L6 Salt Water Intrusion

8/12/2019 L6 Salt Water Intrusion

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Seawater Intrusion &Submarine Groundwater Discharge

• Today

– SubmarineGroundwater Discharge

– Seawater Intrusion

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Historic observations

3,000 years ago, off Ruad, Syria, the Phoenicians had built a submarine GWcollection system that supplied fresh water to the City of Amrit.

Now proposed off Jeddah: Time to Tap Submarine Fresh Water Springs by EssamAl-Ghalib Arab News July 11, 2004

Project SubGATE

CH 4

cit.: L. Sonrel (1868) Le Fond de la Mer

Courtesy of P. Swarzenski and W. Burnett, USGS



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Pliny, the Elder:… from “Natural History,” written prior to 79 A.D.

“... Black Sea : ~37 miles fromland… springs of fresh waterbubbling out as if from pipes onthe seashore...”

“In fact fresh water may bedrawn from the sea in a great

many places, as at the SwallowIslands and at Aradus [ Syria ]and in the Gulf of Cadiz[Spain].”

More observations …

NYMPHEA WATER


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Submarine Groundwater Discharge

Submarine Groundwater Discharge (SGD) =any flow out across the seabed of the continental shelf,regardless of composition or driving force.So, SGD is not defined solely on basis of components, origin, or driving forces.


SGD is a ubiquitous, often distributed discharge source along the coastline.

In some areas, such as karst and fractured areas , it becomes a focuseddischarge, often with high discharge rates.

Great Bay Estuary, NH

ciceet.unh.eduLake Superior

Thermal images

http://www.fws.gov/midwest/greatlakes/Groundwater.htm



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SGD components

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Example: Biscayne Aquifer

(SZ,

2003)

Highly permeable limestone and less-permeable sandstone and sand.

http://capp.water.usgs.gov/gwa/ch_g/G-text4.html



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SGD scales – karst systems27 miles off Jacksonville,FLJOIDES test hole

Fresh water head ~5 mabove sea level

F. Manheim

Reduced GW discharged intooxic Tampa Bay, FL water:

Role for microbially- mediatedredox processes in SGD

(Swarzenski et al., 2004)


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Processes driving SGD

• Terrestrial – hydraulic gradient

• Marine – tidal pumping, wave set-up, current-induced topographic flow, convection (e.g., saltfingering), sea-level differences (e.g., acrossbarrier islands)



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Example: water level differencesacross a barrier island

Difference in water levels sets up hydraulicgradient that results in subsurface flow

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Global Freshwater SGD estimates

Milliman (pers. comm .)1,000-3,000

Zekster (2000)2,400

Berner & Berner ’872,200

COSOD II (1987)100

ReferenceDischarge*(km3/y)

*estimates for fresh water; river discharge = 35,000-40,000 km 3 /y (< 16%)

SGD variable in time & space !!

SGD high in areas with: high rainfall, high relief,fractured rock, poorly developed river systems, karst




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SourceInputs

10 13 moles/y

Reference

Rivers 1.2-1.5 Meybeck (1979)Berner and Berner (1987)Morse & Mackenzie (1990)Milliman (1993)

Hydrothermalactivity

0.2-0.3 Wolery and Sleep (1988)

Aeolian inputs 0.005 Milliman (1993)Groundwater 0.5 COSOD II (1987)Groundwater 0.5-1.6 Milliman (1993)

Contribution toGlobal Ocean Ca 2

+ Budget

>30% river input!(Burnett, 2004)


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What happens when people interfere with the

freshwater discharge to the sea?

Let’s focus on the phreatic aquifer

Then, we replace the mixing zone between

freshwater and saltwater, where transversedispersion is very important, with the

assumption of an immiscible interface.

(Fitts, 2002)

Underlying the freshwater discharge isa so-called “salt water wedge.”



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Simple sharp-interface conceptual model

Q’ = freshwater discharge to sea [L 2/T] z + h f = thickness of freshwater

h f = water table elevation (above MSL) b + h f = thickness of freshwater b = depth to bottom of aqufier above wedge “toe”

z = depth to interface (below MSL) L = length of “sea water intrusion”

for a phreatic aquifer

z z

h f

bQ’

L

toe wedge saltwater

freshwater sea

MSL

x

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f f f s

f f f s

f f s

h z z

g h g z g z

h z z

! ! !

! ! !

" "

+=

+=

+=

f

f s

f

h z ! !

!

"=

026.1,0.1 22 cm g

cm g

s f == ! !


Assume hydrostatics in saltwater and essentially horizontalflow in freshwater. On the sharp interface p s = p f

Assume steady flow and sharp interface.

( ) f f f

s s

h z p

z p

+=

=

!

!

Ghyben-Herzberg Equation

f f f hhh z 40380.1026.1

0.1!=

"

=

equate

Solve for depth to interface:

Typically:

If you observe and map h f you can infer z . If you also

map the bottom of theaquifer you can infer thelocation of the toe.



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z z

h f

bQ’

L

toe wedge saltwater

freshwater sea

MSL

x

dx

dh K

dx

dh Kh

dx

dh Kh

dx

dhh z K Q

f

f s

s

f

f s

s f

f

f s

f f

f f

2

)(2

)(

)1(

)('

! !

!

! !

!

! !

!

""

=

""=

+"

"=

+"=

22

2

2

2

,0

2

2

,0

2

2

,0

22

2

'2'2

iprelationshGHfrom

0,atsince'2

'2

'2

bQ

K b

Q K

L

bh

h L x K

LQh

hhconst h K

xQ

dhdx K Q

f

f s

f

s

f

f s

f s

s

f

f s

toe x f

f s

f s

toe x f

f toe x f f s

f s

f s

f s

!

! !

!

!

!

! !

! !

!

!

! !

!

! !

!

! !

!

! !

"=

##$

%&&'

( ""

=

##$

%&&'

( "=

=="

=

"=+"="

"="

=

=

=


f

f s

f h z ! !

!

"= bh

f

f s

toe x f !

! ! "=

= ,0


Ghyben-Herzberg Equation:

For our simple model, to find intrusion length, L, solvethe flow problem:

At the toe: x= 0, z=b.

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for a confined aquifer of thickness b, & top at depth d .


Q’ = freshwater discharge to sea [L 2/T] z - d = thickness of freshwater h f = freshwater head (above MSL) b = aquifer thickness = thickness of d = depth to top of the aquifer freshwater above wedge “toe”

z = depth to interface (below MSL) L = length of “sea water intrusion”

z z

h f

bQ’

L

toe wedge saltwater

freshwater sea

MSL

x

d

Potentiometric surface

Confining Bed





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Simple sharp-interface conceptual models

2'

Aquifer Phreatic2 Kb

LQ f

f s

f

s

!

! !

!

! "=

2'

:Aquifer Confined2 Kb

LQ f

f s

!

! ! "=

Notes:

Intrusion increases with- higher K

- lower Q’ - greater density difference

- greater aquifier thickness

Some Implications:

Intrusion increases - during drought - with reduced recharge

caused by water diversion - because of pumping

Intrusion decreases with - artificial recharge

Coastal Aquifer

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Island or Pennesula

(Schwartz and Zhang,2003)

Net Recharge





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23http://capp.water.usgs.gov/gwa/ch_g/G-text4.html

Control of Seawater Intrusionin the Biscayne Aquifer, Florida

Documents

L6 Salt Water Intrusion