Geoelectric Sounding for Evaluating Soil Corrosivity and the Vulnerability of Porous Media Aquifers in Parts of the Chad Basin Fadama Floodplain, Northeastern Nigeria

A geoelectrical survey was carried out in parts of the Chad Basin Fadama Floodplain as a means of evaluating both the soil corrosivity and protective capacity. One hundred and six Schlumberger Vertical Electrical Sounding data were collected at the corners of a 225 x 225 m square grid network. Topsoil resistivity and topsoil longitudinal unit conductance maps were generated from the first and second order geoelectric parameters respectively. Areas considered as high corrosivity are the north central, southwestern, southern and northern parts with (ρ< 180 Ω-m). Part of the study area characterized by materials of poor to weak protective capacity has longitudinal conductance values of less than 0.1 and (0.1 0.19) mhos respectively. Values between (0.2 0.79 mhos sandy clay cover) and (0.8 4.9 mhos clay cover) correspond to moderate and good protective capacity respectively. It can thus be concluded that the flanks of the floodplain underlain by appreciable clayey topsoil thickness columns are susceptible to corrosion tendency. These same flanks are characterized by materials of moderate to good protective capacity and serve as sealing potential for the underlying hydrogeological system in the area.


Introduction
Underground storage tanks for petroleum products, septic tanks, agricultural activities, municipal landfills, military installations, nuclear sites, waste infiltration systems and abandoned hazardous waste sites are generally considered major threats to groundwater/hydrogeologic system or porous media aquifers (Mohammed, 2007).
Fadamas floodplain is part of the wetland regions of the West Chad Basin located between Azare and Jama'are towns, northeastern Nigeria.In recent times, efforts by the Federal government and other governmental agencies in areas of water supply and crop production have led to improvement of the lives of the people around the area through farming and animal husbandry.Agricultural activities in many parts of the area may portend serious environmental/hydrogeological threats, particularly in respect of the accessibility of porous/alluvial sand water table aquifers to pollution.The vulnerability of these aquifers to pollution may be considerably high as the continuous and extensive use of chemical fertilizers/pesticides, the drainage or wash off of animal solid wastes disposal from nomadic grazing activities, and flood activities remained.Man's activities ranging from land fill solid wastes disposal to liquid wastes disposal etc. may also remain a contributory factor.However, contamination of the hydrogeologic system in most porous media areas is a common global feature (Oladapo et al., 2004;Mohammed, 2007).The World Health Organization estimated that more than 20% of the world population (around 1.3 billion people) has no safe drinking water and that more than 40% of all population lack adequate sanitation (Oastridge & Trent, 1999).Many developing countries are still faced with difficult choices as they find themselves caught between finite and increasing polluted water supplies on one hand and rapidly rising demand from urbanization on the other hand (Forum Umwelt & Entwickling, 2001).The need then arises to assess the protective capacity of the superficial materials (topsoil overlying the alluvium aquifer) of aquifer to enable the evaluation of the level of protection of the hydrogeological systems against surface sourced pollution.Also, the degree of soil corrosivity may be evaluated should metal pipes be required for reticulation works in the groundwater development within the area.
Geoelectrical resistivity method has been adopted for this study since it is one of the most effective geophysical tools for groundwater and environmental investigations.The superiority of the method over other methods is confirmed in the work of Zohdy (1973); Zohdy, Eaton and Mabey (1974); Ako and Olorunfemi (1989); Mbonu, Ebeniro, Ofoegbu and Ekine (1991); Olayinka and Olorunfemi (1992); Olorunfemi and Fasuyi (1993); Mohammed (2007).

