to corroborate the results.