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

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2018-08
Authors
Hudson, Catherine Y.
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Geology & Geophysics
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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.
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