Field description

This section describes the characterisation of sedimentology in the field along River Benue outcrops and in the sediment cores in the floodplain, in order to assess the sediments of the research area for application with the hand drilling method. Alluvial sedimentology is useful for 1) assessing the suitability of the hand drilling method to abstract the shallow floodplain groundwater, 2) understanding the origin of the sediment and its 3-D distribution and 3) quantifying groundwater flow.

3.4.1.1 Sediments sampling along River Benue Valley outcrops

A total of 5,500 g of sediment from 191 sediment samples were collected along the outcrops of River Benue valley bank, for the purpose of laboratory analysis. Sampling technique was

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followed in accordance with the British Standard guideline (BS EN ISO 22475 – 1, 2006). A hand-held Global positioning System (GPS) system was used for fixing the position of sediments collected at each sampling location in the field. The primary purpose of collecting sediment samples here is for characterisation of the subsurface lithology and stratigraphy of the area (Bob, 2008). At each sampling point, samples were collected at every change of sediment/soil type from the top to the bottom of River Benue outcrops bank (Figure 3.2). The sedimentological description of outcrops using the lithological table/chart was used to describe the samples during sampling. Photographic record with label, scale and colour chart was used for the purpose of description. A small plastic coated (to avoid rust) hand trowel was used for sediment sampling. Samples were put into sealed sampling bags for laboratory analysis.

Figure 3.2: Sampling sediment using a plastic trowel along River Benue Valley Bank, Yola outcrops at sampling location G along transect 3 (see Figure 2.28) (The photograph taken by Mohammed Abana Girei on 19^th April 2011).

Samples were also taken in Cameroon in Benue and Faro Rivers. Figure 1.1 shows the sampling locations. It was hypothesised that the volcanic fields (see Figure 2.4) found in

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Benue and Faro Rivers may have left an impact on the sediment of Benue and Faro Rivers, which contributes sediment to the Nigerian portion of the Benue River. It is hypothesised that magnetic susceptibility measurement would be able to show a volcanic component in the sediment (Dearing, 1999).

Preliminary identification for the sediment/soil grain size in the field was made by feeling the texture of sediment between fingers. For each sediment collected, a small amount was rubbed between fingers in order to determine the difference between sand, silt and clay. Sand feels gritty, silt feels smooth and clays feel sticky.

3.4.1.2 Field sediments sampling using hand augering method

Fieldwork was carried out in April to May 2011, in order to determine the maximum depth of groundwater levels to be available for farmers to irrigate farm land at the peak period of the dry season. The technique used for the sampling and understanding of the groundwater levels at each drill hole followed the British Standard guideline (BS EN ISO 22475 – 1, 2006). The survey campaign was completed in April and May 2011, which is the peak dry period of the region. The hand augering technique (Figure 3.3) was locally constructed to test in the floodplain. The maximum depth obtained was 18 m deep during this fieldwork period.

During the fieldwork survey, I had an informal interview with farmers on the usage of the hand drilling techniques. The farmers’ prefer using augering drilling method than other methods of hand drilling especially jetting, because augering is affordable and easy to use.

During the fieldwork survey, it was also observed that some boreholes fail in the floodplain according to my interaction with the farmers. Therefore, it is useful to understand the causes for the boreholes failure across the floodplain in order to improve the sustainability of the floodplain groundwater for irrigation activities.

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Figure 3.3: Borehole drilling using hand augering method for sediment sampling in the floodplain at borehole location 4 transect 2 (see Figure 2.8) (The photograph taken by Mohammed Abana Girei on 4^th May 2011).

The visual description of the colour of the fresh sediment samples was described according to the Munsell soil colour chart during sampling. Colour patterns of sediment or soil are extremely important for lithological analysis. It is essential to identify the colour of sediment during sampling because some sediments change their colour very quickly in air. An example of this is fine sediment containing iron oxide compounds which, in the fresh-water saturated condition, often has an olive green colour but which rapidly oxidizes to red on exposure to air (BS EN ISO 14688 – 1, 2002). Such kinds of the sediments are the predominant in the floodplain. Colour changes such as those due to oxidation or desiccation should be recorded.

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Figure 3.4: Sampling sediment samples from the drill boreholes using augering hand-drilling method at borehole location 6 along transect 3 (see Figure 2.8) (The photograph taken by Mohammed Abana Girei on 4^th May 2011).

Sediment samples were obtained at the different drill points in the floodplain for laboratory analysis and to estimate the maximum depth with hand drilling until water is reached during the dry season period. Twelve boreholes were drilled using locally made augering methods at twelve points at approximately 500, 1500 and 2500 m intervals along five transects (Figure 3.4) perpendicular to the River Benue and approximately 500 m between the transects. This seems a reasonable spacing to cover the area of the Upper Benue Irrigation Project. A total of 4,800 g of sediment samples in a total number of sixty-five sediment samples were collected from twelve cores for detailed analysis of particle size, magnetic susceptibility, loss on ignition and permeability.

Sampling bags (Figure 3.5) are used to store and to avoid moisture loss of samples for the laboratory analysis. Clear marks were labelled on each sample.

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Figure 3.5: Sediment samples in sampling bags to prevent moisture loss from the sediments (The photograph taken by Mohammed Abana Girei on 29^th May 2011).