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

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 NO₃^- stable 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 NO₃^- concentrations from 43 - 69 µM. The isotopic composition of NO₃^- in 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.