Noble gas signatures in the Island of Maui, Hawaii: Characterizing groundwater sources in fractured systems

Abstract

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.

Global Isotope Hydrogeology―Review

“…For example, deuterium excess values can increase with elevation in some areas (e.g., Dong et al, ; Gonfiantini et al, ; Liotta et al, ); deuterium excess may provide a useful tracer of recharge elevations in some areas. Further, noble gas concentrations can provide insights into the temperature of recharge that can be used to approximate recharge elevations using local lapse rates (Niu, Castro, Hall, Gingerich, et al, ; Peters et al, ; Warrier et al, ). Noble‐gas‐based approaches have been applied to mountain glaciers (Niu, Castro, Hall, Aciego, et al, ) and ice sheets (Niu et al, ) to understand meltwater sources and flow paths.…”

Upwelling of Mantle‐derived Material in Southeast China: Evidence from Noble Gas Isotopes

“…He and Ne were sequentially analyzed on a Thermo Scientific Helix SFT mass spectrometer, whereas Ar, Kr, and Xe were sequentially entered into an ARGUS VI mass spectrometer using a computer-controlled double-headed cryogenic separator. The abundances of elements He, Ne, Ar, Kr, and Xe have typical uncertainties of 1.5%, 1.3%, 1.3%, 1.5%, and 2.2%, respectively, and all uncertainties were within ±1σ level (Niu et al, 2017b). The results are shown in Table 2.…”

“…1/Xe is only affected by altitude (barometric pressure), whereas Ne/Xe is affected both by altitude and temperature (Niu et al, 2017b), so the 1/Xe vs Ne/Xe diagram (Fig. 2) can reflect the altitude of the noble gas when the groundwater is recharged with temperature effect.…”

No Direct Linkage Between Event‐Based Runoff Generation and Groundwater Recharge on the Maimai Hillslope

“…We analyzed gas concentrations in groundwater samples taken from two bedrock wells located in the toe slope and riparian zone within the lower part of the M8 catchment to determine NGT (Lindsay, ; Niu et al, ; Figure ). NGT reflects environmental conditions during seasonally focused recharge periods and provides a means to directly identify seasonal patterns in groundwater recharge by matching NGT values to seasonal temperature variations.…”