A density-dependent multi-species model to assess groundwater flow and nutrient transport in the coastal Keauhou aquifer, Hawai‘i, USA

Fresh groundwater is a critical resource supporting coastal ecosystems that rely on low-salinity, nutrient-rich groundwater discharge. This resource, however, is subject to contamination from point- and nonpoint-sources such as on-site sewage disposal systems (OSDS) and urban developments. Thus, the significance of flow and transport processes near the coastline due to density effects and water circulation in a complex hydrogeologic system was investigated. A three-dimensional, density-dependent groundwater model was developed for the Keauhou basal aquifer (Hawai‘i Island, USA), where hydraulic head, salinity, nutrient concentrations, and submarine spring flux rates were used as calibration variables to best constrain parameters and produce a comprehensive aquifer management tool. In contrast, a freshwater-only model failed to properly simulate nutrient transport, despite the reasonable success in calibrating hydraulic head measurements. An unrealistic value for hydraulic conductivity was necessary for freshwater-only calibration, proving that hydraulic conductivity is a process-based variable (i.e., depends on model conceptualization and the simulated processes). The density-dependent model was applied to assess relative contaminant source contributions, and to evaluate aquifer response concerning water levels and quality due to changing environmental conditions. Nutrients detected in the aquifer are primarily sourced from OSDS, which was supported by a nitrogen isotope mixing model. Additionally, effects of sea-level rise emphasized the complexity of the study site and the importance of model boundaries. While the model is developed and applied for West Hawai‘i, the adapted approaches and procedures and research findings are applicable to other coastal aquifers.