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.

Large-scale surface water-groundwater origins and connectivity in the Ordos Basin, China: Insight from hydrogen and oxygen isotopes

The sustainability of water resources is a major challenge for the Ordos Basin and Loess Plateau of China. The basis of effective water management is an understanding of the water cycle process. This study investigated the surface water-groundwater origins and connectivity using stable isotopes (δD and δ18O) of surface water and groundwater in 11 river basins in the Ordos Basin. It was found that the surface water-groundwater origins and hydraulic connection were characterized by regional differences, mainly induced by climatic characteristics, hydrogeological conditions and human activities. Specifically, the impact of thick loess deposits caused surface water and groundwater to take long time to produce a hydraulic connection. In contrast, areas with thin loess deposits and frequent human activities showed a good connectivity between surface water and groundwater. As for water origins, summer precipitation was a common source of surface water and groundwater in the study area, and groundwater discharge was another source of surface water. However, surface water and groundwater were subjected to different degrees of evaporation during receiving precipitation recharge. Notably, thick loess deposits had an impact on groundwater evaporation because both the recharge of precipitation to groundwater and the discharge of groundwater to surface water took a long time. In addition, it was found that frequent human activities (mining, irrigation and urban construction) could weaken the impact of evaporation. This large-scale analysis provided new insights into the origins and connectivity of surface water and groundwater in areas with thick unsaturated zones for water resources management.

Hydraulic connection affects uranium distribution in the Gas Hure salt lake, Qaidam Basin, China

The widespread hydraulic connection is necessary for the formation of a salt lake. However, only limited studies have ever been carried out to investigate the influence of the hydraulic connection on the distribution of elements around certain salt lake. In this study, a total of 66 water samples (including river water, stream water, spring water, brine, intercrystalline brine, well water, and drilling brine) were collected around the Gas Hure salt lake (GSKLH) to investigate the relationship between hydraulic connection and uranium (U) distribution via hydrochemistry and isotope (²³⁴U/²³⁸U, δ¹¹B) techniques. The results suggested that the GSKLH was recharged by water from the Kulamulekesay and Atetikan rivers, groundwater (borehole brine and some intercrystalline brine), and deep fluid (some intercrystalline brine), with each contributing 44.03%, 14.95%, and 41.02% of total recharge, respectively. The U-bearing rock was dominated mainly by silicates, carbonates, and evaporites in the high mountain area (region 1), overflow area (region 2), and plain area (region 3) of the GSKLH, respectively. In the GSKLH, the U distribution was strongly correlated with hydraulic connection and the U concentration was influenced by both groundwater flow system and flow velocity (represented by the γCl^-/γCa²⁺ ratio). Thus, U was enriched under the conditions of regional groundwater flow system and slow velocity in the GSKLH.