Groundwater Geochemistry(Hydrogeochemistry)

Wan Zuhairi Wan Yaacob (PhD, Assoc. Prof)

Program Geologi, UKM

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Geochemistry

• Water / rock interactions in unsaturated/saturated zones

• Geochemistry is important to groundwater studies:-– Characterizing the natural system (or

groundwater composition)

– Understanding contaminant migration

– Designing remediation programs

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Simple geochemical model

• Aqueous geochemistry

– Water/rock interactions

– To control the groundwater composition and the movement of dissolved constituents

Deutch WJ. 1997. Groundwater Chemistry. CRC Press

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Fresh water without any dissolved constituents

(disequilibrium)

Reactions that dissolve gases and minerals and

changes the solution composition

Dynamic geochemical system consisting(i) Solid phase(ii) Gas phase(iii) Aqueous

solution phase

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disequilibrium

Groundwater solution• Definitions and concentration units

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• Concentration of solute in solution• Milligram per liter (mg/L)

• Part per million (ppm)

mg/L ppmmg/L ppm

mg/L------- = solution density (g/cc) – TDS (g/cc)ppm

Solution density = 1.008 g/ccTDS = 10,000 mg/L (0.01 g/cc)

Then, 1 ppm = 0.998 mg/L (0.2 % difference!)

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• Groundwater solutes

– Major Ions (concentration > 1 mg/L)

– Minor ions (concentration < 1 mg/L)

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• Converting measured concentration (mg/L or ppm) to electrical equivalent unit (meq)

= 4.6 meq / L

(1000 miliequivalents = 1 equivalent)

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• Conversion to meq

• Electrical balance

Electrical balance:

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Electrical balance for Table 1-2 = +0.5%+ (excess cations; insufficient anions)- (excess anions; insufficient cations)Reasonable balance for routine analysis < 5%

Several possible reasons that create an electrical imbalance in the data

1. The design of the sampling program neglected a major dissolved species

2. Laboratory error

3. Using unfiltered water samples

4. Precipitation of a mineral in the sample

5. In certain cases, the dissolved species may not correspond to the typical species used in the making the ion balance calculation

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Groundwater types• Classify groundwater based on dominant

cations and anions

• Ca-HCO3 type (dominant with Ca and HCO3)

• Displayed graphically by several methods

– Bar graph

– Circular diagram

– Stiff diagram

– Trilinear or Piper diagram

– Durov diagrams

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Bar Diagram Circular Diagram

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Stiff Diagram Piper Diagram

Durov Diagram

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STIFF DIAGRAM

1. Ion concentrations in meq L-1 are plotted on the horizontal axis.

2. Cations are plotted to the left, anions to the right, of a vertical axis.

3. The data are plotted in four rows and the points are connected to form a polygon.

4. Advantage: each water type produces a distinct shape.

5. Disadvantage: each analysis requires its own plot; only a limited number of data can be shown on a single plot.

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P i n e Cre e k , CDA V a l l e y , Id a h oM ine Wat er s

Cat io ns m e q / l Anio n s

1 5 1 0 5 5 1 0 1 5

C l

H C O 3 + C O 3

S O 4M g

C a

N a + K

AD0 0 2

C l

H C O 3 + C O 3

S O 4M g

C a

N a + K

AD0 0 4

C l

H C O 3 + C O 3

S O 4M g

C a

N a + K

AD0 0 5

C l

H C O 3 + C O 3

S O 4M g

C a

N a + K

AD0 0 7

C l

H C O 3 + C O 3

S O 4M g

C a

N a + K

S97 - 3

C l

H C O 3 + C O 3

S O 4M g

C a

N a + K

SP0 0 2

C l

H C O 3 + C O 3

S O 4M g

C a

N a + K

SPNEW

An example of a Stiff diagram drawn for mine waters from the Pine Creek district, Coeur d’Alene Valley, ID. The anions are mostly dominated by sulfate, with lesser bicarbonate, whereas the cations are dominated by calcium and magnesium.