Location and Areal Extent
The study area is the River Jama'are floodplain in the West Chad Basin.It is situated on the northern side of the Azare -Jama'are highway, about 30 km West of Azare town in Katagum Local Government Area of Bauchi-State (Figure1).It is confined within longitudes 9°56 '30"E and 9°58'00"E and latitudes 11°39'15"N and 11°41'15"N (FSN, 1978)  The study area covers an areal extent of about 4.53 Kilometers square and situated on approximately flat terrain with average surface elevation of about 370 m above sea level.A local topographic high exist as the northern edge of the area at Yola settlement.This makes the entire area vulnerable to periodic flooding at the peak of rainy season.

Geology and Hydrogeology
The study area is underlain by Cretaceous-Tertiary Chad Formation and Recent alluvial Formation of Pleistocene age.The two formations directly rest on the basement bedrock rock (Figure 2).However, rocky hills and inselbergs of the basement rock occur around Geidam, Gumel and Shira, about 30 km southwest of the study area.This suggests that the study area is located within a transitional sedimentary/basement terrain.The major geological features in the area include approximately NW-SE trending geophysically identified suspected deeply buried regional parallel faults/fractured zones in the basement bedrock.

Chad Formation
Recent Alluvium

Data Collection Techniques
Apparent resistivity data were collected at One hundred and six stations (Figure 3) by the use of the Schlumberger Vertical Electrical Sounding (VES) technique.The inter-electrode spacing (AB/2) of the array used was varied from 1-225 m with a maximum spread length of 450 m.The PASI 16 Digital Resistivity Meter was used for the data collection.
For n parallel layers of resistivities ρ i , , ……… ρ n , and thicknesses h i, ……… h n as shown in a typical geoelectrical section (Figure 4).
The total longitudinal unit conductance (S) is defined as: The total transverse unit resistance (T) is defined as: The average longitudinal resistivity (ρ L ) is defined as: The average transverse resistivity (ρ t ) is defined as: The coefficient of anisotropy (λ) is defined as: The subscript i indicates the position of the layer in section.

Geoelectric Results
The results are presented as vertical electrical sounding curves displaying the geoelectric parameters (layer resistivities and thicknesses), corrosivity and longitudinal conductance maps.
The topsoil resistivity shows values that range from 0-250 ohm-m with the higher frequency in the 0-250 ohm-m range.The mean is 325 ohm-m while the standard deviation is 892 ohm-m.This indicates a very high degree of dispersion with a coefficient of variation of 274.5%.This variation may be explained by the wide textural/compositional variation in the topsoil.Seasonal variations in the amount of available recharge and topographic elevation/depth of water table may have also affected the soil resistivity.
The topsoil resistivity values that are less than 100 ohm-m typify clay while high resistivity (ρ >100 ohm-m) may suggest sandy clay, clayey sand, sand, compact sand or lateritic column (Ako & Olorunfemi, 1989;Olayinka & Olorunfemi, 1992;Olorunfemi & Okhue, 1992;Omosuyi et al., 2003;Oladapo et al., 2004).Resistivity values in the range of (ρ>1000 ohm-m) white lateritic sand (hard pan) and sand observed at few places, particularly in the topographical high areas in the northern and western edges of the site.The thickness of the topsoil ranges from 0.4 to 6.7 m, but is generally less than 3 m with the most frequently occurring values in the 1.0-2.0m range.

Soil Corrosivity Evaluation
Soil resistivity values can be classified in term of soil corrosions as shown in Table 1.
The first layer resistivity values obtained from the interpretation results were utilized in generating corrosivity map (Figure 6).The map is used in the evaluation of the degree of soil corrosivity, at shallow depth, in the area, should metal pipes/buried utilities be required for reticulation works in the groundwater development and other engineering utilities.
The areas considered to be of high corrosivity are the north central, southwestern southern and northern parts of the area.These areas are characterized by relatively low resistivity values (ρ< 180 Ω-m).Areas with high resistivity values (>180 ohm-m) are precisely non corrosive.These areas include the eastern, part of the southern and western parts of the area.The eastern flank is particularly overlain by lateritic hardpan with relatively high resistivity values.
More than 60% of the study area displays a relatively low topsoil resistivity values with high tendency for corrosivity.Hence, metallic utilities/pipes etc buried within the areas with high degree of corrosivness are susceptible to corrosion.

