Stable isotopes of precipitation and groundwater provide new insight into groundwater recharge and flow in a structurally complex hydrogeologic system: West Hawai'i, USA

Local meteoric water lines (LMWL) and corresponding relationships between delta H-2 and delta O-18 values in precipitation and elevation are useful tools for assessing groundwater recharge areas and flow paths. The LMWL and relationship between delta O-18 values in precipitation and elevation for the West Hawai'i region of Hawai'i (USA) were determined utilizing a network of eight cumulative precipitation collectors sampled at 6-month intervals over a 2-year period. Additionally, delta H-2 and delta O-18 values for groundwater samples across the study area were determined. These data were then utilized to develop new conceptual models of groundwater flow and characterize groundwater flow paths in this complex and poorly understood hydrogeologic setting. The West Hawai'i LMWL indicates a primary source of oceanic moisture from the lee of the island, while the delta O-18-elevation relationship resembles that determined for the trade-wind portion of the Hawai'i Volcano region. Conceptual models incorporating the effects of subsurface geological features on groundwater occurrence and flow in the West Hawai'i region were developed and subsequently utilized in conjunction with delta O-18 values for groundwater samples to determine that groundwater flow paths in the West Hawai'i region generally originate at high elevations located far within the island's interior. This study demonstrates the utility of considering subsurface structural characteristics in conjunction with H and O isotopic content of precipitation and groundwater to better understand groundwater flow in regions with poorly characterized hydrogeology and has important implications for future development and scientific investigation of water resources in West Hawai'i.