Assessment of groundwater age and transport in West Hawai'i aquifers

Abstract

Groundwater is a freshwater resource that is essential for human existence and supports coastal ecosystems. In Hawai‘i, groundwater provides nearly all domestic water demands and supplements irrigation and industrial needs. As an island state, however, Hawai‘i is completely surrounded by the Pacific Ocean and the fresh groundwater is underlain by saltwater. The groundwater capacity is therefore confined to the boundaries of each island. Additionally, Hawai‘i’s fresh groundwater is susceptible to natural contamination, such as saltwater intrusion caused by over-pumping or sea level rise, as well as anthropogenic contamination sourced, for example, from cesspools, agriculture fields, and urban land development. It is therefore imperative to sustainably manage Hawai‘i’s aquifer systems to ensure these groundwater resources are not depleted. This dissertation examines groundwater flow and transport in the West Hawai‘i aquifers using a combination of groundwater age tracers (CFCs, SF6, 3H/3He, 14C), nutrient tracers (δ15N of NO3-), and numerical models. Sustainable water management requires the understanding of both groundwater quantity and quality, which rely on flow and transport. Apparent groundwater ages help to characterize aquifer storage and groundwater flow properties, which can be combined with nutrient tracers to assess contamination transport. Additionally, numerical models are valuable tools developed to test new conceptual flow models and to assess contaminant transport. The objectives of this dissertation are to 1) estimate the apparent ages of young and old groundwater mixed within the West Hawai‘i aquifers, 2) test a new conceptual model of the area to support the need of groundwater age corrections, and 3) develop a robust groundwater model for the basal Keauhou aquifer to investigate nutrient source and transport. Groundwater age is the time interval between the moment of recharge and the moment of discharge, and is an underlying parameter used for groundwater management. The distribution of apparent ages sheds light on groundwater flow and aquifer storage, which inform groundwater sustainability. Groundwater age can generally be divided into three categories: modern groundwater, old groundwater, and very old groundwater. With proper understanding of aquifer properties, each groundwater age category can be sustainably utilized. Further, groundwater ages can be used to inform how natural and anthropogenic contaminants are transported and dispersed through the aquifer systems. Natural and anthropogenic contaminants are of growing concern in Hawai‘i, where groundwater resources are limited to island areas and vulnerable to environmental changes. Nutrients sourced from on-site sewage disposal systems (OSDS), wastewater treatment plants (WWTP), and urban development can negatively impact groundwater resources and coastal ecosystems that rely on fresh terrestrial water. Groundwater models can therefore be utilized to assess how nutrients are transported through aquifer systems and validate apparent ages. The second chapter of this dissertation estimates the apparent age of young groundwater detected in the West Hawai‘i aquifers using chlorofluorocarbon (CFC), sulfur hexafluoride (SF6), and tritium/helium-3 (3H/3He) tracers. This multi-tracer approach infers the mixing of young and old groundwater across the West Hawai‘i aquifers. It also suggests that volcanic CO2 influences apparent groundwater ages. The third chapter of this dissertation complements the second chapter by assessing the apparent age of old groundwater from West Hawai‘i using radiocarbon (14C). An isotope mass balance based on δ13C values of dissolved inorganic carbon (DIC) is employed to correct the apparent groundwater ages for the influence of volcanic CO2. Additionally, a two-dimensional groundwater model was developed to support apparent age results using a new conceptual model. The fourth chapter of this dissertation investigates groundwater flow and transport with the development of a three-dimensional density-dependent groundwater model of the basal Keauhou aquifer. The model demonstrates the need of a density-dependent model when investigating coastal aquifers with submarine groundwater discharge. Using δ15N of nitrate (NO3-) measurements, the model estimated the relative contributions of major nutrient sources and tested future scenarios. Findings from this dissertation suggest the mixing of young and old groundwater that are sourced from various distances and elevations. It further demonstrates the complexity of the West Hawai‘i aquifer systems, which requires careful consideration of the subsurface geology and the inclusion of density-dependent flow properties. These findings can be utilized in West Hawai‘i’s groundwater resource management and serve as a catalyst for future work to be done across the state. Further, the methods and implications from this work provide a possible model for studies elsewhere with similar conditions and resource concerns.