What Is Salt Water Intrusion?
Fresh water from rainfall and surface water percolates into the earth and accumulates in permeable rock strata (limestone, sandstone) or deposits of sand, gravel and clay. At the coast, these formations encounter salt water from the sea. When fresh water flow is blocked or water is pumped out of the aquifer more rapidly than the aquifer is recharged by rainfall, then salt water enters the aquifer.
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Biscayne Aquifer
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In Everglades National Park, Florida, salt water intrusion threatens the Biscayne aquifer, a limestone and calcium-rich sand formation that supplies fresh water for Miami-Dade County. Saline (salt) water conducts more electricity than fresh water, so detailed images of this county were made using electromagnetic sensors. Airborne sensors suspended below helicopters collected data at 10 to 15 meter intervals along flight paths 400 meters apart. The study found salt water where roadbeds blocked fresh water flow. Seawater also migrated inland along soil banks dredged from canals.
Port Washington Aquifer
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Great Neck, New York, is a densely populated peninsula of Long Island. More than 4 million gallons of fresh water were being pumped from the Port Washington aquifer each day to meet Great Neck's needs. After 1960, some wells were abandoned as a 20-foot deep wedge of salt water seeped along bedrock into the aquifer. A survey of Time Domain Electromagnetic (TDE) soundings at 14 locations plus a subsequent drilling program measured the intrusion. Computer graphic models were used to locate new, less vulnerable well sites, and pumping was reduced to sustainable levels.
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NAGROM
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Netherlands is located on river deltas, with much of its territory below sea level, reclaimed from the sea during the past 2,000 years. In the 1990s, the Netherlands developed National Groundwater Model (NAGROM) to monitor three-dimensional flow among multiple aquifers at 3,000 points. In particular, the Netherlands wants to predict how water will flow among fresh water and different densities of salt water as sea level changes. NAGROM predictions of vertical movement of salt water are especially important where clay forms the confinement layer between fresh water and deeper saline water.
Los Angeles Basin
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Local wells provide 30 percent of the water for the 10 million people living in Los Angeles. Starting in the 1920s, well pumping reversed the flow of water in the aquifer, so that salt water intruded. In the 1950s, injection wells were constructed to pump fresh water into the aquifer, creating a "barrier" to salt water intrusion. Salt water intrudes where aquifers are exposed on the seafloor, along buried ancient streams, and through crushed rock at fault lines.
Israel
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The sandstone Quaternary aquifer that underlies coastal Israel and the Gaza Strip is being "mined." Pumping exceeds the recharge from surface water entering the aquifer in the eastern Israeli hills. The Mediterranean Sea intrudes along a salt water front. Drip irrigation uses saline water for salt-tolerant crops. Water is desalinated for domestic use. Concrete or grout is injected into the aquifer at a line of boreholes to create underground dams to confine fresh water and keep salt water out.
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References
- Everglades National Park, Florida Measured by Airborne Electromagnetic Surveys, David V. Fitterman and Maryla Deszcz-Pan, 2001
- Computer Modeling and Surface Geophysics Unravel the Mystery of Salt Water Intrusion on Long Island, Mark Maimone, 2001
- Accounting for 3-dimensional density differences in the analytic element groundwater flow model of the coastal zone of the Netherlands, Willem J. de Lange, 2001.
