GEOCHEMICAL AND STABLE ISOTOPE SOURCE TRACKING OF TERRESTRIAL NUTRIENT POLLUTION TO THE COASTAL WATERS OF WAIALUA BAY, NORTH SHORE, OAHU THESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI‘I AT MĀNOA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN GEOLOGY AND GEOPHYSICS December 2020 By Lucas M. Ellison Thesis Committee: Craig R. Glenn, Chairperson Brian Popp Celia Smith Acknowledgments I would first like to thank my advisor Craig Glenn for all his help developing this project, securing its funding, and providing support and guidance throughout the past two years. His advice and leadership were indispensable in the completion of this project. I would also like to thank my committee members Brian Popp and Celia Smith for their expertise and support. This project would not have been possible without fellow graduate student and project partner Jordan Mason, who provided vital support throughout the project. His help with field work and throughout the entire graduate school process was invaluable and this project could not have been completed without him. I also thank Veronica Gibson, Vanessza Lopez, Jake Harrington, Jiana Blaha, Nerissa Barling, and Sherine S. Boomla for assistance with field work, and Natalie Wallsgrove for assistance in the lab. We thank the private landowners in the Waialua-Haleiwa region who granted access to sample their groundwater wells. We thank Sayo Costantino, Emily Geske, and everyone at Loko Ea Fishpond access. This research was supported by grants to Craig Glenn from the USGS State WRRIP and a grant/cooperative agreement from the national Oceanic and Atmospheric Administration, Project R/HE-33, which is sponsored by the University of Hawaii Sea Grant College Program, SOEST, under Institutional Grant No. NA18OAR4170076 from NOAA Office of Sea Grant, Department of Commerce. The views expressed herein are those of the author and do not necessarily reflect the views of NOAA or any of its subagencies. i Abstract 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 15 18 11 this region. Nutrient concentrations, δ Nnitrate and δ Onitrate values, and δ B 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 15 decrease in the extent of natural excess nutrient remediation processes. Elevated δ Nnitrate and 18 δ Onitrate 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. ii Table of Contents 1. Introduction ...................................................................................................................................1 2. Background ....................................................................................................................................3 2.1 Hydrogeology ...........................................................................................................................3 2.2. Land use ..................................................................................................................................9 2.2.1 On site disposal systems .....................................................................................................9 2.2.2 Historic and current agriculture ........................................................................................ 10 2.3 Climate and hydrology ............................................................................................................ 13 2.4 Beaches .................................................................................................................................. 14 3. Materials and methods ................................................................................................................ 15 3.1 Water sampling strategy ......................................................................................................... 18 3.2 Geochemical source-tracking of water ..................................................................................... 18 3.2.1 Nutrient concentrations .................................................................................................... 19 - 3.2.2 Dual isotopic composition of NO3 ..................................................................................... 19 3.2.3 Boron concentrations and isotopic compositions for agricultural differentiation ................ 20 3.3 Algal surveys .......................................................................................................................... 20 3.5 Salinity correction ................................................................................................................... 21 4. Results ......................................................................................................................................... 21 4.1 Geochemical Source-tracking of water .................................................................................... 21 4.1.1 Nutrient Concentrations ................................................................................................... 21 - 4.1.2 Dual isotopic composition of NO3 ..................................................................................... 26 4.1.3 Boron concentrations and isotopes ................................................................................... 28 4.2 Algal Surveys .......................................................................................................................... 28 5. Discussion .................................................................................................................................... 30 5.1 Endmembers .......................................................................................................................... 31 5.1.1 Values and processes of nitrogen in endmembers.............................................................. 32 5.1.2 Boron concentrations and isotope values in endmembers .................................................. 35 5.2 Processes affecting the concentration and isotopic composition of nitrate ............................... 36 5.3 Nitrogen characteristics of the regional Waialua stream system ............................................... 37 5.3.1 Nutrient sources in the Anahulu River ............................................................................... 38 5.3.2 Seasonal nitrification trends ............................................................................................. 39 5.3.3 Nutrients in the Kiikii and Paukauila streams .................................................................... 40 iii 5.3.5. Denitrification assessment of the streams ........................................................................ 41 5.3.5 Comparison to other Hawaiian streams ............................................................................ 43 5.4 Nutrients and Waialua beaches ............................................................................................... 44 5.4.1 Coastal nutrient transport as a component of SGD ............................................................ 44 5.4.2 The effect of OSDS spatial density on the source of nutrients in SGD .................................. 44 5.4.3 Nutrient loading into Waialua beaches ............................................................................. 45 6. Summary and conclusions ...........................................................................................................