Evaluation of microcontaminants as tracers for groundwater nutrient sources within mixed-use watersheds along the South Kohala coast of the Island of Hawaii

McKenzie, Casey D.

Evaluation of microcontaminants as tracers for groundwater nutrient sources within mixed-use watersheds along the South Kohala coast of the Island of Hawaii

Files

McKenzie_hawii_0085O_11627.pdf (26.16 MB)

Date

2022

Authors

McKenzie, Casey D.

Advisor

Dulai, Henrietta

Department

Earth and Planetary Sciences

Abstract

Anthropogenic nutrient loading of coastal environments is a known contributor to marine ecosystem health decline in Hawai‘i, and submarine groundwater discharge (SGD) has been shown to serve as a pathway to fuel nutrient delivery to coastal zones. Anthropogenic sources that may supply nitrogen (N) to groundwater include agricultural practices, resorts and golf courses that utilize fertilizers in landscaping applications, an abundance of homes outfitted with cesspools, and input from wastewater infrastructure. SGD provides a means of conveying land-derived nutrients to the coastal ocean and hence facilitates N-transport to coastal zones. Analysis of SGD enabled the exploration of the spatial distribution of groundwater composition and its relation to land-use. Previous research identified the presence of SGD in the study area, and we targeted and sampled 45 coastal springs and 20 upland wells along a 14 km long coastline in South Kohala, Hawai‘i. A wide range in nutrient concentrations was observed in SGD and well samples, with N loads well above 100 μM (1000-fold above marine levels). Some, but not all, δ15N-NO3- signatures suggest that N observed in SGD is of wastewater origin. We utilized known microcontaminants as novel tracers for identifying which land-use categories arehydrologically and chemically connected to SGD and, therefore, may be nutrient sources to the coastline. Microcontaminants of interest included pharmaceuticals commonly present in cesspool waste, restricted use pesticides and herbicides utilized in landscaping practices, and common detergents used in commercial industry that utilize public sewage infrastructure. To determine groundwater flow paths, δ18O-H2O allowed calculation of estimated recharge elevations to estimate upstream end members of flow lines. Spatial analysis of land uses and practices (LUP) along flow lines was utilized to quantify LUP overlying groundwater flow. δ18O-H2O of groundwater also contributed evidence of where groundwater flow paths from Kohala, Mauna Kea and Mauna Loa volcanoes may potentially converge along the South Kohala coastline. When comparing flow path analysis to developed scoring methods, upstream contribution of microcontaminants to groundwater pathways confirmed that microcontaminants could serve as land-use tracers throughout the length of the flow path. Microcontaminant scoring geochemically connected up-stream land uses to coastal SGD seeps along hydrological flow paths. Spatial analysis of microcontaminant distribution in SGD and LUP scoring revealed that caffeine, carbamazepine, sulfamethoxazole has strong correlation with presence of OSDS, confirming their suitability as wastewater specific tracers.

Keywords

Environmental management, Environmental geology, Land use planning, groundwater tracer, mixed-use watershed, nitrate contamination, submarine groundwater discharge

URI

https://hdl.handle.net/10125/104682

Extent

104 pages

Rights

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Collections

M.S. - Earth and Planetary Sciences

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