A comprehensive and detailed study was conducted for the first time in the Benichab Region (western part of Mauritania) unique groundwater resource to identify the main process driving groundwater salinization using major and minor ions together with environmental isotopes. One hundred and nineteen samples were collected from 45 points (dug-wells and boreholes) during the period 2015 to 2017, where physico-chemical, chemical and isotope parameters were analyzed. pH values indicate circumneutral to basic (6.8 to 8.6) values while Electric Conductivity (EC) values show an increasing gradient from fresh groundwater lense (EC
The Faidherbia tree (Faidherbia albida) is frequently used as an intercrop in Sahelian agroforestry parklands due to its multi-purpose advantages and reverse phenology. However, its effect upon the water balance remains unclear, due to the challenges in directly measuring water fluxes in the underlying vadose zone. Mechanistic hydrological models can be inversely calibrated on transient observations and used to partition different hydrological components, but the computational burden of the analysis can become impractical if the model itself is computationally expensive. To overcome this limitation, and to provide novel insights into the hydrological role of Faidherbia, we combine a low-fidelity, one-dimensional hydrological model (HYDRUS-1D) with a kriging-based correction function to emulate the response of a high-fidelity, two-dimensional axisymmetric description of the system (HYDRUS-2D). Multiannual measurements of soil moisture and sap flow in a Senegal agroforestry parkland are used in conjunction with Bayesian inference to calibrate the resulting validated multifidelity surrogate, and to inversely estimate soil hydraulic and root water uptake parameters. Results show that the model can reproduce observations with good accuracy and limited uncertainty for both the calibration and the validation phases, and also confirm the phreatophytic behaviour of Faidherbia by indicating the existence of a moderately compensated root water uptake. Moreover, a local sensitivity analysis suggests that a fully compensated uptake could potentially reduce groundwater recharge by 13%. Interestingly, estimated soil hydraulic parameters hint at the possibility of root-induced changes in soil hydraulic properties that mimic preferential and/or macropore flow, resulting in sustained recharge fluxes (≈ 26% of the annual precipitation). The analysis indicates that overall, Faidherbia could have a net positive effect upon the water balance in arid areas.
A regional groundwater quality evaluation was conducted in the deep Maastrichtian aquifer of Senegal through multivariate statistical analysis and a GIS-based water quality index using physicochemical data from 232 boreholes distributed over the whole country. The aim was to 1) identify the water types and likely factors influencing the hydrochemistry, and 2) determine the suitability of groundwater for drinking and irrigation. Results showed that sodium, chloride, and fluoride are highly correlated with electrical conductivity (EC) reflecting the significant contribution of these elements to groundwater mineralization. The principal component analysis evidenced: 1) salinization processes (loaded by Na+, K+, EC, Cl-, F- and HCO3-) controlled by water/rock interaction, seawater intrusion and cation exchange reactions; 2) dolomite dissolution loaded by the couple Ca2+ and Mg2+ and 3) localized mixing with upper aquifers and gypsum dissolution respectively loaded by NO3- and SO42-. The hierarchical clustering analysis distinguished four clusters: 1) freshwater (EC = 594 μs/cm) with mixed-HCO3 water type and ionic contents below WHO standard; 2) brackish (Na-mixed) water type with moderate mineralization content (1310 μs/cm), 3) brackish (Na-Cl) water type depicted by high EC values (3292 μs/cm) and ionic contents above WHO and 4) saline water with Na-Cl water type and very high mineralization contents (5953 μs/cm). The mapping of the groundwater quality index indicated suitable zones for drinking accounting for 54% of the entire area. The occurrence of a central brackish band and its vicinity, which were characterized by high mineralization, yielded unsuitable groundwater for drinking and agricultural uses. The approach used in this study was valuable for assessing groundwater quality for drinking and irrigation, and it can be used for regional studies in other locations, particularly in shallow and vulnerable aquifers.
