Freshwater-Saltwater Mixing Zone in Coastal Aquifers: Biased vs. Reliable Monitoring Eyal Shalev Yossi Yechieli } – Geological Survey of Israel Haim Gvirtzman – Hebrew University, Jerusalem
Outline • Long – screened boreholes • Groundwater flow in coastal aquifers • Field data • Numerical simulations • Conclusions
Hydraulic Head face ter In Interface
Elci et al. , 2003
Molz et al. , 1994
Britt, 2005
Konokow and Hornberger, 2006
Konokow and Hornberger, 2006
T/143 -3 T/143 -4 41. 56 44. 43 75. 13 73. 13 81. 56 116. 00
South Korea Kim et al. , 2005 J. Hydrology
Kim et al. , 2005 J. Hydrology
Kim et al. , 2005 J. Hydrology
Jeju Island Kim et al. , 2006 J. Hydrology
Well Top of Bottom of Altitude of identification screened or land surface number open datum (ft) interval (ft) Aquifer 18 K 049 330. 00 270 620 19 E 009 217 200 342 Upper Floridan 350 ft = 106. 7 m Upper Floridan 142 ft = 43. 3 m 21 T 001 259 89 123 Upper Floridan 32 L 015 74 545 750 Upper Floridan 205 ft = 62. 5 m 74 320 340 Upper Brunswick 32 L 017 74 200 215 Surficial 32 R 003 120 134 155 Miocene 35 P 094 18. 7 15 15 Surficial 36 Q 008 9. 91 250 406 Upper Floridan 32 L 016 Priest, S. , 2004, USGS Scientific Investigations Report 2004 -5265
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Conclusions • Observations monitored in long-screen boreholes do not accurately represent natural phenomena • Boreholes may act as a “short circuit” along a vertical gradient, connecting between the higher and lower hydraulic head zones • This is shown to occur in coastal aquifers both in steady state and transient (tide) conditions • In steady state conditions, the borehole interference is higher for the anisotropic case
Conclusions (continued) • When tides are introduced, the high amplitude of the freshwater-saltwater mixing zone occurs only in the borehole. • In the aquifer, the freshwater-saltwater mixing zone fluctuations are dampened, and tens of meters inland from the shoreline the fluctuations are in the order of few centimeters. • Groundwater flow directions are affected by tides to a large distance from the shoreline.
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