Sources and spatial variability of groundwater-delivered nutrients in Maunalua Bay, O'ahu, Hawaii

Study region: Maunalua Bay, Oahu, 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 (166171 mu M) and delta N-15-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 delta N-15-NO3- values ranging from 5.7-5.9%. and NO3- concentrations from 43-69 mu M. 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. (C) 2015 The Authors. Published by Elsevier B.V.