Estimation of aquifer productivity and groundwater recharge from single - well pumping test

Due to the abrupt variations in lithology and structures associated with crystalline basement terrains of southwestern (SW) Nigeria, reliable evaluations of aquifer properties are essential for the management and protection of groundwater resource. For this purpose, single-well pumping and recovery tests were carried out in wells within the various bedrock terrains of Ibarapa region of SW Nigeria to evaluate the discharge, drawdowns, storage capacity, transmissivity, recovery rate and other well inventory with the aim of evaluating sustainable groundwater yield within the diverse geological terrains of the region of study. From the results, the average (av.) groundwater discharge (in m3/day) for wells in areas underlain by amphibolite was 72.05; 53.45 in gneisses; 68.27 in migmatite and 64.33 in granitic terrains. In the respective order of geological settings, the total drawdowns in wells were 10.70 m, 21.99 m 17.69 m and 15.58 m and transmissivities (in m2/day) were 6.85, 2.57, 0.76, 1.72, while specific capacity values (in m3/d/m) were correspondingly; 7.67, 3.10, 4.00 and 5.54.  The recovery tests showed that wells in granitic terrains were characterized by better water recharge than in other terrains. However, groundwater yield in amphibolite and to a lesser extent in gneisses were quite sustainable and the rates of groundwater recovery though moderate sustained continuous discharge for hours of continuous pumping. It is therefore mandatory to develop a reticulation system in this region so as to cater for water supply in areas underlain by granite and migmatite at the central and north-eastern part of the study area. &nbsp

Groundwater Recharge and Aquifer Transmissivity of Crystalline Basement Terrain

It is known that substantial groundwater bearing zones can develop within the weathered and fractured units of impermeable crystalline rocks if there is a good and recurrent source of recharge. On account of this, eighty-two geo-electric soundings were carried out to determine the degree and extent of weathering and bedrock fracturing; and in addition, twenty-one single –well (constant discharge) pumping tests were also conducted for the estimation of the transmissivity of the water-bearing zones across the various bedrock terrains of Ibarapa areas of southwestern Nigeria. Weathering development was found to be more pronounced in areas underlain by amphibolite and gneisses bedrocks with a regolith thickness range of 4.4 - 47.5m (av.18.41 m) as compared to the relatively shallower soil horizon range of 2.4 - 23.6 m (av. 12.03 m) that developed upon light coloured rocks of migmatite and granites. The recharge potentials as inferred from the total longitudinal conductance indicated that there is likelihood of better water infiltration in areas underlain by migmatite and granites with 0.02 – 0.60 mhos. The discharging areas underlain by amphibolite and gneisses with higher conductance of 0.01 – 1.42 mhos were characterized by thicker and finer grained overburden unit and more numerous bedrock fractures that sustain reliable groundwater yield. Only one-third of the boreholes were sustainable with transmissivities of 7.75 – 0.50 m2/day; these prolific boreholes were mostly within amphibolite and gneisses terrains. Change in the drawdowns during pumping was between 1.7 and 14m, while the groundwater yield was between 93.58 and 32.78 m3/day. Keywords: Groundwater recharge, transmissivity, sustainable yield DOI: 10.7176/JEES/9-10-13 Publication date:October 31st 201

Hydro-Chemical Studies and Assessment of Trace Elements and Bacterial Contamination of Shallow Groundwater of Oyo Area, Southwestern Nigeria

Groundwater from shallow hand-dug wells is the only reliable source of water supply in the ancient town of Oyo in the southwestern region of Nigeria. The present work studied the water quality in twenty-five wells across this area by measuring the ancillary parameters, and analyzing the major and trace elements and taking the inventory of the wells as well as assessing the total coliform and Escherichia coliform in sampled water. The field measurement showed that the groundwater is slightly acidic with an average pH of 6.6 and fresh from the total dissolved solids values between 70 and 630 mg/L. From the average concentrations of major ions, the dominance order of the cations constituents in groundwater is in the order- Mg2+ > Ca2+ > K+ > Na+ and HCO- > CI- > NO3- > SO42- for anions. The ranges of trace elements concentrations were; Iron II- 0.01 – 0.06; Chromium VI: 0 - 0.34; manganese 0 – 1.2 in mg/L, while Copper was between 0.38 – 32.7; Cadmium 0.03 – 1.87; lead 0.02 – 3.57; Zinc 0.01 – 7.79; and Arsenic 0.05 – 7.35 in µg/L. The total coliform count ranges from 6 – 1860 cfu/100 mL while Escherichia coliform (E. coli) units was between 2  and 1640 cfu/100 mL with frequency occurrence from nil in three wells representing 12% of the wells, 1 – 50 units (28%), 51 – 100 (20%), 101 – 500 (20%) and > 500 units (20%). The groundwater is grossly contaminated with nitrate and coliform bacteria while four wells were found to be contaminated with manganese and chromium. It is mandatory that the water be treated for metal and bacterial contaminations prior to consumption and public enlightenment on drinking water guidelines be in place. Keywords: Groundwater. Major ions. Trace elements. Coliform Bacteria. DOI: 10.7176/JEES/13-3-02 Publication date: April 30th 2023

