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

Abstract

Study regionMaunalua Bay, Oʻahu, Hawai’i. Study focusWe 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 regionRegionally elevated NO3- concentrations (166–171μM) 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 43–69μM. 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.