Geochemical Evolution of Hawaiian Groundwater

Abstract

Ph.D. University of Hawaii at Manoa 2016.Includes bibliographical references.Groundwater in Hawaiʻi is heavily utilized for domestic, industrial, and agricultural purposes and additionally serves as a delivery mechanism of dissolved nutrients and inorganic C to coastal waters via submarine groundwater discharge (SGD). An understanding of the factors that control dissolved nutrient and inorganic C concentrations in groundwater is vital to sustainable use of this economically and ecologically important resource. In order to better understand the dynamics of dissolved nutrients and inorganic C in Hawaiian groundwater I investigated the biogeochemistry of a subsurface wastewater effluent plume in West Maui and used H and O isotopic composition of water to develop groundwater conceptual models and flow paths for the West Hawaiʻi region which I then used to evaluate relationships between terrestrial controls and groundwater geochemical parameters. I utilized N and C species concentration data along with δ15N values of NO3- and δ13C values of dissolved inorganic C to evaluate the stoichiometry of biogeochemical reactions (mineralization, nitrification, anammox, denitrification) occurring within a subsurface wastewater plume that originates as treated wastewater injection and enters the coastal waters of West Maui as SGD via several submarine springs. Additionally, I compared wastewater time- series data, injection rates, and treatment history with submarine spring time-series data to assess correlation between input and output variables. I found that heterotrophic denitrification is the primary mechanism of N loss within the groundwater plume and that chlorination for pathogen disinfection suppresses microbial activity responsible for N loss, resulting in increased coastal ocean N loading. Replacement of chlorination with UV disinfection may restore biogeochemical reactions responsible for N loss within the aquifer and return N-attenuating conditions in the effluent plume, reducing N loading to coastal waters. I characterized the local meteoric water line (LMWL) and relationship between δ18O values in precipitation and elevation for the West Hawaiʻi region utilizing a network of 8 cumulative precipitation collectors sampled at 6-month intervals over a 2 year period. Additionally, I determined δ2H and δ18O values for groundwater samples across the study area. I then utilized these data to develop new conceptual models of groundwater flow and characterized groundwater flow paths in this complex and poorly understood hydrogeologic setting. The West Hawaiʻi LMWL indicates a primary source of oceanic moisture from the lee of the island, while the δ18O ned for the trade-wind potion of the Hawaiʻi Volcano region. I developed updated conceptual models on groundwater occurrence and flow in the West Hawaiʻi region incorporating subsurface geological features that I utilized in conjunction with δ18O values for groundwater samples to determine that groundwater flow paths in the West Hawaiʻi region generally originate at high elevations in the island’s interior I measured PO43-, SiO44-, NO3-, and DIC concentrations as well as δ15N of NO3- and δ13C of DIC values for groundwater samples collected throughout the West Hawaiʻi study area. I then used the Spearman’s rank correlation test to aid in the assessment of the effects of land use/land cover, wastewater effluent discharge, and geothermal activity along flow paths determined for each groundwater sample on the measured parameters. I found that geothermal activity was significantly correlated to elevated groundwater SiO44-, NO3-, and DIC concentrations and that wastewater effluent discharge along with urban and park land use was significantly correlated to elevated groundwater NO3- concentrations. Additionally, land use and land cover types associated with greater precipitation and soil development were significantly correlated to elevated PO43