A two-dimensional numerical groundwater flow model was established and calibrated for the hyperarid Najd region in southern Oman. The results indicate that recent recharge rates are required to sustain the observed groundwater heads in the Najd. The model was also used to estimate possible ranges of past recharge rates and the effective porosity of the main aquifer unit. Recharge rates during past humid periods were estimated to be no more than 1–3 times modern rates. The effective porosity was estimated to be between 0.06 and 0.093. Insight into the nature of the long-term transport within the aquifer was gained by using transient model runs over the last 350 ka and (1) varying the recharge intensity (from 0.1 to 2.5 times modern), and (2) the timing and duration of humid and dry periods. Finally, results indicate that although recharge rates and the flow conditions have likely changed over time, a steady-state model is capable of reproducing the observed groundwater residence times in the Najd based on carbon-14, helium and chlorine-36 dating.

In the High Plains (HP) region of northeastern New Mexico (NE NM), USA, underlying bedrock aquifers are utilized where the High Plains Aquifer is thin, absent, or unsaturated. These usage patterns, aquifer depletion, and increasing regional aridity imply that NE NM is a possible analogy for more easterly portions of the central HP. To examine the relationship between recharge, residence time, and hydrogeologic setting, 85 well and spring samples were analyzed for environmental tracers (δD, δ¹⁸O, δ¹³C, and limited tritium and carbon-14 activities). Approximately half of the wells were open to strata of the Dakota Group. δD was −105.0 to −41.7‰ (median −58.2‰) and δ¹⁸O was −13.7 to −4.4‰ (median −8.1‰). Overall, isotopic composition is correlated with elevation and influenced by hydrogeologic setting. Ten anomalously depleted waters, most near volcanic-capped mesas, may represent higher-elevation or winter-biased recharge, a different modern precipitation source, or recharge from a cooler climate. Recharge, estimated by chloride mass balance using groundwater chloride concentrations, averages 6 mm/year below 2,000-m elevation and 16 mm/year above 2,000 m. Tritium (nondetectable to 5.7 tritium units) and carbon-14 activities (modern carbon fraction 0.23–1.05) suggest that Holocene to modern waters occur, possibly as mixtures, and that alluvial channels and other surficial features promote recharge, likely at higher rates than regional averages. It is noteworthy that isotopically depleted waters in this study tended to be tritium-free. Additional residence time tracers and seasonal precipitation isotopic sampling could address recharge and the origin of depleted waters.

Hydrogeochemistry and environmental isotopes were used to gain insight into the recharge processes, water–rock interactions, and groundwater residence time, and to identify groundwater flow systems (GFSs) in an interfluve between the Han River and Yangtze River in the eastern Jianghan Plain (China), an alluvial-lacustrine plain in the middle reaches of the Yangtze River. Because of carbonate mineral weathering, groundwater in the plain is predominantly HCO₃-Ca or HCO₃-Ca-Mg type. The decrease in typical ions and isotopic depletion with increasing depth indicates that the GFSs were divided into local and regional GFSs with an approximate depth limitation of 20 m. The consistent variations are attributable to complex anthropogenic activities, water–rock interactions and groundwater flow patterns. The multiple independent local GFSs exhibited a pattern in which groundwater was discharged into surface waters during the non-flood season. Groundwater age of local GFSs is modern according to the ³H concentrations, so the hydrodynamic circulation is active. Furthermore, the regional GFS pattern is controlled by slow lateral flow from west or northwest to east, eventually discharging into the Yangtze and Han rivers. The distribution of δ¹⁸O indicated three zones in regional GFSs that are likely dominated by the altitude effect of recharge areas. The groundwater age of regional GFSs varied from hundreds of years to 5000 years, estimated by ¹⁴C isotope data, revealing that the hydrodynamic circulation of regional GFSs is slow to relatively stagnant. The hydrodynamic characteristics and hydrochemical distributions corroborated the mixing zones of differently hierarchical GFSs in the discharge area of the Jianghan Plain.

The effective deep recharge to the Hutton Sandstone, a major confined aquifer of the Surat Basin, Australia, has been quantified for the first time with the aid of environmental tracers. A factor of ten discrepancy was found when deriving groundwater flow velocities from applying the environmental tracers ¹⁴C and ³⁶Cl. It was possible to reconcile these contradictory results describing the Hutton Sandstone as a dual porosity system, in which a significant part of the tracer is not only lost by radioactive decay, but also by diffusion into stagnant zones of the aquifer. The conceptual and mathematical description of this process allowed for quantification of the effective deep recharge into this aquifer. The resulting recharge value is only a small percentage (~3%) of earlier estimates using chloride mass balance. The chloride mass balance probably gives a correct shallow infiltration rate but most of that infiltration is diverted to springs and surface water nearby (“rejected recharge”). Only a small fraction of recharge finally reaches the deeper system. These results are significant for water resource quantification from groundwater in deep confined systems. The presented dual porosity reconceptualization is likely applicable to a significant number of earlier studies that apply environmental tracers to old groundwater, and indicates that those original results may actually give too small values for groundwater velocity and too large estimates of recharge. This reconceptualization may be particularly valid for systems that include old groundwater and that have limited spatial and temporal coverage of tracer data such as the Great Artesian Basin.