8.7 Freshwater/Saltwater Interaction

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8.7 Freshwater/Saltwater Interaction

• in coastal aquifers

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Coastal aquifers

• Density difference,
• zone of mixing (diffusion)
• Interface
• Saltwater front

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Island in an ocean

• Freshwater lens
• Moving interface

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Saltwater intrusion

• Pumping of groundwater ? diverts groundwater from
  flowing toward sea ? seawater movement toward
  aquifer
• Causes deterioration of water quality in coastal
  aquifers

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Locating the Interface

• Ghyben-Herzberg relation

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Ghyben-Herzberg Relation

• Hydrostatic conditions

• Weight of column of freshwater extending from
  water table to interface

weight of column of saltwater extending from sea
level to the same depth

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Ghyben-Herzberg Relation

• ?f freshwater density 1.0g/cm3
• ?s saltwater density 1.025g/cm3
• z height of saltwater column
• hf hydraulic head above sea level
• hf z height of freshwater column

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Ghyben-Herzberg Relation

• The depth to the interface between freshwater and
  saltwater is approximately 40 times the height of
  the water table above sea level

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Position of the interface
x
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Example 8.4

• ?f 1.0 g/cm3, ?s 1.025
• Water levels in two wells far from shoreline
• 0.5 m , 1.0 m
• Distance between two wells 1000 m
• K 10 m/d, b 50 m
• Calculate
• Length of saltwater wedge
• Position of interface

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Example 8.4, contd.
L
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Example 8.4, solution

• Discharge from aquifer to the sea per unit length
  of shoreline
• Q Kb dh/dx
• (10 m/d)(50 m)(1.0 m -0.5 m)/1000 m
• 0.25 m3/d/m
• Interface 0.5 z2
• Length of protrusion 1250 m

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Methods for limiting seawater intrusion

• Controls on pumping
• Reducing pumping rates
• Reducing number of pumped wells
• Artificial recharge through ponds
• Using imported water or reclaimed wastewater

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Methods for limiting seawater intrusion

• Pumping trough barriers
• Using a series of pumped wells to remove
  saltwater and form a potentiometric barrier
• Freshwater injection
• To establish a seaward hydraulic gradient

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Case study saltwater intrusion control in
Biscayne Aquifer, Florida
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Upconing of Interface

• Upconing rise of interface as a result of
  groundwater pumping
• Rise of interface (upconing) z
• Maximum permitted pumping rate Qmax

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Example 8.5

• The distance from the base of a pumping well to
  the freshwater-saltwater interface is 100 m, the
  pumping rate is 3000 m3/day, and the hydraulic
  conductivity is 10 m/d.
• What will be the position of the interface?
• Whats the maximum permitted pumping rate?
• --------------------SOLUTION------------------
• Rise z (3000 m3/d)(1.0 g/cm3)/
• (2x3.1416)(100m)(10 m/d)(1.025 g/cm3 1.0 g/cm3)
• 13.26 m
• Q max 4.7x103 m3/day

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HW

• 8.4
• 8.5