Sources and spatial variability of groundwater-delivered nutrients in Maunalua Bay, Oʻahu, Hawai‘i

Abstract

Study region: Maunalua Bay, Oʻahu, Hawai’i. Study focus: We examined submarine groundwater discharge (SGD), terrestrial groundwater, and nearshore marine water quality in two adjacent aquifers (Waialae East and Waialae West) with differing land-use and hydrogeologic characteristics to better understand the sources and spatial variability of SGD-conveyed nutrients. Nutrient concentrations and NO3− stable isotope ratios were measured and integrated with SGD flux, land-use, and recharge data to examine SGD nutrient loads and potential sources in each aquifer. New hydrological insights for the region: Regionally elevated NO3- concentrations (166–171 μM) and δ15N–NO3− values (10.4–10.9‰) were apparent in SGD in the Waialae West Aquifer, an area with high on-site disposal system density (e.g., cesspools). Coastal sites sampled in the neighboring Waialae East Aquifer exhibited significantly lower values for these parameters, with δ15N–NO3− values ranging from 5.7–5.9‰ and NO3− concentrations from 43–69 μM. The isotopic composition of NO3− in SGD originating from the Waialae West Aquifer was consistent with wastewater. Modeled recharge data corroborated the NO3− stable isotope source designation. SGD emanating from Waialae West Aquifer was primarily influenced by two-component mixing of a wastewater source with low nutrient groundwater as wastewater effluent accounted for more than 4% of total recharge and 54–95% of total N and P loads in the aquifer.

Keywords

• Submarine groundwater discharge
• Wastewater
• Nitrate stable isotope ratios
• Nutrients
• Island hydrogeology
• On-site disposal systems