Freshwater-Saltwater Mixing Zone Biased vs. Reliable Monitoring in Coastal Aquifers: Biased vs. Reliable Monitoring Eyal Shalev Yossi Yechieli Haim Gvirtzman – Hebrew University, Jerusalem } – Geological Survey of Israel

Outline Long – screened boreholes Groundwater flow in coastal aquifers Field data Numerical simulations Conclusions

Hydraulic Head Interface Interface

Elci et al., 2003

Molz et al., 1994

Britt, 2005

Konokow and Hornberger, 2006

Konokow and Hornberger, 2006

T/143-4 T/143-3 41.56 81.56 31.57 34.44 73.13 116.00

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 identification number Altitude of land surface datum (ft) Top of screened or open interval (ft) Bottom of screened or open interval (ft) Aquifer 18K049 330.00 270 620 Upper Floridan 19E009 217 200 342 21T001 259 89 123 32L015 74 545 750 32L016 320 340 Upper Brunswick 32L017 215 Surficial 32R003 120 134 155 Miocene 35P094 18.7 15 36Q008 9.91 250 406 350 ft = 106.7 m 142 ft = 43.3 m 205 ft = 62.5 m Priest, S., 2004, USGS Scientific Investigations Report 2004-5265

anisotropy 1:30

anisotropic anisotropic

anisotropy 1:30

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.