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We examine groundwater recharge processes and their relationship to rainfall intensity in the semi-arid, southwestern Lake Chad Basin of Nigeria using a newly compiled database of stable isotope da...

The present study is to explore the feasibility of GRACE-based estimation of a groundwater storage change in a data-poor region using a case study of the Ngadda catchment in the Lake Chad Basin. Although the Ngadda catchment has only one set of in situ time series data of groundwater from 2006 to 2009 and a limited number of groundwater measurements in 2005 and 2009, GRACE-based groundwater storage change can be evaluated against the in situ groundwater measurements combined with specific yield data. The cross-correlation analysis in the Ngadda catchment shows that maximum rainfall reached in July and August, whereas both the maximum total water storage anomaly and the maximum groundwater storage anomaly occurred 2months later. Whereas the mean annual amplitude of total water storage anomaly is about 17cm from both the average total water storage anomaly from three mascon products and the one from three spherical harmonic products, the mean annual amplitude of soil moisture storage anomaly is substantially varied from 5.58cm for CLM to about 14cm for NOAH and Mosaic. The goodness-of-fit tests show that CLM soil moisture produces the closest estimation of groundwater storage anomaly to the in situ groundwater measurements. The present study shows that GRACE-based estimation for groundwater storage anomaly can be a cost-effective and alternative tool to observe how groundwater changes in a basin scale under the limitation of modelling and in situ data availability.

Groundwater forms the basis of water supplies across much of Africa and its development is rising as demand for secure water increases. Recharge rates are a key component for assessing groundwater development potential, but have not been mapped across Africa, other than from global models. Here we quantify long-term average (LTA) distributed groundwater recharge rates across Africa for the period 1970–2019 from 134 ground-based estimates and upscaled statistically. Natural diffuse and local focussed recharge, where this mechanism is widespread, are included but discrete leakage from large rivers, lakes or from irrigation are excluded. We find that measurable LTA recharge is found in most environments with average decadal recharge depths in arid and semi-arid areas of 60 mm (30–140 mm) and 200 mm (90–430 mm) respectively. A linear mixed model shows that at the scale of the African continent only LTA rainfall is related to LTA recharge—the inclusion of other climate and terrestrial factors do not improve the model. Kriging methods indicate spatial dependency to 900 km suggesting that factors other than LTA rainfall are important at local scales. We estimate that average decadal recharge in Africa is 15 000 km3 (4900–45 000 km3), approximately 2% of estimated groundwater storage across the continent, but is characterised by stark variability between high-storage/low-recharge sedimentary aquifers in North Africa, and low-storage/high-recharge weathered crystalline-rock aquifers across much of tropical Africa. African water security is greatly enhanced by this distribution, as many countries with low recharge possess substantial groundwater storage, whereas countries with low storage experience high, regular recharge. The dataset provides a first, ground-based approximation of the renewability of groundwater storage in Africa and can be used to refine and validate global and continental hydrological models while also providing a baseline against future change.

The rapid development of groundwater systems as part of urban water supplies around the globe is raising critical questions regarding the sustainable management of this essential resource. Yet, in many major cities, the absence of an effective policy regime means that the practice of groundwater exploitation is driven by the actions of domestic households and drilling contractors. Understanding what shapes the decisions and practices of these actors, their understandings of the groundwater resource and the extent to which scientific knowledge shapes this understanding, is an area of critical importance that is currently under-researched. Using a mixed-methods methodology, the paper explores domestic practices of groundwater abstraction in Lagos, Nigeria. It finds that there is a disjuncture between the households who are actively shaping exploitation of the groundwater resource on a day-to-day basis and science and state actors. This disjuncture results in household decisions that are influenced by commonly held, but potentially outdated, perceptions of the groundwater resource rather than scientific evidence or policy instruments. The unseen nature of groundwater resources effectively renders the scale of changing groundwater conditions invisible to households and the state, adding to the challenge of influencing practice. Addressing this disjuncture requires not just more scientific knowledge, but also the active construction of interfaces with, and between, non-state actors through which knowledge can be confronted, discussed and shared.

