H11F-1414
A multi-tracer approach for determining the sources and spatial variability of groundwater-delivered nutrients to coastal waters: Maunalua Bay, Oahu, Hawai'i

Monday, 14 December 2015
Poster Hall (Moscone South)
Christina M Richardson, University of Hawaii at Manoa, Dept. of Geology and Geophysics, Honolulu, HI, United States
Abstract:
Nutrient pollution of coastal waters commonly arises from terrestrial non-point sources of N and P such as on-site disposal systems (OSDS) and fertilizer leachate. Elevated nutrient loading of submarine groundwater discharge (SGD) has been documented in the western edge of Maunalua Bay, Oahu, an area with high OSDS density. We examined coastal groundwater and nearshore marine water quality in two adjacent aquifers (Waialae West and Waialae East) within the study region with differing land-use and hydrogeological characteristics to better understand 1) the spatial variability of SGD nutrient and water fluxes and 2) the reasons for this spatial variability. Nutrient concentrations and NO3stable isotope ratios were measured in coastal and terrestrial groundwater as well as nearshore marine water and integrated with SGD flux, land-use, and recharge data to examine potential nutrient sources in each aquifer. 

Regionally-elevated NO3- concentrations (169 µM) and δ15N-NO3- values (10.9 ‰) were apparent in SGD in the Waialae West Aquifer where OSDS density is highest. 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 NO3concentrations from 43 - 69 µM. The isotopic composition of NO3in SGD originating from the Waialae West Aquifer was primarily influenced by mixing of a wastewater source, with wastewater effluent accounting for nearly 4.4% of total recharge and 79 - 97% of total N and P loads within the aquifer. These findings illustrate the utility of synthesizing nutrient concentrations and stable isotope parameters together with SGD flux determination, and aquifer-scale land-use and recharge data in determining the contribution of terrestrial sources to coastal nutrient loading via SGD.