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Base Exchange Indices in Assessing Aquifer Salinization/Freshening

Explore the historical context, exchange reactions, correction methods, and interpretations of Base Exchange Indices in coastal aquifers, crucial for detecting salinization or freshening trends over time. Correcting for marine cations, examining chemical water types, and evaluating hydrochemical facies provide insights into groundwater evolution toward salinization or freshening.

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Base Exchange Indices in Assessing Aquifer Salinization/Freshening

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  1. 2 1 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

  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] Examples CEC = 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

  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

  7. BEX: Base EXchange index In chem watertype coding. Examples: CaHCO3+, CaHCO3-, CaHCO3.

  8. Algorithm for interpreting base exchange index BEX

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

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

  11. 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

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

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

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

  15. 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

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

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

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