Partner for progress

Pieter J. Stuyfzand 1,2

SWIM-20, Naples FL USA, 23-27 June 2008

Base exchange indices as indicators of salinization or freshening of (coastal)

aquifers

12

2

General interest in base exchange of groundwaters

In the early days (1890-1920s): Speculations about origin of soft (NaHCO3) waters

(weathering of Na-silicates?) Not Renick, USA (1924) but Versluys, Neth’s (1916)

Later on connection with: Migration of fresh-salt interface Precipitation or dissolution of limestone (cave

development) Formation of dolomite Mobilization of DOC and F in NaHCO3 groundwaters

Flocculation and deflocculation of clay permeability reduction

3

The classical exchange reaction and its duration upon fresh or salt water intrusion

The reaction:

Ca-EXCH + 2Na+ Na-EXCH-Na + Ca2+

Its duration in aquifers:

REQ = tEQ / tH2O = 1 + CEC ρS (1 – n) / [n ΣC]

ExamplesCEC = 10 meq/kg d.w.; ρS = 2.65 kg/L; n = 0.35

Fresh water intrusion: ΣC = 6 meq/L REQ = 9.2

Salt water intrusion: ΣC = 520 meq/L REQ = 1.1

4

The various Base Exchange Indices and their inherent problems

No. Author Index [meq/L]

1 Versluys (1916, 1931) Na / (Na + Ca + Mg)2 Schoeller (1934) [Cl - (Na + K)] / Cl3 Schoeller (1956) (Na + K) / Cl4 Schoeller (1956) (Ca + Mg) / (HCO3 + CO3 + SO4)5 Delecourt (1941) Na + K - Cl6 Stuyfzand (1986) # Na + K + Mg - 1.0716 Cl7 This publication ## Na + K - 0.8768 Cl

Problems:1-7: other processes may influence values 1: CaCO3 dissolution influences ratio1-5: bias due to analytical errors, no threshold 2: ocean water already positive1-4: do not quantify amount of cation exchange 5: ocean water already negative

#: for aquifers without dolomite ##: for aquifers containing dolomite

5

Basic assumptions behind BEX: Na, K, Mg, Cl in coastal fresh/salt groundwater from SMOW

All Na, K, Mg, Cl- ions are principally of marine origin (no halite, silicate weathering negligible; if dolomite then exclude Mg)

Fractionation of main constituents of sea water during spray formation can be ignored (spray the only source of Na, K, Mg, Cl in rain water)

Cl behaves conservatively (no losses, no gains)

The 3 marine cations (Na, K, Mg) exchange together for Ca

6

Correcting individual ions for contribution of sea salt losses or gains by hydrogeochemical reactions

X* = X - αX Cl- with: αX = X/Cl in SMOW

Example:

Na = 50, Cl = 100 mg/L Na* = 50 – 56 = -6 mg/L

Major Trace constituent mg/L mol/L constituent mg/L mol/L

Na+ 0.5564 0.8581 B 2.3 10-4 7.541 10-4

K+ 0.0206 0.0187 Br- 3.48 10-3 1.543 10-3

Ca2+ 0.0213 0.0188 F- 7.19 10-5 1.341 10-4

Mg2+ 0.0668 0.0974 Li+ 9.34 10-6 4.773 10-5

SO42- 0.1401 0.0517 Mo 5.17 10-7 1.913 10-7

TotH (Ca2+ + Mg2+) 0.1162 Rb+ 6.06 10-6 2.513 10-6

BEX = Na*+K*+Mg* meq/L 1.0716 Sr2+ 4.21 10-4 1.702 10-4

factor factor

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BEX: Base EXchange index

BEX = Na + K + Mg - 1.0716 Cl (all in meq/L)

with: 1.0716 = (Na+K+Mg)/Cl in mean ocean water

Class BEX CONDITIONSCode If base exchange if other process(es)

only process relevant- negative Salinized Marine cations deficit <-(0.5+0.02Cl) and <1.5(Σk-Σa)• zero no base exchange Marine cations equilibrium >-(0.5+0.02Cl) and <+(0.5+0.02Cl) and #+ positive Freshened Marine cations surplus >+(0.5+0.02Cl) and >1.5(Σk-Σa)

# and abs(BEX + {(Σk-Σa)/abs(Σk-Σa)} (0.5+0.02Cl)) > 1.5 abs(Σk-Σa)

[aCa]-EXCH + bNa+ + cK+ + dMg2+ aCa2+ + [bNa,cK,dMg]-EXCH

2a = b + c + 2d Freshening Salinization

for BEX, in meq/L

INTERPRETATION

In chem watertype coding.

Examples: CaHCO3+, CaHCO3-, CaHCO3.

