1. 8.7 Freshwater/Saltwater Interaction
in coastal aquifers

2. Coastal aquifers Density difference, zone of mixing (diffusion)
Interface
Saltwater front

3. Island in an ocean
Freshwater lens
Moving interface

4. Saltwater intrusion
Pumping of groundwater  diverts groundwater from flowing toward sea  seawater movement toward aquifer
Causes deterioration of water quality in coastal aquifers

5. Locating the Interface
Ghyben-Herzberg relation

6. 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 =

7. 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

8. 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

9. Position of the interfacex

10. Example 8.4
f = 1.0 g/cm3, s = 1.025
Water levels in two wells far from shoreline :
0.5 m , m
Distance between two wells = 1000 m
K = 10 m/d, b = 50 m
Calculate:
Length of saltwater wedge
Position of interface

11. Example 8.4, contd. L

12. 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

13. 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

14. 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

15. Case study: saltwater intrusion control in Biscayne Aquifer, Florida

16. Upconing of Interface
Upconing = rise of interface as a result of groundwater pumping
Rise of interface (upconing) = z =
Maximum permitted pumping rate =Qmax

17. (2x3.1416)(100m)(10 m/d)(1.025 g/cm3 – 1.0 g/cm3)
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)
= m
Q max = 4.7x103 m3/day

18. HW
8.4
8.5