NATURAL AND ANTHROPOGENIC CONTROLS ON GROUNDWATER NUTRIENT AND DISSOLVED INORGANIC CARBON CONCENTRATIONS: WEST HAWAII, USA

Groundwater in the western portion of the island of Hawaii serves as the region’s primary water supply and delivery mechanism of dissolved nutrients and inorganic carbon to the coastal ocean via submarine groundwater discharge (SGD). Despite the economic and ecological importance of groundwater in this region, the relationships between natural and anthropogenic terrestrial factors and groundwater nutrient and dissolved inorganic carbon (DIC) concentrations are poorly understood. We measure PO₄^3-, SiO₄^4-, NO₃^-, and DIC concentrations as well as δ¹⁵N of NO₃^- and δ¹³C of DIC values for groundwater samples collected throughout the West Hawaii study area. We then use 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. We find that geothermal activity is significantly correlated to elevated groundwater SiO₄^4-, NO₃^-, and DIC concentrations and that wastewater effluent discharge as well as urban and park land use are significantly correlated to elevated groundwater NO₃^- concentrations. Additionally, land use and land cover types associated with greater precipitation and soil development are significantly correlated to elevated PO₄^3- concentrations.