A new geometric model of hydrogeological systems in Cameroun's Douala Coastal Sedimentary Basin (DCSB) was constructed. The model is based upon the basin's known geology, plus data from recent field campaigns that collected rainwater and groundwater samples for analyses of stable isotopes (δ2H, δ18O, δ13C), radiogenic isotopes (3H, 14C), and water chemistry. Aquifer characteristics that were thereby deciphered include recharge, isotopic distributions, residence times, and mixing processes. Rainfall samples (mean δ18O = -2.0‰; mean δ2H = -6.80‰; weighted mean = -2.4‰ δ18O, -9.85‰ δ2H) scatter along two distinct lines, thus indicating that local rainfall events undergo fractionation induced by convective activities, high humidity, amount effects, and seasonal variations. Stable isotope values of river water samples are closed to the weighted mean of local precipitation, with some downstream enrichment. The Quaternary/Mio-Pliocene superficial aquifer system (depth < 70 m) and the intermediate Oligocene/Upper Eocene aquifer system (depth: 70 to 200 m) exhibit evidence of similar fractionation processes through an enrichment gradient of δ-values. The enrichment is more pronounced at the top of the superficial aquifer, which is very exposed to direct modern water infiltration, evaporation, and amount effects. The depth profiles of δ-values coupled to water chemistry and tritium contents, evidence leakage between (a) the superficial system's Quaternary alluvium sands and Mio-Pliocene sands; and (b) the superficial and intermediate systems. Thus, the aquifers that contain modern, post nuclear groundwater are characterized by flow exchanges and direct recharge from rainfall events. In contrast, the Upper Eocene system has depleted δ-values and lower bicarbonate contents, suggesting not only that this system was recharged by rapid infiltration (without evaporation), but that this recharge occurred during a cooler time in the past. The transit times (computed from 14C dates) indicate that uncorrected ages ranging from hundreds to thousands of years.
Abstract Sustainable water management in semi‐arid agriculture practices requires quantitative knowledge of water fluxes within the soil‐vegetation‐atmosphere system. Therefore, we used stable‐isotope approaches to evaluate evaporation ( E a ), transpiration ( T a ), and groundwater recharge (R) at sites in Senegal's Groundnut basin and Ferlo Valley pasture region during the pre‐monsoon, monsoon, and post‐monsoon seasons of 2021. The approaches were based upon (i) the isothermal evaporation model (for quantifying E a ); (ii) water and isotope mass balances (to partition E a and T a for groundnut and pasture); and (iii) the piston displacement method (for estimating R). E a losses derived from the isothermal evaporation model corresponded primarily to Stage II evaporation, and ranged from 0.02 to 0.09 mm d −1 in the Groundnut basin, versus 0.02–0.11 mm d −1 in Ferlo. At the groundnut site, E a rates ranged from 0.01 to 0.69 mm d −1 ; T a was in the range 0.55–2.29 mm d −1 ; and the T a / ET a ratio was 74%–90%. At the pasture site, the ranges were 0.02–0.39 mm d −1 for E a ; 0.9–1.69 mm d −1 for T a ; and 62–90% for T a / ET a . The ET a value derived for the groundnut site via the isotope approach was similar to those from eddy covariance measurements, and also to the results from the previous validated HYDRUS‐1D model. However, the HYDRUS‐1D model gave a lower T a / ET a ratio (23.2%). The computed groundwater recharge for the groundnut site amounted to less than 2% of the local annual precipitation. Recommendations are made regarding protocols for preventing changes to isotopic compositions of water in samples that are collected in remote arid regions, but must be analysed days later. The article ends with suggestions for studies to follow up on evidence that local aquifers are being recharged via preferential pathways.
Soil hydraulic parameters (SHPs) required as inputs for numerical models are scarce in Sahelian regions. Instead, they are estimated using pedotransfer functions (PTFs), but their ability to simulate soil water dynamics has not been evaluated. This study aims to parameterize SHPs with seven different PTFs and inverse modelling to examine their ability to simulate water fluxes in Senegal's Groundnut basin. We used four years of field measurements of soil water content (SWC) and actual evapotranspiration (ETa) under pearl millet and groundnut crop rotation for model evaluation. Inverse modelling for SWC (root mean square error [RMSE] ≤ 0.015 cm3 cm−3) and ETa (RMSE ≤ 0.62 mm d−1) yielded the best model performance compared to PTFs (0.024–0.175 cm3 cm−3 and 0.68–0.96 mm d−1, respectively). Where field measurements are lacking for inverse estimation, three of the seven tested PTFs yielded good modelling results and could be used as a parsimonious approach for cultivated Sahelian soils.
