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