Kiwa Nieuwegein

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Base exchange indices as indicators of
salinization or freshening of (coastal) aquifers
Pieter J. Stuyfzand 1,2
SWIM-20, Naples FL USA, 23-27 June 2008
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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, Neths
  (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

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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 ?S (1 n) / n SC
• Examples
• CEC 10 meq/kg d.w. ?S 2.65 kg/L n 0.35
• Fresh water intrusion SC 6 meq/L ? REQ 9.2
• Salt water intrusion SC 520 meq/L ? REQ 1.1

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The various Base Exchange Indices and their
inherent problems
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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

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Correcting individual ions for contribution of
sea salt ? losses or gains by hydrogeochemical
reactions
X X - aX Cl- with aX X/Cl in SMOW
Example Na 50, Cl 100 mg/L ? Na 50 56
-6 mg/L
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BEX Base EXchange index
In chem watertype coding. Examples CaHCO3,
CaHCO3-, CaHCO3.
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Algorithm for interpreting base exchange index BEX
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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
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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
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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
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(No Transcript)
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Changes in Na, K, Ca, Mg and Cl during first
freshening then salinization (PHREEQC-2)
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Changes in BEX, Cl and X during first freshening
then salinization (PHREEQC-2)
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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