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Changes of freshwater-lens thickness in basaltic island aquifers overlain by thick coastal sediments

Hydrogeology Journal

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Abstract

Freshwater-lens thickness and long-term changes in freshwater volume in coastal aquifers are commonly assessed through repeated measurement of salinity profiles from monitor wells that penetrate into underlying salt water. In Hawaii, the thickest measured freshwater lens is currently 262 m in dike-free, volcanic-rock aquifers that are overlain by thick coastal sediments. The midpoint depth (depth where salinity is 50% salt water) between freshwater and salt water can serve as an indicator for freshwater thickness. Most measured midpoints have risen over the past 40 years, indicating a shrinking lens. The mean rate of rise of the midpoint from 1999–2009 varied locally, with faster rates in highly developed areas (1.0 m/year) and slower rates in less developed areas (0.5 m/year). The thinning of the freshwater lenses is the result of long-term groundwater withdrawal and reduced recharge. Freshwater/salt-water interface locations predicted from measured water levels and the Ghyben-Herzberg principle may be deeper than measured midpoints during some periods and shallower during other periods, although depths may differ up to 100 m in some cases. Moreover, changes in the midpoint are slower than changes in water level. Thus, water levels may not be a reliable indicator of the amount of freshwater in a coastal aquifer.

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Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Changes of freshwater-lens thickness in basaltic island aquifers overlain by thick coastal sediments
Series title:
Hydrogeology Journal
Volume
18
Issue:
6
Year Published:
2010
Language:
English
Publisher:
Springer
Publisher location:
Amsterdam, Netherlands
Contributing office(s):
Pacific Islands Water Science Center
Description:
12 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Hydrogeology Journal
First page:
1425
Last page:
1436
Time Range Start:
1999-01-01
Time Range End:
2009-12-31
Country:
United States
State:
Hawai'i