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Most published solutions for aquifer responses to ocean tides focus on the one-sided attenuation of the signal as it propagates inland. However, island aquifers experience periodic forcing from the entire coast, which can lead to integrated effects of different tidal signals, especially on narrow high-permeability islands. In general, studies disregard a potential time lag as the tidal wave sweeps around the island. We present a one-dimensional analytical solution to the ground water flow equation subject to asynchronous and asymmetric oscillating head conditions on opposite boundaries and test it on data from an unconfined volcanic aquifer in Maui. The solution considers sediment-damping effects at the coastline. The response of Maui Aquifers indicate that water table elevations near the center of the aquifer are influenced by a combination of tides from opposite coasts. A better match between the observed ground water head and the theoretical response can be obtained with the proposed dual-tide solution than with single-sided solutions. Hydraulic diffusivity was estimated to be 2.3 x 10(7) m(2)/d. This translates into a hydraulic conductivity of 500 m/d, assuming a specific yield of 0.04 and an aquifer thickness of 1.8 km. A numerical experiment confirmed the hydraulic diffusivity value and showed that the y-intercepts of the modal attenuation and phase differences estimated by regression can approximate damping factors caused by low-permeability units at the boundary.

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FRONT COVERSchematic cross section of deep monitor wells penetrating through the freshwater lens into the underlying saltwater in a volcanic-rock aquifer, showing how upward and downward borehole flow affects the salinity distribution with depth in the wells. This report and any updates to it are available online at: http://pubs.usgs.gov/sir/2010/5058/ For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1-888-ASK-USGS For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod Effects of Groundwater Withdrawal on Borehole Flow and Salinity Measured in Deep MonitorTo order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.Suggested citation: Rotzoll, Kolja, 2010, Effects of groundwater withdrawal on borehole flow and salinity measured in deep monitor wells in Hawai'i-implications for groundwater management: U.S. Geological Survey Scientific Investigations Report 2010-5058, 42 p.Vertical salinity profiles measured in deep monitor wells commonly are used to estimate the volume of freshwater in aquifers in Hawaiÿi. Water-resource managers use information from salinity profiles to monitor changes in the freshwater volume over time and determine appropriate groundwater withdrawal rates. Natural or anthropogenic factors can cause water in deep monitor wells to flow vertically within the borehole, and this may cause the observed salinity distribution in the monitor well to be a poor indicator of the salinity distribution in the aquifer, resulting in an underestimation or overestimation of the volume of freshwater stored in an aquifer. Thus, a better understanding of how borehole flow affects the measured vertical distribution of salinity in monitor wells is needed for proper analysis and interpretation of vertical salinity profiles and sound management of the groundwater resources. Indicators of Borehole FlowSalinity profiles from paired shallow and deep monitor wells may differ over the common depth interval of the wells. Observed differences can be explained by borehole flow moving brackish water upward in the deep well, an interpretation supported by flow measurements. Additionally, steplike changes in salinity or temperature with depth may be indicative of borehole flow over a discrete depth interval, and such changes are evident to some extent in all available profiles. Vertical flow may affect hundreds of feet of a salinity profile, and this occurs commonly in monitor wells located in southern Oÿahu. Steplike changes in salinity that do not affect long sections (less than 70 ft) of a profile may be r...

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For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.