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The Source and Magnitude of Submarine Groundwater Discharge Along the Kona Coast of the Big Island, Hawaii.

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Title:The Source and Magnitude of Submarine Groundwater Discharge Along the Kona Coast of the Big Island, Hawaii.
Authors:Hudson, Catherine Y.
Contributors:Geology & Geophysics (department)
Date Issued:Aug 2018
Publisher:University of Hawaiʻi at Mānoa
Abstract:Submarine groundwater discharge (SGD) is present along the coastline where freshwater flows
from the land, combines with brackish water circulation, and enters the sea. The purpose of
this study is to quantify SGD along the shoreline and to identify the origin of the groundwater in
SGD in the Hualalai aquifers on the Kona Coast of the Big Island, Hawaii. We collected SGD
samples at coastal springs spanning along 82 km of shoreline of the combined Kiholo and
Keauhou aquifers, and then analyzed samples to determine their oxygen isotope values. The
stable oxygen isotopes of water trend to more negative values with increasing duration and
altitude of precipitation and were therefore used to predict recharge elevation. After
determining the oxygen isotopic composition of samples, we calculated the expected oxygen
isotopic composition of precipitation in the form of snow, rain, or fog drip. As an
approximation, it is assumed that such values are applicable to infiltrated water that percolates
through permeable rock to enter the subsurface aquifer. By performing this calculation at
increasing distances upslope along assumed flow paths, the elevation of SGD recharge was
determined based on the integrated recharge isotopic signature. After identifying this
elevation, we then projected possible groundwater areas of recharge to the points of discharge
at the coastline. While the exact flow paths cannot be determined using this method, we
identified five separate possible water recharge regions within the two aquifers, some which
span outside of the aquifer boundaries. In the north Kiholo Aquifer, the water mass balance of
SGD discharge-recharge volume within north Kona suggest that only about 37% of water
originates from recharge within the Kiholo Aquifer boundary. Findings also suggest that, despite
geological barriers, SGD signatures are very similar across the Kiholo-Keauhou boundary,
implying similar recharge areas and flow paths. In Keauhou aquifer, recharge to the basal lens
makes up only 9-39% of SGD. The rest of the water is sourced from the high-level aquifer. In the
case of south Keauhou Aquifer, SGD signatures suggest significant recharge contributions from
elevations beyond the aquifer boundary. This study concludes that there are complex recharge
and flow patterns in the Hualalai aquifers, suggesting recharge contributions from neighboring
upstream aquifers and the occurrence of lateral flow to adjacent neighboring aquifers. This
study was not able to quantify the exact recharge-discharge water balance due to missing SGD
values in the south Keauhou aquifer. It was also not able to directly quantify, only imply, that
some recharge is channeled to deeper aquifer layers, perhaps discharging farther offshore.
Nevertheless, the study confirmed past findings and provided new insights into the
interconnectivity of the aquifers in the Hualalai region.
Description:M.S. Thesis. University of Hawaiʻi at Mānoa 2018.
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.
Appears in Collections: M.S. - Geology and Geophysics

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