CHARACTERIZING GROUNDWATER SOURCES ON THE ISLAND OF MAUI, HAWAII USING NOBLE GASES AND STABLE ISOTOPES

Previous work in fractured groundwater systems and rainwater (Warrier et al., 2012; 2013) suggests that noble gas temperatures (NGTs) reflect the temperature of the ground air at the time of rainwater infiltration rather than the mean annual air temperature (MAAT) in sedimentary systems where NGTs are used as indicators of past climate. This suggests that NGTs in fractured areas may provide a seasonality record, and thus, information on timing of recharge in addition to location. Many fractured, volcanic islands such as Maui, have complex internal structures and poorly understood hydrological systems. Developing this new NGT application will lead to a better understanding of fractured groundwater flow systems and will contribute to improved water resource management plans in these regions.

The first Maui noble gas study on samples collected from the basal (wells) and perched (springs) aquifers as well as rainwater in June 2014 suggests that perched and basal aquifers are relatively separate entities (Niu et al., 2017). All samples are consistent with binary mixing between air-saturated water (ASW) and an ice-like signature. Here, we present the results of noble gas and stable isotopes from samples collected in February 2016 from the basal and perched aquifers as well as rain events. Comparing the June 2014 and February 2016 datasets, noble gases in rainwater and perched springs generally display distinct seasonal signatures, while samples from the basal aquifer display similar seasonal patterns, indicating that the basal aquifer is less prone to seasonality. Stable isotope data show that all samples have deuterium excess ranging from 13 to 29, while 2016 samples have generally greater deuterium excess (17 to 29) than those from 2014 (ranging from 13 to 20). Preliminary results indicate that for most samples, recharge for both types of aquifers takes place at an altitude significantly higher than that of the collection point, possibly between 2 and 7 km.

References:

Niu, Y., M. C. Castro, C. M. Hall, S. B. Gingerich, M. A. Scholl, R. B. Warrier (2017). Water Resour. Res., 53, doi:10.1002/ 2016WR020172.

Warrier, R. B., et al. (2012). Water Resour. Res., 48, W03508 (2012), doi:10.1002/2015GL065778.

Warrier, R. B., et al. (2013). Geophys. Res. Lett., 40, 1 – 5. doi:10.1016/j.apgeochem.2016.11.015.