1 / 18

Karst Chemistry II

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/

colman
Download Presentation

Karst Chemistry II

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Karst Chemistry II

  2. 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10-5Spc

  3. 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)

  4. Bjerrum Plot

  5. 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

  6. 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.

  7. 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.

  8. 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

  9. 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?

  10. 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

  11. 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.

  12. 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

  13. 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.

  14. 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.

  15. 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.

  16. Activity Coefficient vs. Ionic Strength

More Related