Time Scale of Water-Rock Interaction Processes in the Fresh-Saline Water Interface

of Coastal Aquifers

1Department of Geological and Environmental Sciences, Ben Gurion University, Beer Sheva, Israel2Geological Survey of Israel, Jerusalem, Israel3Institute of Earth Sciences, Hebrew University, Jerusalem, Israel

Amos Russak1,2, Orit Sivan1, Yoseph Yechieli2 and Boaz Lazar3

http://water.usgs.gov/ogw/gwrp/saltwater/fig2.gif

Fresh-saline water interface (FSI)

1. Global sea level change.

2. Big sea storms.

3. Over exploitation of groundwater.

4. Seasonal changes in groundwater flow rate.

5. Tide. (?)

Changes in the FSI can happen due to:

Research Goals

• Identifying seasonal and diurnal controls on the chemical composition of water at the FSI.

• Quantifying the time scale and the chemical processes in the FSI from field observations and laboratory simulations.

(Netanya)

Research Area • Poleg Well located 70 meters from the shore.

• Poleg well depth is about 45 meters and perforated all along the length of the pipe.

Poleg

Poleg Well

Mediterranean

Sea

1) Seasonal electrical conductivity profiles.

2) Seasonal high resolution (15cm) chemical profiles by multi-layer-sampler (MLS). Equilibration with groundwater is less than 1 month.

3) Diurnal chemical profiles by 3mm diameter pipes and peristaltic pump. Sampling water was done from 6 depths every 2 hours for 28 hours.

Field Methods

MLS rodMLS Cell

Ion Exchange Simulations in the LaboratoryI. A column was filled with sediments

from the research area.

II. The column was saturated with local fresh groundwater.

III. Seawater was pumped through the column in three experiments using different flow velocities. Experiment are somewhat similar to those of Appelo, with addition of other parameters.

IV. The column was flushed with freshwater after each salinization experiment.

5 experiments

15

20

25

30

0 20 40 60Electrical Conductivity (mS·cm-1)

Dep

th (m

)

20

25

30

0 10 20 30 40 50 60

DecemnberJanuaryFebruaryMayjuneJulyseptember

The FSI zone moves vertically.

January

July

Field Results

0

20

40

60

80

100

120

140

0 100 200 300 400 500 600 700

Most of the results are on a simple mixing line between fresh water and saline water.

The magnesium shows a conservative behavior.

0

20

40

60

80

100

120

140

0 100 200 300 400 500 600 700

Mg2+

(meq

·L-1)

Cl- (meq·L-1)

01020304050

0 100

200

300

400

500

600

700

SeawaterFresh WaterMLS 1 (Russak)MLS 2 (Russak)MLS 3 (Russak)MLS 1 (Sivan)MLS 2 (Sivan)MLS 3 (Sivan)28 Hr

Fresh water in the

well

saline water in the

well

Seawater

Most of the results are above the mixing line between fresh water and saline water, indicating that cation exchange occur.

The cation exchange time scale is seasonal at most, and possibly also on diurnal scale.

0

10

20

30

40

50

0 100 200 300 400 500 600 7000

10

20

30

40

50

0 100 200 300 400 500 600 700

Ca2+

(meq

·L-1)

Cl- (meq·L-1)

01020304050

0 100

200

300

400

500

600

700

SeawaterFresh WaterMLS 1 (Russak)MLS 2 (Russak)MLS 3 (Russak)MLS 1 (Sivan)MLS 2 (Sivan)MLS 3 (Sivan)28 Hr

Fresh water in the well

saline water in the

well

Sea water

Experiments Results

• Magnesium show a conservative behavior.

• Calcium and strontium are enriched.

• Negative δ13CDIC imply on organic matter oxidation.

• Potssium seem to be delay.

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3

-12.0

-6.0

0.0

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3

-12.0

-6.0

0.0

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3

-12.0

-6.0

0.0

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3-12.0

-6.0

0.0

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2 2.5 3-12

-10

-8

-6

-4

-2

0

Con

cent

ratio

n

(rel

ativ

e to

max

imum

)

Pore Volume

Seawater end

member

Fresh water end member

δ13C

DIC

Cl

Mg

CaK

Na

SO4

δ13CDIC

Sr

Values of δ13CDIC in seawater and

saline groundwater

Experiment Results in different

velocities

Pore Volume

Con

cent

ratio

n (r

elat

ive

to m

axim

um)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.5 1.0 1.5 2.0 2.5

Na K Ca Mg

Sr Cl SO4

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2

~50 m·y-1

~200 m·y-1

~1000 m·y-1

Slope of -1 means that the enrichment of calcium is equal to the depletion in potassium and sodium.

The cation exchange calculated as the difference between the observed cation concentration and the expected concentration (if conservative).

Cation exchange calculation

-40

-35

-30

-25

-20

-15

-10

-5

0

5

-10 0 10 20 30 40

y = -1.02x + 0.14R2 = 0.83

-40

-35

-30

-25

-20

-15

-10

-5

0

5

-10 0 10 20 30 40

[N

a obs

-Na e

xp[ +

]K

obs-K

exp

])m

eq·L

-1(

]Caobs – Caexp[ (meq·L-1)

• Most of the field results (including the diurnal sampling) show seawater intrusion pattern.

Comparison Between Field and Experiments Results

• There is a good correlation between the field results and the laboratory experiments.

0

10

20

30

40

50

60

0 100 200 300 400 500 600 7000

10

20

30

40

50

60

0 100 200 300 400 500 600 7000

10

20

30

40

50

60

0 100 200 300 400 500 600 7000

10

20

30

40

50

60

0 100 200 300 400 500 600 700Cl- (meq·L-1)

Ca2+

(meq

·L-1)

Salinization exp.

Flushing exp.

0204060

0100200300

400

500600700

Slainization 1Salinization 2Salinization 3Freshening 1Freshening 2

0102030405060

0100200300400500600700

Slainization 1Salinization 2Salinization 3Freshening 1Freshening 2SeawaterFresh WaterMLS 1 (Russak)MLS 2 (Russak)MLS 3 (Russak)MLS 1 (Sivan)MLS 2 (Sivan)MLS 3 (Sivan)28 Hr

Black – exp.

Color - field

Conclusions1. Salinization and flushing experiments simulate

very well the field observations in the FSI.

2. Flushing is expressed as a simple mixing lines between all ions.

3. Exchange of Ca with (Na + K) is a major process occurring in the FSI during salinization.

4. Cation exchange is a very fast process occurring even at very high flow rates (1,000 m·y-1 in laboratory experiments).