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Algorithm for interpreting base exchange index BEX

Problems and Interpretation of BEX Fresh Salt

0 Ad- and desorption of Na, K and Mg not simultaneous CR CR1 False positive BEX:

Dissolution of minerals like dolomite (+Mg), albite (+Na) C CMineralization of fresh biomass (+K) R RRLeaching of fertilizers or manure (+K) C RR

2 False negative BEX:Dissolution of halite (lack of K and Mg) R CMineral transformations like dolomitization (-Mg) R CRNew formation of minerals (-K, -Na, -Mg) R RSynthesis of biomass (rapid growing forest; -K) R RRSignificant atmosferic deposition of Cl2 gas (+Cl) RR RR

3 BEX (not false) indicates results of past shifts in fresh / salt interfacePositive BEX at specific point does not mean system is actually freshening C CNegative BEX at specific point does not mean system is actually salinizing C C

4 If chloride decreasing or constant and BEX trend:from positive to 0, then Freshening CC RRfrom 0 to positive, then Salinization or False pos BEX CR? RRmore negative, then False negat BEX R RR

5 If chloride increasing and BEX trend negative, then Salinization CC CC6 If chloride increasing and BEX trend positive, then Salinization by special source water# R RR

CC = very common; C = common; RC = uncommon, not rare; R = Rare; RR = very rare / nonexistent

# = for instance brackish surface water from a canal, with inputs from exfiltrating freshened groundwater or from agriculture

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BEX and chemical watertypes along a freshening flow line: the Bergen dune water body

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Quality evolution along dune system flow path towards reclaimed lake, near Bergen

-2

0

2

4

6

8

10

12

14

0 1000 2000 3000 4000 5000 6000 7000

Flow distance [m]

X*

, B

EX

[

meq

/L]

Na*

K*

Mg*

Ca*

BEX

F2CaHCO3 F2CaHCO3+ F2MgHCO3+ F3NaHCO3+B4NaHCO3+

B4NaCl+

F*NaCl.

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Prograde hydrochemical facies chain during resp. freshening and salinization (modified after Stuyfzand 1993 p.152)

[aCa]-EXCH + bNa+ + cK+ + dMg2+ aCa2+ + [bNa,cK,dMg]-EXCH

Freshening Salinization

BEX BEX

HySA Chem. Watertype meq/L HySA Chem. Watertype meq/L

Dap F*NaCl 0.2 S S2NaCl 0

Dar F0NaCl 0.2 Sr S2NaCl 1

Dr F1CaMix 0.2 Srs S3NaCl- -37

Dd F2CaHCO3 0 B3NaCl- -63

F2CaHCO3+ 1 B2CaCl- -105

F2MgHCO3+ 2 Dds F2CaCl- -3.8

F3NaHCO3+ 6.2 Dd F3CaHCO3 0.5

F4NaHCO3+ 7.7

(D+L)df B5NaCl+ 9

Ldf B6NaCl+ 21.8

FRESHENING DUNE HYDROSOME

Low alkalinity start

Ddf

INTRUDING NORTH SEA in DUNE

Medium alkalinity start

(D/S)rs

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Plot of sea salt corrected cation concentrations (X*) and TIC versus BEX, for 36 deep anoxic groundwater samples coastal dunes Netherlands

During fresh water intrusion:

Ca* = Ca*0 – f BEX

TIC = TIC0 + 2f BEX

with f = 0.5

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Aquifer Cell Cell Porosity Disp. CEC CaCO3 FeCO3 MnCO3Unit No. No. α [m] meq/kg Fresh Salt

Dune, beach and Step 1: Flushing with fresh anoxic dune groundwater shallow, marine sandsFlow = 2 m/year downwards, for 240 years

1-5 1-5

Pleistocene, fluvial +15-20 15-20 marine sands

2 0.38 0.12 14 yes no no 10.0 1.12

35-40 35-40

55-60 55-60 Pleistocene, fluvial loamB 0.37 0.12 140 yes yes yes 95.1 2.20

75-80 75-80 Pleistocene, fluvial sands

3 0.35 0.12 8 no no no 6.9 1.08

95-100 95-100

m - MSLStep 2: Flushing with salt, anoxic North Sea groundwaterFlow = 2 m/year upwards, for 60 years

Retard. factor

Holocene marine clays20

0

A

1

120

40

60

80

100

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Changes in Na, K, Ca, Mg and Cl during first freshening then salinization (PHREEQC-2)

0.001

0.01

0.1

1

10

100

1000

0 50 100 150 200 250 300

Time

Na

, K

, C

a,

Mg

[m

mo

l /

L]

0

100

200

300

400

500

600

Cl

[m

mo

l /

L]

Na

K

Ca

Mg

Cl

Freshening: downward movement F/S-interface

Salinization: F/S upward movement

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Changes in BEX, Cl and X* during first freshening then salinization (PHREEQC-2)

-100

-80

-60

-40

-20

0

20

40

60

80

0 50 100 150 200 250 300

Time

BE

X, c

on

cen

trat

ion

of

X*

[me

q/L

]

0

100

200

300

400

500

600

Cl

[me

q/L

]

Na*

K*

Ca*

Mg*

2 BEX

Cl

Freshening: downward movement F/S-interface

Salinization: F/S upward movement

1

2 3 4 5

6

7

9

109

8

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Conclusions

BEX is the best of 7 available Base EXchange indices, but needs a correction for Mg in dolomitic systems.

BEX (and other indices) need to be interpretated with great care: actual – palaeo, false positive or negative!

Be aware of ‘disguised’ salinization or ‘pseudofreshening’: an increasingly positive BEX ( freshening) with stable low Cl, is an early-warning of on-going salinization

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Various fresh and various brackish/salt hydrosomes and their HYFA (Stuyfzand, 1993 p128)

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Cation exchange due to salinization, in a monitor well sampled in 1905 and 1939