INTEGRATING PRECIPITATION AND GROUNDWATER CHEMISTRY TO INTERPRET GROUNDWATER FLOW IN THE HAWAIIAN ISLANDS

Hawaiʻi is dependent on groundwater resources, yet how water moves through the subsurface is not well understood in many locations across the state. ‘Ike Wai, a 5 year NSF-funded EPSCoR project, incorporates analyses of precipitation and groundwater chemistry in its effort to better understand Hawai‘i’s hydrologic systems. The focus areas of the project are the Pearl Harbor aquifer (Oʻahu) and the Hualālai aquifer (Hawai‘i Island). Previous work on the Hualālai aquifer has prompted us to look further inland for the source of groundwater recharge. Groundwater models currently use water level elevations to model groundwater flow downslope, but in many cases, there is a lack of data in the critical upslope regions where groundwater sources and recharge occurs. The integration of groundwater chemistry along flow paths will provide constraints in these groundwater flow models. We use the known correlation between precipitation δ¹⁸O and δ²H isotopic values and elevation of rainfall to evaluate the source location of each groundwater sample taken from wells within each aquifer, and also submarine groundwater discharge sampled from the coast. We also use major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃^-, SO₄^2-, Cl^-, etc.) and trace metals (Fe, Pb, Zn, Cs, Cr, Sr, Ba, etc.) as natural tracers by relating respective concentrations to the subsurface geology. As such, the isotopes and various chemical species are then used as tracers to develop a better understanding of the movement and distribution of groundwater. Preliminary data and interpretations will be presented.