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Field Assessment and Groundwater Modeling of Pesticide Distribution in the Faga'alu Watershed on Tutuila, American Samoa

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Title:Field Assessment and Groundwater Modeling of Pesticide Distribution in the Faga'alu Watershed on Tutuila, American Samoa
Authors:Welch, Eric
Contributors:Dulai, Henrietta (advisor)
El-Kadi, Aly I. (advisor)
Oceanography (department)
Global Environmental Science (department)
Date Issued:2018
Publisher:University of Hawaiʻi at Mānoa
Place of Publication:Honolulu
Abstract:Pesticide use is prevalent worldwide and their dispersal across water resources has been gaining
increased attention. Pesticides in natural waters can be toxic not only to terrestrial and aquatic biota,
but also to humans. Pesticide studies usually focus on surface waters since surface runoff and
sediments are considered their primary migratory pathways. Groundwater pathways, however, have
not been as thoroughly investigated since most pesticides are not commonly considered mobile and
persistent enough for subsurface dispersal. Pesticide transport through aquifers may take several
decades, eventually reaching drinking water sources or the marine environment.
Groundwater and surface water interactions were investigated as a vector of pesticide migration on
the island of Tutuila in American Samoa in August 2016. For the island-wide study that looked at
pesticide spatial variation, samples were collected from groundwater wells and springs as well as
streams, for selected pesticides (glyphosate, imidacloprid, azoxystrobin, DDT/DDE). A more detailed
pathway-specific study was conducted in the Faga`alu watershed, where field data were integrated
into a groundwater model to reconstruct flow paths and pesticide discharge from the groundwater
aquifer to the stream and the coastline. In combination with land-use maps, groundwater models
were used to identify potential pesticide sources.
All measured pesticide concentrations on Tutuila were well below EPA regulated limits. They were
found at the same frequency and in the same concentration ranges in surface and groundwater
samples. DDT was uniformly distributed, but there was a variation in glyphosate and imidacloprid
spatial distribution that followed population density. In Faga`alu, groundwater pathways were
responsible for a majority (~70%) of glyphosate and DDT discharge (350 & 482 mg/d and 2990 & 5519
mg/d) to the stream and coastline, respectively, in comparison to surface runoff according to my
model and field measurements during dry season. The timescale of the transport spans from many
decades (DDT applications were banned in the 1970s) to most recent applications of glyphosate that
were traced back to populated regions, where this herbicide is probably applied in backyard-scale
agriculture rather than large-scale farming operations. This study shows the significance of
groundwater flow for pesticide transport and discharge into the coastal ocean.
Pages/Duration:110 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.
Rights Holder:Welch, Eric
Appears in Collections: Global Environmental Science (GES)

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