Karst Chemistry II. Conductivity – Specific Conductance Conductance – the electrical conductivity of aqueous solution, and is directly related to the.

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Presentation transcript:

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/ 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 =1.88  Spc

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 CaCO 3 ) = [HCO 3 - ]+2[CO 3 2- ]+[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 CaCO 3 )  1.22=HCO 3 - (mg/L)

Bjerrum Plot

Derived Parameters The raw data can be difficult to compare systems, thus more “useful” parameters were developed by using the measured parameters: –Hardness –CO 2 Partial Pressure (PCO 2 ) –Saturation Indices (SI) –Ca/Mg Ratio

Hardness (Hd) The concentration of ions that prevent soaps from lathering, Ca 2+ and Mg 2+ are the most important. Hd=50  (epm Ca 2+ + epm Mg 2+ ) Hd=2.5 ppm Ca ppm Mg 2+ 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 CaCO 3 ) = A + B*Spc (S/m) A and B are best fit parameters that will vary with the different systems.

CO 2 Partial Pressure (P CO 2 ) P CO 2 is the pressure of a hypothetical coexistent gas phase that is exactly in equilibrium with the analyzed water Not actually the CO 2 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 Ca 2+ 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 1.Equilibrium 2.Undersaturated 3.Oversaturated The SI provides a references to a condition. SI = log (K iap /K sp ) For calcite

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. Aggressive (Undersaturated) Negative 0 Supersaturated Positive

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 CaCO 3 or will be saturated and precipitate CaCO 3. The amount of CaCO 3 that will be dissolved or precipitated can be determined by the mixing ratio.

Ca/Mg Ratio The atomic ratio of calcium to magnesium, [Ca 2+ ]/[Mg 2+ ] 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.

Activity Coefficient vs. Ionic Strength