This work represents the first comprehensive noble gas study in rainwater. It was carried out in southeast Michigan. Results show that all rainwater samples are in disequilibrium with surface conditions. Two noble gas patterns are identified. The first one, associated with low‐pressure systems, presence of fog and light rainfall, displays a relative Ar enrichment together with Ne, Kr, and Xe depletion. The second one, associated with the passage of frontal systems, displays a mass‐dependent depletion pattern. Precipitation is characterized by thunderstorms, heavy rainfall, and high cloud ceiling heights. A diffusion mass‐transfer model suggests that noble gas patterns originate from ice. Complete re‐equilibration with surface conditions should occur within hours. For the first time, this study establishes a direct correlation between the noble gas composition in rainwater and weather patterns and highlights their potential to identify timing and location of recharge in shallow aquifer systems where infiltration is rapid.

[1] Through a combined noble gas and stable isotope study carried out in the Galapagos Islands of Santa Cruz and San Cristobal, we demonstrate the utility of atmospheric noble gases in identifying recharge areas and timing of recharge in fractured, basaltic systems. Timing of recharge obtained through noble gas temperatures (NGTs) for all samples is corroborated by stable isotopes. Except for one sample, combined NGTs and stable isotope analyses point to recharge during the hot season for all low-altitude (<$420 m) spring samples from San Cristobal and all basal aquifer samples in Santa Cruz. Stable isotope comparisons also indicate that San Cristobal springs located at high altitudes (>420 m above sea level) are recharged during both the ''garúa'' and hot seasons. Preservation of seasonality independently recorded by NGTs and stable isotopes is further reinforced by estimated young water ages. Samples located at high-altitude display systematic deviations of dissolved noble gases from expected air saturated water values and lead to inconsistent recharge altitudes and temperatures using standard NGT models. Existing degassing models are unable to account for the observed noble gas pattern for most samples. We explore various mechanisms to assess their potential at reproducing the observed noble gas signature. In particular, the potential impact of fog droplets during the cooler ''garúa'' season on dissolved noble gas concentrations in groundwater and the effect of mixing highaltitude (!1500 m) rainwater with low-altitude ($400 m) fog droplets is explored. This mixing hypothesis is capable of explaining Ne and Xe concentrations for most high-altitude samples.Citation: Warrier, R. B., M. C. Castro, and C. M. Hall (2012), Recharge and source-water insights from the Galapagos Islands using noble gases and stable isotopes, Water Resour. Res., 48, W03508,

Stable isotopes (δD, δ18O) and 14C derived ages in the Saginaw aquifer in southern Michigan suggest subglacial meltwater contributions from the Laurentide Ice Sheet of up to 36% in the late Pleistocene, following the Last Glacial Maximum. Contributions of up to 74% from previous glaciation periods are observed. Together with the Marshall record [Ma et al., 2004], noble gas temperatures (NGTs) and excess air (EA) from the Saginaw aquifer capture, for the first time, the onset of the Younger Dryas (∼12.9 kyr BP) with a ∼3.3°C cooling accompanied by drier conditions. Mid‐Holocene (MH) climatic shifts are also identified, with warming (∼2.9°C), increased aridity starting at ∼5.4 kyr BP followed by reversal to cooler, humid conditions at ∼4.1 kyr BP. Except for the last MH reversal, the stable isotope record mimics the NGT and EA records. Contrasting trends displayed byδ18O and deuterium‐excess in the last MH reversal suggests enhanced vapor transport from the Gulf of Mexico.

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Uneven distribution of rainfall and freshwater scarcity in populated areas in the Island of Maui, Hawaii, renders water resources management a challenge in this complex and ill‐defined hydrological system. A previous study in the Galapagos Islands suggests that noble gas temperatures (NGTs) record seasonality in that fractured, rapid infiltration groundwater system rather than the commonly observed mean annual air temperature (MAAT) in sedimentary systems where infiltration is slower thus, providing information on recharge sources and potential flow paths. Here we report noble gas results from the basal aquifer, springs, and rainwater in Maui to explore the potential for noble gases in characterizing this type of complex fractured hydrologic systems. Most samples display a mass‐dependent depletion pattern with respect to surface conditions consistent with previous observations both in the Galapagos Islands and Michigan rainwater. Basal aquifer and rainwater noble gas patterns are similar and suggest direct, fast recharge from precipitation to the basal aquifer. In contrast, multiple springs, representative of perched aquifers, display highly variable noble gas concentrations suggesting recharge from a variety of sources. The distinct noble gas patterns for the basal aquifer and springs suggest that basal and perched aquifers are separate entities. Maui rainwater displays high apparent NGTs, incompatible with surface conditions, pointing either to an origin at high altitudes with the presence of ice or an ice‐like source of undetermined origin. Overall, noble gas signatures in Maui reflect the source of recharge rather than the expected altitude/temperature relationship commonly observed in sedimentary systems.