Assessment of Modeling Uncertainties for Nutrient Transport Models of the Keauhou Aquifer, Hawai'i

Abstract:

Groundwater from the Keauhou aquifer is the primary source of freshwater supplied to residents of West Hawai‘i Island as well as coastal ecosystems. Numerical models have been developed to simulate groundwater flow through West Hawai‘i aquifer systems, but many uncertainties persist due to a complex and poorly understood hydrogeology and lack of available data, thus making it difficult to properly manage groundwater resources. The objective of this study was to develop a density-dependent Keauhou basal-aquifer model to assess the potential impact of future developments on the coastal environment. To do so, we simulated spatial and temporal changes in salinity, nitrogen, and phosphorus concentrations in the aquifer and at its discharge points along the coastline, under various development scenarios and pumping stressors. The model, however, is sensitive to several uncertainty issues, including parameterization non-uniqueness, model conceptualization, spatial data representation, and capturing spatial distribution of observed discharged points at different scales. In an attempt to minimize these uncertainties, we implement a multi-species calibration approach utilizing sampled nitrogen concentrations from production wells and coastal springs, along with submarine groundwater discharge measurements, to constrain various flow and transport parameters. These uncertainties and the underlying assumptions are all important to consider when using modeling results to guide the management process of groundwater resources.