Hydrogeological characterisation and prospect of basement Aquifers of Ibarapa region, southwestern Nigeria

Abstract The present study involved the use of 82 geo-electric soundings, and the measurement of well inventory and conduct of yield tests in 19 wells across the various bedrock terrains of Ibarapa region of southwestern Nigeria. The aim is to proffer solution to the unsustainable yield of the available boreholes in order to effectively exploit the existing groundwater resource in the area. From the geological reports, the area is underlain by four principal crystalline rocks that include porphyritic granite, gneisses, amphibolite and migmatite. The geo-electric studies revealed that the degree and extent of development of the weathered–fractured component varied, leading to diversity in groundwater yield and in aquifer vulnerability to contamination. The thickness of the weathered layer is greater than 18 m in areas underlain by amphibolite and gneisses and less than 13 m within migmatite and porphyritic granite terrains. High groundwater yield greater than 70 m3/day was recorded in wells within the zones of rock contacts and in areas with large concentration of bedrock fractures and elevated locations across the various bedrock terrains. Aquifer vulnerability is low in amphibolite, high in granitic terrains, low to moderate in gneisses and high to moderate in migmatite. Also, wells’ depths and terrain elevation have a moderate to strong indirect relationship with groundwater yield in most bedrock terrains, except in high topographic areas underlain by porphyritic granite. Therefore, there is need for modification of well depth in accordance with the terrain elevation and hydrogeological complexity of the weathered–fractured components of the variuos bedrock terrains, so as to ensure a sustainable groundwater yield

Groundwater Occurrence from Hydrogeomorphological Study of Hard Rock Terrain of Part of Southwestern Nigeria

Studies of structural and hydrogeomorphological units (HGU) that are indicators of groundwater occurrence were carried out across an area extent of more than 700 km2 within the hard rock terrain of southwestern Nigeria. These studies integrated geological remote sensing techniques (RST) and geographical information system (GIS) methods to generate thematic maps that included elevation, drainage, lineaments and vegetation index for characterising the attributes of groundwater occurrence across the area. The results revealed that the lineament system is mainly rectilinear with major trends of NNW-SSE and NE-SW on the gneiss, NW-SE and NE-SW on porphyritic granite and NNE-SSW, NW-SE and E-W on migmatite. The discharge zones in the area are the lowland terrains underlain by gneiss and amphibolite. Similarly, variably directional discontinuities that are related to rock contacts are equally laden with groundwater. Conversely, the recharge areas are the high-lying terrains characterised by higher fracture density and underlain by porphyritic granite and migmatite. Additionally, there are evidences of groundwater seepage along the major river channels. Therefore, besides the rock structures, landform is another crucial factor that guides groundwater distribution in the study area

Sustainability and Conceptual Groundwater Hydraulic Models of Basement Aquifers

Groundwater flow of the basement terrains of the Ibarapa region was studied by carrying out pumping test and measurement of borehole inventory. The view was to identify the associated aquifer systems from the time-drawdown curves, quantify the estimable hydraulic properties and develop hypothetical models for the understanding of the groundwater flow in the area underlain by diverse crystalline bedrocks. Three aquifer types were identified namely, dual, leaky and regolith. The yield of groundwater in dual and leaky aquifers that dominated terrains underlain by amphibolite and gneisses was sustainable, but the discharge of regolith aquifers mainly associated with migmatite and granite terrains declined at late pumping stage. The transmissivities of the dual and leaky aquifers were between 2.02 and 11.65 m2/day, while those of regolith aquifers were mostly less than 1.00 m2/day. The average aquifer transmissivities in m2/day by bedrocks were: 6.85, 2.57, 0.76 and 1.72, correspondingly. The inter-relationships between transmissivities and groundwater discharge showed diverse aquifer representations, from sustainable high-yielding to unsustainable low-yielding types. Conscientious effort is, therefore, required for well construction in the area