Abstract
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.
Highlights
Groundwater is a critical natural resource required for daily human use such as potable water supply, industrial uses, and agricultural irrigation (Kemper 2004; Alley 2006)
It was concluded that such an effort is only useful if appropriate representation of various processes is included in the conceptual model
As is the case for most coastal aquifers, groundwater from West Hawai‘i Island is vulnerable to contaminants derived from point- and nonpoint-sources such as on-site sewage disposal systems (OSDS), land use and land cover (LULC), and Wastewater Treatment Plant (WWTP)
Summary
Groundwater is a critical natural resource required for daily human use such as potable water supply, industrial uses, and agricultural irrigation (Kemper 2004; Alley 2006). As Hawai‘i’s population increases, future land development and higher water demand are inevitable, along with an expected overall decline in recharge rates due to projected climate change conditions (Elison-Timm et al 2015) These combined factors will further exacerbate saltwater intrusion (Ferguson and Gleeson 2012), emphasizing the need to understand how future circumstances may affect groundwater resources. OSDS associated with development are considered one of the major sources of nutrient contamination (e.g., Lapointe et al 1990; Harris 1995; Reay 2004) The majority of these OSDS are cesspools, where wastewater effluent is not treated before entering the groundwater system (Whittier and El-Kadi 2014). Most of Hawai‘i’s OSDS are located near the shoreline (Whittier and El-Kadi 2014), and overlooking freshwater–saltwater interactions can lead to inaccurate interpretations of resulting contaminant loads and poorly inform water management decisions
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