TRACING GROUNDWATER CONNECTIONS AND NUTRIENT FLOW BETWEEN LAND AND SEA USING UAV INFRARED MAPPING, RADON, AND NUMERICAL GROUNDWATER MODELING: WAIALUA REGION, NORTH SHORE, O‘AHU

Mason, Jordan Paul

TRACING GROUNDWATER CONNECTIONS AND NUTRIENT FLOW BETWEEN LAND AND SEA USING UAV INFRARED MAPPING, RADON, AND NUMERICAL GROUNDWATER MODELING: WAIALUA REGION, NORTH SHORE, O‘AHU

Files

Mason_hawii_0085O_10773.pdf (4.19 MB)

Date

2020

Authors

Mason, Jordan Paul

Advisor

Glenn, Craig R.

Department

Earth and Planetary Sciences

Abstract

Submarine groundwater discharge (SGD) is now well recognized as a significant source of excess nutrient loading to the nearshore environment. Locating and quantifying SGD is important, and equally important is identifying the sources from which SGD nutrient fluxes were derived. The Waialua region within the North-Central Groundwater Region on O‘ahu, Hawai‘i, is susceptible to excess nutrient loading to the nearshore due to the long history of agriculture as well as the presence of many cesspools and other on-site sewage disposal systems (OSDS) near the coast. In this study, SGD was located and mapped using unmanned aerial vehicle thermal infrared (UAV-TIR) imaging and coastal salinity surveys. Radon mass balance modeling was used to calculate local-scale SGD, which was combined with in situ nutrients to estimate nutrient fluxes. Finally, numerical groundwater modeling was used to calculate and upscale to basin-scale SGD rates and regional-nutrient fluxes. SGD seeps were successfully identified with UAV-TIR imaging and salinity surveys. For the three beaches within the Waialua region, radon mass balance modeling calculated local-scale total SGD rates and nitrate fluxes that were generally less than those calculated by the numerical groundwater model. Worst-case and typical-case OSDS-loading scenarios were modeled. It was found that in the worst-case scenario OSDS delivered almost twice the amount of nitrate to the coast as agriculture, but in the typical-case loading scenario two of the beaches within the Waialua region received up to 92% of their nutrient load from agricultural contributions. The typical-case OSDS scenario was found to most accurately match observed nutrient fluxes. Locally, the nitrate concentration from agriculture was low when compared to the nitrate concentration from OSDS, but regionally due to its spatial distribution agriculture contributes up to 38% of the nitrate flux for the North-Central Groundwater Region. Both scenarios showed agriculture as a significant source of nutrients to the nearshore of the North-Central Groundwater Region. This study shows the benefit of using multiple SGD locating and quantifying methods for accurately understanding the contribution of nutrient loading within SGD for a coastal region affected by nearby anthropogenic sources of nutrients.

Keywords

Environmental geology, Geology, Groundwater, Modeling, Nutrients, Submarine Groundwater Discharge

URI

http://hdl.handle.net/10125/70334

Extent

65 pages

Rights

All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.

Collections

M.S. - Earth and Planetary Sciences

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