The scarcity of groundwater storage change data at the global scale hinders our ability to monitor groundwater resources effectively. In this study, we assimilate a state-of-the-art terrestrial water storage product derived from Gravity Recovery and Climate Experiment (GRACE) satellite observations into NASA's Catchment land surface model (CLSM) at the global scale, with the goal of generating groundwater storage time series that are useful for drought monitoring and other applications. Evaluation using in situ data from nearly 4,000 wells shows that GRACE data assimilation improves the simulation of groundwater, with estimation errors reduced by 36% and 10% and correlation improved by 16% and 22% at the regional and point scales, respectively. The biggest improvements are observed in regions with large interannual variability in precipitation, where simulated groundwater responds too strongly to changes in atmospheric forcing. The positive impacts of GRACE data assimilation are further demonstrated using observed low-flow data. CLSM and GRACE data assimilation performance is also examined across different permeability categories. The evaluation reveals that GRACE data assimilation fails to compensate for the lack of a groundwater withdrawal scheme in CLSM when it comes to simulating realistic groundwater variations in regions with intensive groundwater abstraction. CLSM-simulated groundwater correlates strongly with 12-month precipitation anomalies in low-latitude and midlatitude areas. A groundwater drought indicator based on GRACE data assimilation generally agrees with other regional-scale drought indicators, with discrepancies mainly in their estimated drought severity.

Maiduguri and environs lie in the semi-arid region of Nigeria, characterized by low rainfall of approximately 600 mm per annum and high evapotranspiration of over 2000 mm per annum. Surface water is largely seasonal with rivers flowing for about three months in a year, although the Alau dam is an exception whose water is treated for supply to about 30% of the citizens of Maiduguri. Therefore, perennial water supply to the inhabitants is from groundwater, with the shallow Upper aquifer the dominant source. Assessment of quality of this water is thus fundamental. This is because of its susceptibility to contamination as a result of poor waste disposal from a rapidly growing population and industrialization. Water samples from 128 shallow boreholes were used to measure the EC, NO3 and temperature using appropriate meters, in addition to taking the site coordinates. The values of the EC and NO3 were used to evaluate the quality of this shallow groundwater. The results show a linear relationship between EC and NO3, which indicate that NO3 is a major contribution to the EC. NO3 concentrations in some boreholes are over 700 ppm; considerably higher than the WHO and NSDWQ permissible limit of 50 ppm. These boreholes are in wards of older settlements and with high population density. Most of the water from boreholes having lower than 50 ppm average NO3 are from wards that have low population density and usually of recent settlement. Although, for borehole sites close to the river channel, their NO3 concentrations are low due to dilution. Distribution of the NO3 concentrations over the study area indicates that the age of settlement and population density the major controls. There is the need to assess and constrain the source and mechanism of NO3 leaching into the groundwater of this area. Ultimately, there will be a need to develop modern human waste disposal systems that will enhance protection and/or minimize groundwater contamination. Keywords: Groundwater, Nitrate, Contamination, Maiduguri

International audience; Douala, Cameroon has been experiencing repeated cholera epidemics due to contaminated groundwater. The present study is to characterize the sedimentary aquifer system of Douala using time domain electromagnetic sounding (TDEM) and magnetic resonance sounding (MRS) for better understanding of the aquifer system to explore cleaner groundwater resources in the region. The TDEM and MRS results show good agreement of aquifer structure with the observed water tables from nearby wells and spring water. The existence of a deeper aquifer from the survey may provide a solution to develop a new fresh groundwater well field for the prevention of future cholera outbreaks in the region. As part of the Geoscientists Without Borders project, the present study also emphasizes capacity building of geophysical applications in the region. The project successfully trained eight African graduate students to master the TDEM and MRS techniques with involvement of undergraduate students from the St. Jerome Catholic University of Douala.

International audience; 36Cl measurements in groundwaters of the deep confined aquifer of the Lake Chad Basin (LCB) were performed in order to constrain groundwater geochemical ages and residence times. Twenty-seven wells were sampled in Nigeria, Niger and Chad in the southern parts of thelarge (700 000 km2) multilayered aquifer of the LCB. 36Cl/Cl values range between 11±1.10-15 to 148±8.10-15 at/at. The highest ratios are observed near the recharge zone of the Nigerian part of the Continental Terminal aquifer, while the lowest ones are found in wells located near the southern fringe of the present-day lake Chad. Chloride concentrations are low (below 100 mg/l) and not correlated to the 36Cl/Cl values, indicating negligible dissolution of evaporites in most samples. Reliable36Cl ages can be calculated along the different flow paths investigated, suggesting residence times of the deep groundwaters larger than 300 000 years. These results are consistent with new AMS-14C data below the detection limit but are in contradiction with previous 14C data obtained in the area.

No Abstract. Journal of Mining and Geology Vol. 42(1) 2006: 51-55

No Abstract.Journal of Mining and Geology 2005, Vol. 14(1): 123-130