Freshwater - Saltwater
Interactions
outflow face
Freshwater - Saltwater Interface
• Coastal plain and coastal surficial aquifers
• Fresh water mixes poorly with salt water due
to density difference.
• Salt water wedges beneath fresh water.
• Interface = narrow zone of diffusion /
dispersion, some mixing but a barrier to flow.
• Freshwater discharges to the surface along the
interface, emerging along an outflow face.
Ghyben-Herzberg principle
water table
h
Depth to interface is 40X
water table elevation.
z
salty
face
fresh
inter
z = h (ρw / ρs - ρw)
z = 40 h
ocean
Outflow Face
• Freshwater is not static at interface.
• Freshwater is flowing upward along the
interface.
• Creates a zone of freshwater discharge at the
shoreline = outflow face.
Ghyben-Herzberg principle
water table
h
salty
fresh
face
z
Ideal
inter
z = h (ρw / ρs - ρw)
z = 40 h
ocean
Outflow Face
water table
fresh
Low Tide
freshwater
outflow face
Cape Ann, Mass.
inter
Due to groundwater
movement.
face
ocean
salty
Actual
outflow face
outflow face
Paulson et al., 1998
Effect of tide on outflow seepage
Salt water intrusion
• Movement of the fresh-salt interface landward.
• Passive encroachment
– reduction in freshwater discharge
– lowering of water table
– interface moves slowly inland
• Active encroachment
– reversal of hydraulic gradient
– interface moves rapidly inland
– usually caused by pumping
Passive encroachment
Active encroachment
Chloride ion concentration in water from a
public supply well in Great Neck, Long Island.
Stumm, 2001
Chloride ion concentration in water from a public
supply well in Great Neck, Long Island (well was
shut off due to salt contamination).
Stumm, 2001
Saltwater wedge in North Shore Aquifer, Great Neck Long Island
fresh
salt
Stumm, 2001
Saltwater upconing