Hydrology of three Loko Iʻa, Hawaiian fishponds, on windward Hawaiʻi Island, Hawaiʻi
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2018-08
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Groundwater is a primary source of nutrients for loko iʻa, Hawaiian fishponds, in Hawaiʻi. Freshwater inputs are a key component to these dynamic coastal ecosystems yet flow rates are variable. The focus of this study was to (1) understand the changes in groundwater flow through time, (2) determine differences in groundwater composition among shoreline and loko iʻa springs and (3) analyze how climate change may impact these groundwater dependent ecosystems. Three groundwater-fed loko iʻa kuapā were the focus of this study: Honokea, Hale o Lono and Waiāhole loko iʻa, in Keaukaha, HI. Through time series measurements and the application of mass balance equations, groundwater flow over a 12-month period was found to significantly vary at monthly time scales. Daily and 3-day sum rainfall amounts and groundwater flow were positively correlated. Sampling of three loko iʻa springs and 12 additional shoreline springs characterized major ion chemistry and nutrient concentrations. The highest NO2 + NO3 concentrations were at Waiāhole and the highest PO4 concentrations were at Honokea. Based on 18O, the mean rainfall recharge elevation that contributed to regional recharge of aquifers discharging into loko iʻa and shoreline springs ranged between 400 and 900 m, with the elevation of source water increasing at springs farther east. Predicted increases in rainfall between 20-40% could increase groundwater flow equivalent to that observed after >75 mm of rain over a 48 hr period. Future sea level rise, 0.4 m by the year 2040, could result in the daily high tide salinity increasing from 3-8 to >16. This study provides baseline information and predictions for managers restoring these unique environments to prepare for future changes in loko iʻa hydrology. Furthermore, the methods used here can be applied to larger groundwater dependent ecosystems throughout Hawaiʻi.
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Hydrologic sciences
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60 pages
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