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Isocon of TDS

Stiff pattern

Stiff pattern are centered over locations of wells

PIPER DIAGRAMS1. Consists of two triangles (one for cations and one for

anions), and a central diamond-shaped figure.

2. Cations are plotted on the Ca-Mg-(Na + K) triangle as percentages.

3. Anions are plotted on the HCO3--SO4

2--Cl- triangle as percentages.

4. Concentrations are in meq L-1.

5. Points on the anion and cation diagrams are projected upward to where they intersect on the diamond.

6. Many water analyses can be plotted on the same diagram and can be used to classify waters.

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Cations meq/L % of total

Anions meq/L % of total

Ca2+ 1.15 36 Cl - 0.27 9

Mg2+ 0.39 12 SO42- 0.02 1

Na+ + K+ 1.64 52 CO32- +

HCO3 -

2.80 90

Total 3.18 Total 3.09

Percentage of cations and anions as percentage of the total(Step 2 and 3)

Figure 1-6 fromKehew (2001). Water analyses plotted on a Piper diagram. Cationpercentages in meq L-1

plotted on the left triangle, and anion percentages in meq L-1

plotted on the right triangle.

Ca = 22.3 %Mg = 13.7 %Na+K = 64 %HCO3 = 31.3 %SO4 = 54.5 %Cl = 14.2 %

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Figure 1-7 fromKehew (2001). Classification of hydrochemical facies using the Piper plot.

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Pine Cr ee k , CDA Va lle y , I d ah o

M in e W a t e r s

C A T I O N S A N I O N S% m e q / l

N a + KH C O + C O3 3 C l

M g S O4

C aC a lc iu m ( C a ) C h lo r id e ( C l)

Su

l fa

te

(S

O4

)+

Ch

l or

i de

(C

l )

Ca

l ci u

m(

Ca

)+

Ma

gn

es

i um

(M

g)

Ca

rb

on

ate

(C

O3

)+

Bi c

ar

bo

na

te

(H

CO

3)

So

di u

m(

Na

)+

Po

ta

ss

i um

(K

)

Su

l fa

te

(S

O4

)

Ma

gn

es

i um

(M

g)

8 0 6 0 4 0 2 0 2 0 4 0 6 0 8 0

80

60

40

20

20

40

60

80

20

40

60

80

80

60

40

20

20

40

60

80

20

40

60

80

80

60

40

20

80

60

40

20

AD002

AD004

AD005

AD007

S97- 3

SP002

SPNEW

An example of a Piper diagram drawn for mine waters from the Pine Creek district, Coeur d’Alene Valley, ID. These may be characterized as Ca-Mg sulfate-bicarbonate-type waters.

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Ke he w ( 2 0 0 1)

C A T I O N S A N I O N S% m e q / l

N a + KH C O + C O3 3 C l

M g S O4

C aC a lc iu m ( C a ) C h lo r id e ( C l)

Su

l fa

te

(S

O4

)+

Ch

l or

i de

(C

l )

Ca

l ci u

m(

Ca

)+

Ma

gn

es

i um

(M

g)

Ca

rb

on

at

e(

CO

3)

+B

i ca

rb

on

at

e(

HC

O3

)

So

di u

m(

Na

)+

Po

ta

ss

i um

(K

)

Su

l fa

te

(S

O4

)

Ma

gn

es

i um

(M

g)

8 0 6 0 4 0 2 0 2 0 4 0 6 0 8 0

80

60

40

20

20

40

60

80

20

40

60

80

80

60

40

20

20

40

60

80

20

40

60

80

80

60

40

20

80

60

40

20

T o t a l D is s o lv e d S o lid s

( P a r t s P e r M illio n )

0 1,

00

0

2,

00

0

3,

00

0

4,

00

0

5,

00

0

1 1

1

2

2

2

3

3

3

4

4

4

5

5

5

6 6

6

7

7

7

8

8

8

An example of a Piper diagram with TDS circles.

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Plot the radius of TDS using suitable scale(5000 ppm)

TDS – represents overall salt content of the water

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karst rivers systems Can you guess the type of aquifer of this groundwater ?

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Thank you

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