Overburden Protective Capacity Evaluation
The ability of an earth medium to retard and filter percolating fluid is a measure of its protective capacity (Olorunfemi et al., 1999).Henriet (1976) described the protective capacity of an overburden exerted by retardation and filtration of percolating pollutants as being proportional to its thickness and inversely proportional to its hydraulic conductivity.Clayey material content is generally characterized by low permeability, low resistivity, low hydraulic conductivity and longitudinal unit conductance values.Hence the protective capacity can be considered as being proportional to the longitudinal conductance (S).That is, the higher the overburden longitudinal conductance of an area, the higher its protective capacity.
The modification of Henriet (1976) and Oladapo et al. (2004) classifications shown in Table 2 is adopted to suite the evaluation of the protective capacity of this basement/sedimentary transition environment.Figure 7 is the map of the longitudinal unit conductance (S) of the superficial materials (topsoil) constructed from the topsoil longitudinal unit conductance (s) values.The values vary between 0.00044 and 0.61 mhos.Values between 0.2 and 0.79 mhos typically correspond to moderate protective zones (sandy clay cover), such zones are distinguished as small pockets or closures found scattered within the map region, while pockets of blank found at the southeastern and northern ends typify good protective capacity (0.8 and 2.2 mhos) on the map.The remaining segments of the map constitute about 95 % of the total areal extent of the study area are characterized by materials of poor to weak protective capacity (< 0.1 and 0.1 -0.19) mhos respectively.This area may be highly proned to pollution.

Conclusion
The north central, southwestern, southern and northern parts of the study area contain corrosive topsoils with low resistivity (ρ< 180 Ω-m), while the eastern, the southern and western parts of the area with high resistivity topsoils are precisely non corrosive.Hence, metallic utilities/pipes etc required for reticulation works buried within the superficial layer in the area are susceptible to corrosion suggesting a significant contribution of clayey matrix in the layer.
The longitudinal conductance map reveals that above 95% of the study area are characterized by topsoil with poor to weak protective capacity.This implies that alluvium aquifers, the most prolific in the basin are not protected enough from surface pollutant(s) in most part of the area, while the remaining other few areas even at relatively shallow depth are significantly protected by variably thick clay/sandy clay topsoil which is annually reinforced by the flood activities of the meandering Jama'are river.
The study, therefore, has helped to evaluate the superficial topsoil materials overlying the alluvium aquifer in the study area and enables the assessment of the vulnerability of aquifer(s) to surface pollutants such as chemical fertilizers frequently used by farmers within the Fadama floodplain basin.

Recommendation
However, the challenges of long -term safe rate of the aquifers vis-à-vis ever changing climatic factors are important considerations in the assessment of the vulnerability of aquifer(s) to surface pollutants in any parts of the basin.While it is recommended that further study be carried out to assess the long -term safe rate of aquifers from a combination of hydrometerological, geophysical and geochemical techniques, the geochemical tool can be used in such a study.

Appendix
Dar Zarrouk Parameters obtained from First Order Geoelectric Parameters.

VES Station
Topsoil

Figure 1 .
Figure 1.Map of the study area showing the floodplain of River Jama'are

Figure 2 .
Figure 2. Geological/Hydrogeological map of the Northern Bauchi State showing the study area Figure 4.A typical geoelectrical section

Figure 5
Figure 5 (a).Computer interpretation results and curve types

Figure 7 .
Figure 7. Topsoil longitudinal unit conductance map which approximate eastings and northings of 602631.843mE and 605356.818mE and 1288367.009mN and 1292062.197mN of the Universal Traverse Mercator (UTM) Mina Zone 31 coordinates respectively.

VES Point + Longitudinal Conductance Value Contour Line 0.2 River Preliminary Borehole Abandonned Borehole Foot Path Major Road Minor Road
h