Geochemical and stable isotope source tracking of terrestrial nutrient pollution to the coastal waters of Waialua Bay, North Shore, Oahu

Ellison, Lucas Macrae
Glenn, Craig R.
Earth and Planetary Sciences
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Anthropogenic nutrient loading has long been identified as potentially harmful to coastal waters surrounding the Hawaiian Islands, with excess nutrients being delivered by stream runoff as well as by inputs to groundwaters that exit the coast as submarine groundwater discharge (SGD). Identifying, understanding, and quantifying where excess anthropogenic nutrients are introduced to surface and subsurface waters, properly deciphering how they may chemically evolve during transport, and understanding their history and final composition as they are discharged is therefore of vital importance for protecting aquatic resources and the surrounding environments. The Waialua region on the north shore of Oahu is particularly at risk of experiencing harmful effects from nutrient loading due to both its high density of onsite sewage disposal systems (OSDS) and its decades-long history of agricultural nutrient fertilization. We thus used a multi-tracer approach to determine the amount and to differentiate the sources of nutrients entering surface waters, groundwaters and the coastal waters of this region. Nutrient concentrations, δ15Nnitrate and δ18Onitrate values, and δ11B values were used to establish geochemical and hydrological connections between terrestrial nutrient sources and the groundwater, stream, and coastal waters. Geochemical surveys of benthic macroalgae were performed along major beaches in the region to help confirm sources of nutrients, the extent of nutrient loading at each location, and the distance away from shore that nutrient pollution extended. Results indicate that nutrient concentrations in Waialua region streams are elevated relative to other impaired and pristine streams throughout the Hawaiian Islands. All Waialua region streams showed a seasonal shift in the form of nitrogen with a higher proportion of ammonium in the dry season, suggesting a seasonal decrease in the extent of natural excess nutrient remediation processes. Elevated δ15Nnitrate and δ18Onitrate values in the region were documented to be associated with the highest OSDS density and indicate significant leaching of wastewater into streams and coastal waters. However, widespread denitrification was found in several beaches and streams, particularly near locations with the highest densities of OSDS. Along the coast, the highest nutrient concentrations were found near the locations with the most OSDS, however the relationship between OSDS density and overall nutrient loading is not simple, as it is clear regions with few OSDS are also experiencing significant nutrient loading from agricultural sources.
Marine geology, Hydrologic sciences, Geochemistry, Nitrogen, Nutrients, OSDS, SGD, Stable isotopes
96 pages
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