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Karst Chemistry II. 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/
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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) I=1.8810-5Spc
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-]+2[CO32-]+[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)1.22=HCO3- (mg/L)
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
Hardness (Hd) • The concentration of ions that prevent soaps from lathering, Ca2+ and Mg2+ are the most important. Hd=50(epm Ca2+ + epm Mg2+) Hd=2.5 ppm Ca2+ + 4.1 ppm Mg2+ • No useful theoretical significance, but is a useful measure of the amount of dissolved carbonate rock.
Relation between Hd and Spc • A linear relation exists between Hd and Spc Hd (mg/L as CaCO3) = A + B*Spc (S/m) • A and B are best fit parameters that will vary with the different systems.
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. in atmospheres
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?
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
Aggressive (Undersaturated) Supersaturated 0 Negative Positive • 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.
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
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.
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.
Ca/Mg Ratio • The atomic ratio of calcium to magnesium, [Ca2+]/[Mg2+] • For dolomites, the Ca/Mg=1 • The more calcite present the larger the ratio. • Provides information about the type of rock the water flows through.