We explore in-situ fluorescence spectroscopy as an instantaneous indicator of total bacterial abundance and faecal contamination in drinking water. Eighty-four samples were collected outside of the recharge season from groundwater-derived water sources in Dakar, Senegal. Samples were analysed for tryptophan-like (TLF) and humic-like (HLF) fluorescence in-situ, total bacterial cells by flow cytometry, and potential indicators of faecal contamination such as thermotolerant coliforms (TTCs), nitrate, and in a subset of 22 samples, dissolved organic carbon (DOC). Significant single-predictor linear regression models demonstrated that total bacterial cells were the most effective predictor of TLF, followed by on-site sanitation density; TTCs were not a significant predictor. An optimum multiple-predictor model of TLF incorporated total bacterial cells, nitrate, nitrite, on-site sanitation density, and sulphate (r2 0.68). HLF was similarly related to the same parameters as TLF, with total bacterial cells being the best correlated (ρs 0.64). In the subset of 22 sources, DOC clustered with TLF, HLF, and total bacterial cells, and a linear regression model demonstrated HLF was the best predictor of DOC (r2 0.84). The intergranular nature of the aquifer, timing of the study, and/or non-uniqueness of the signal to TTCs can explain the significant associations between TLF/HLF and indicators of faecal contamination such as on-site sanitation density and nutrients but not TTCs. The bacterial population that relates to TLF/HLF is likely to be a subsurface community that develops in-situ based on the availability of organic matter originating from faecal sources. In-situ fluorescence spectroscopy instantly indicates a drinking water source is impacted by faecal contamination but it remains unclear how that relates specifically to microbial risk in this setting.
Agriculture drought is a recurrent phenomenon that affects most parts of Sahel regions. With the advent of various satellite data, drought risk assessment has become more accessible, but the challenge lies in choosing a well-defined time scale. This study aims to contribute to agricultural drought monitoring, based on the calculation of composite indices using the MODIS sensor in the Senegalese Groundnut basin. The approach is based upon vegetation condition and health (VCI, VHI) and temperature (TCI) indices. Subsequently, those indices were evaluated based upon crop yield data, and a Holt-Winter forecasting was performed to determine the driest month in 2020-2021. Here we show that all investigated communes are sensitive to drought, especially Keur-Samba Gueye (in July, September, and October). Based on its significant correlation with VCI and TCI (R2>0.57), VHI was proven to be a reliable water stress indicator. Our approach demonstrates the Groundnut basin's vulnerability to climate change.
Urban groundwater in Sub-Saharan Africa provides vital freshwater to rapidly growing cities. In the Thiaroye aquifer of Dakar (Senegal), groundwater within Quaternary unconsolidated sands provided nearly half of the city's water supply into the 1980s. Rising nitrate concentrations traced to faecal contamination sharply curtailed groundwater withdrawals, which now contribute just 5% to Dakar's water supply. To understand the attenuation capacity of this urban aquifer under a monsoonal semi-arid climate, stable-isotope ratios of O and H and radioactive tritium (3H), compiled over several studies, are used together with piezometric data to trace the origin of groundwater recharge and groundwater flowpaths. Shallow groundwaters derive predominantly from modern rainfall (tritium >2 TU in 85% of sampled wells). δ18O and δ2H values in groundwater vary by >4 and 20‰, respectively, reflecting substantial variability in evaporative enrichment prior to recharge. These signatures in groundwater regress to a value on the local meteoric water line that is depleted in heavy isotopes relative to the weighted-mean average composition of local rainfall, a bias that suggests recharge derives preferentially from isotopically depleted rainfall observed during the latter part of the monsoon (September). The distribution of tritium in groundwater is consistent with groundwater flowpaths to seasonal lakes and wetlands, defined by piezometric records. Piezometric data further confirm the diffuse nature and seasonality of rain-fed recharge. The conceptual understanding of groundwater recharge and flow provides a context to evaluate attenuation of anthropogenic recharge that is effectively diffuse and constant from the vast network of sanitation facilities that drain to this aquifer.
'%3e%3cg id='b'%3e%3cpath id='a' d='M0-100V0h50z' transform='rotate(18 0 -100)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(72)'/%3e%3cuse xlink:href='%23b' transform='rotate(144)'/%3e%3cuse xlink:href='%23b' transform='rotate(216)'/%3e%3cuse xlink:href='%23b' transform='rotate(288)'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cg id='b'%3e%3cpath id='a' d='M0-8v8h4z' transform='rotate(18 0 -8)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(72)'/%3e%3cuse xlink:href='%23b' transform='rotate(144)'/%3e%3cuse xlink:href='%23b' transform='rotate(216)'/%3e%3cuse xlink:href='%23b' transform='rotate(288)'/%3e%3c/g%3e%3c/svg%3e)