Karst Chemistry II

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Karst Chemistry II
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Conductivity Specific Conductance

• Conductance the electrical conductivity of
  aqueous solution, and is directly related to the
  concentration of ionic species in the solution.
• Measured in S/m or mS/
• Conductance is a function of temperature, all
  measurements must be corrected to a standard
  reference temperature (20 ºC or 25 ºC) Specific
  Conductance
• Can provide a measure of Ionic Strength (I)
• I1.88?10-5Spc

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Alkalinity

• The acid-neutralizing capacity of water
• Reflects the presence of carbonate, bicarbonate,
  hydroxyl, other basic constituents
• In carbonate waters, the alkalinity is
• Alk (as CaCO3) HCO3-2CO32-OH--H
• Where the concentrations are molar concentrations
• In karst ground water, the pH dictates that only
  the bicarbonate ion makes a significant
  contribution to Alk
• Alk (as CaCO3)?1.22HCO3- (mg/L)

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Bjerrum Plot
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Derived Parameters

• The raw data can be difficult to compare systems,
  thus more useful parameters were developed by
  using the measured parameters
• Hardness
• CO2 Partial Pressure (PCO2)
• Saturation Indices (SI)
• Ca/Mg Ratio

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Hardness (Hd)

• The concentration of ions that prevent soaps from
  lathering, Ca2 and Mg2 are the most important.
• Hd50?(epm Ca2 epm Mg2)
• Hd2.5 ppm Ca2 4.1 ppm Mg2
• No useful theoretical significance, but is a
  useful measure of the amount of dissolved
  carbonate rock.

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Relation between Hd and Spc

• A linear relation exists between Hd and Spc
• Hd (mg/L as CaCO3) A BSpc (S/m)
• A and B are best fit parameters that will vary
  with the different systems.

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CO2 Partial Pressure (PCO2)

• PCO2 is the pressure of a hypothetical coexistent
  gas phase that is exactly in equilibrium with the
  analyzed water
• Not actually the CO2 pressure present in the
  atmosphere.
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  in atmospheres

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What will happen to the ability of the water to
dissolve calcite over time?

• The water becomes saturated with Ca2
• What should happen to cave/passage development as
  water travels further into a limestone aquifer?

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Saturation Indices (SI)

• A solution containing a mineral will be in one of
  three conditions
• Equilibrium
• Undersaturated
• Oversaturated
• The SI provides a references to a condition.
• SI log (Kiap/Ksp)
• For calcite

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• The SI describes a scale as follows
• However, the SI is dependent upon the boundary
  conditions, and will change with variations in
  the Pressure and temperature.

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What would happen if two waters with different
SIs mix?

• When two waters mix, and both are in equilibrium,
  an undersaturated water is produced.
• Mixing corrosion
• This allows for cave development to continue down
  gradient

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Mixing Corrosion

• When two waters mix, the resulting CO2 and CaCO3
  compositions will plot somewhere in the line
  connecting the previous compositions.
• The exact composition will be based upon the
  ratio of the mixing water.

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Mixing Corrosion cont.

• The mixed water will have either be aggressive
  and dissolve more CaCO3 or will be saturated and
  precipitate CaCO3.
• The amount of CaCO3 that will be dissolved or
  precipitated can be determined by the mixing
  ratio.

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Ca/Mg Ratio

• The atomic ratio of calcium to magnesium,
• Ca2/Mg2
• For dolomites, the Ca/Mg1
• The more calcite present the larger the ratio.
• Provides information about the type of rock the
  water flows through.

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Activity Coefficient vs. Ionic Strength