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ʻĪao Aquifer and Long-Term Availability of Groundwater
ʻĪao Aquifer and Long-Term Availability of Groundwater
The U.S. Department of the Interior and U.S.G.S. prepared two reports discussing the history and future of ʻĪao aquifer regarding it's long-term availibility of groundwater. ʻĪao Aquifer serves as the primary domestic water supply source for the island of Maui.
In 1948, groundwater pumping was introduced and used for agriculture, and later used for domestic purposes in 1955. Due to the introduction of pumping and a decreased recharge rate in 1980, there have been a number of issues facing the aquifer including:
decline in groundwater levels
increased chlorine concentration in pumped wells
thinning of freshwater lens
rise in transition zone between saltwater and freshwater
"Plantation agriculture has been the dominant land use overlying the coastal part of the Iao aquifer since the formation of the Wailuku Sugar Company, which was first organized in 1862" (Meyer Presley, 1). Groundwater levels have also decreased due to the increase of withdrawals to support and increasing population. Between 1940 and 1998, the report by William Meyer and Todd K. Presley states that ʻĪao stream and surrounding areas have seen a water level decline of 18 feet.
Recharge Groundwater recharge related to direct rainfall infiltration occurs over the entire study area. In addition, groundwater recharge may occur in stream channels where local groundwater levels near the stream channel are lower than the stream altitude" (Rotzoll, Oki, Johnson, Souza, 10).
Groundwater Flow "Precipitation is the main source of freshwater on Maui. The precipitation either (1) runs off as surface water to streams, (2) evaporates or is transpired by vegetation, (3) recharges the groundwater system, or (4) is stored in the soil" (Rotzoll, Oki, Johnson, Souza, 9).
Discharge "in the study area occurs in the forms of withdrawals from wells and tunnels, discharge to "streams, and discharge to subaerial and submarine coastal springs and seeps (Rotzoll, Oki, Johnson, Souza, 10).
Freshwater-Lens "exists as a continuous system in the mainly dike-free volcanic rocks and sedimentary deposits that form a caprock over the volcanic rocks. The freshwater lens in the study area forms because of the density difference between freshwater and underlying saltwater" (Rotzoll, Oki, Johnson, Souza, 11).
The purpose of these two reports, Long-Term Groundwater Availability in the Waihe‘e, ‘Īao, and Waikapū Aquifer Systems, Maui, Hawai‘i By Kolja Rotzoll, Delwyn S. Oki, Adam G. Johnson, and William R. Souza and The Response of the Iao Aquifer to Ground-Water Development, Rainfall, and Land-Use Practices Between 1940 and 1998, Island of Maui, Hawaii By William Meyer and Todd K. Presley, is to survey the effects that withdrawals continue to have on the aquifer and water levels, while also analyzing itʻs long-term sustainability.
Rotzoll, Oki, Johnson, and Souza conclude that withdrawals continuing at a rate from years 2017-2018 would "ultimately lead to lower water levels and a higher 50-percent ocean-water salinity surface and produce some groundwater with chloride concentrations above 250 mg/L" (45).
In the report by Meyer and Presley, they state that the Hawaiʻi State Commission on Water Resource Management's estimate of 20.0 Mgal/d as a sustainable yield for the ʻĪao aquifer would be susceptible to saltwater intrusion in the wells. They conclude that to minimize saltwater intrusion and decreasing water-levels, the solution could be to distribute pumpage over the given area.
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