8.7 freshwater/saltwater interaction in coastal aquifers
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8.7 Freshwater/Saltwater Interaction
in coastal aquifers
Coastal aquifers
Density difference,
zone of mixing (diffusion)
Interface
Saltwater front
Island in an ocean
Freshwater lens
Moving interface
Saltwater intrusion
Pumping of groundwater diverts groundwater from flowing toward sea seawater movement toward aquifer
Causes deterioration of water quality in coastal aquifers
Locating the Interface
Ghyben-Herzberg relation
Ghyben-Herzberg Relation
Weight of column of freshwater extending from water table to interface
weight of column of saltwater extending from sea level to the same depth
Hydrostatic conditions:
=
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
s f fgz g h z ( )
z hf
s ff
z h f40
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
z h f40
Position of the interface
xKz
Qs f
f
1
2
2( )
'
LKb
Qs f
f
1
2
2( )
'
x
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:1. Length of saltwater wedge2. Position of interface
Example 8.4, contd.
L
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
xKz
Qs f
f
1
2
2( )
'
LKb
Qs f
f
1
2
2( )
'
Methods for limiting seawater intrusion
1. Controls on pumping: Reducing pumping rates Reducing number of pumped wells
2. Artificial recharge through ponds Using imported water or reclaimed wastewater
Methods for limiting seawater intrusion
3. Pumping trough barriers Using a series of pumped wells to remove
saltwater and form a potentiometric barrier
4. Freshwater injection To establish a seaward hydraulic gradient
Case study: saltwater intrusion control in Biscayne Aquifer, Florida
Upconing of Interface
zQ
Kdf
s f
'
( )
2
Q d K s f
fmax .
( )
0 6 2
Upconing = rise of interface as a result of groundwater pumping
Rise of interface (upconing) = z =
Maximum permitted pumping rate =Qmax
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? What’s 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
HW
8.4
8.5
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