Shallow urban aquifers under hyper-recharge equatorial conditions and strong anthropogenic constrains. Implications in terms of groundwater resources potential and integrated water resources management strategies

Hydrochemical and isotopic studies providing a new functional model for the coastal aquifers in Douala Coastal Sedimentary Basin (DCSB)/Cameroon

A new conceptual model of the hydrogeological systems in Cameroon'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 allowed the collection of 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 processes during convective events, variability in humidity, amount effects, and seasonal variations. Stable isotope values of river water samples are close 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 rainfall water infiltration, evaporation, and amount effects. The depth profiles of δ-values coupled to water chemistry and tritium contents, evidence leakage between (i) the superficial system's Quaternary alluvium sands and Mio-Pliocene sands; and (ii) 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 (with limited effect of evaporation), but that this recharge occurred during a cooler time in the past. The residence times (computed from 14C dates) indicate uncorrected ages ranging from hundreds to thousands of years.

Insight into Groundwater Resources along the Coast of Benin (West Africa) through Geochemistry and Isotope Hydrology; Recommendations for Improved Management

Along the West-African coast, groundwater is under several threats coming from both human activities and climate change. However, hydrogeological studies have so far been conducted in a piecemeal way, city by city. In this paper, a regional study was conducted along the Beninese coast, combining hydrogeochemistry and water stable isotopes. Monthly rainfall samples were analyzed in terms of chemistry and isotopes as well as groundwater from Holocene (upper aquifer) and Mio-Plio-Pleistocene (lower aquifer). This allowed to determine the recharge timing of aquifers (April to October, excluding August). Rainwater then infiltrates the soil with a slight evaporation. The upper aquifer, more heterogeneous, is displaying many different water types while the lower aquifer shows mainly a Na-Cl water type. While the upper aquifer shows many signs of contamination from human activities and saltwater intrusion from lakes and lagoons, the deeper aquifer is more influenced by a geogenic signature. These results are then interpreted regarding the demographic trends and climate change scenario. In the long-term, the groundwater level of the lower aquifer is expected to decrease as the rate of abstraction increases and recharge rate decreases. It is therefore recommended to develop adapted and urgent protection measures of the water resource to ensure sustainable and healthy groundwater exploitation.

Hydrological components of groundwater recharge in leaky aquifers adjacent to semipervious streambank: analytical study

Groundwater resources can be artificially augmented by managed aquifer recharge especially in arid and semi-arid areas. Unlike most of the existing results which focus only on impervious aquifer base and assume perfect hydraulic connection between aquifer and adjacent stream, the present study addresses the recharge process in a more realistic case of leaky aquifer interacting with semipervious streambank. The mathematical model relies on 2D linearized Boussinesq equation with Robin boundary condition accounting for hydraulic resistance owing to semiperviousness of the streambed. The method of Green's function offers an integral representation of hydraulic head in semi-infinite aquifer domain in response to time-varying recharge from a rectangular basin. Hydrological components of groundwater recharge are derived taking into account volumetric rates of leakage, mound storage, and outflow to adjacent stream. Hypothetical test cases illustrate how sum of these components exactly counterbalances the volumetric rate of recharge received by the aquifer at each instant of time, and this is in full conformity with the expected mass balance at aquifer scale. The most influential parameters affecting the model outputs are identified through sensitivity analysis. Furthermore, interaction of pumping wells with groundwater mound is examined. Performance of the present results is assessed against numerical counterparts from finite element method, and extremely good agreement is found for temporal mound peaks.

Hydrogeochemical Assessment of Groundwater and Suitability Analysis for Domestic and Agricultural Utility in Southern Punjab, Pakistan

Groundwater is a critical water supply for safe drinking water, agriculture, and industry worldwide. In the Khanewal district of Punjab, Pakistan, groundwater has severely deteriorated during the last few decades due to environmental changes and anthropogenic activities. Therefore, 68 groundwater samples were collected and analyzed for their main ions and trace elements to investigate the suitability of groundwater sources for drinking and agricultural purposes. Principal component analysis (PCA) and cluster analysis (CA) were employed to determine the major factors influencing groundwater quality. To assess the groundwater’s appropriateness for drinking and irrigation, drinking and agricultural indices were used. The pH of the groundwater samples ranged from 6.9 to 9.2, indicating that the aquifers were slightly acidic to alkaline. The major cations were distributed as follows: Na+ > Ca2+ > Mg2+ > K+. Meanwhile, the anions are distributed as follows: HCO3− > SO42− > Cl− > F−. The main hydrochemical facies were identified as a mixed type; however, a mixed magnesium, calcium, and chloride pattern was observed. The reverse ion exchange process helps in exchanging Na+ with Ca2+ and Mg2+ ions in the groundwater system. Rock weathering processes, such as the dissolution of calcite, dolomite, and gypsum minerals, dominated the groundwater hydrochemistry. According to the Weight Arithmetic Water Quality Index (WAWQI), 50% of the water samples were unsafe for drinking. The Wilcox diagram, USSL diagram, and some other agricultural indices resulted in around 32% of the groundwater samples being unsuitable for irrigation purposes. The Khanewal’s groundwater quality was vulnerable due to geology and the influence of anthropogenic activities. For groundwater sustainability in Khanewal, management strategies and policies are required.