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

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.