Step 4: Carbonate System and Calcite Saturation

Buffer Capacities & Alkalinity

aqion_gw_carbonate_system

All relevat information about the carbonate system, buffer capacities, alkalinity, and water hardness are summarized in the right screenshot.1

The upper part show the calculated buffer capacities (ANC and BNC). For each capacity two values are given:

  • first value – refers to the case when there is no precipitation of minerals
  • second value in parantheses – refers to the case when minerals precipitate

If thermodynamics forbids any precipitation both values are equal.2

Additional information shown in this panel are:

  • M alkalinity in meq/L (i.e. total alkalinity)
  • P alkalinity in meq/L
  • TH – total hardness
  • CH – carbonate hardness
  • water type (soft or hard water)
  • carbonat speciation (HCO3-, CO3-2, CO2)
  • CO2 partial pressure

For more information see here.

Calcite Saturation

aqion_gw_calcite_saturation

This screenshot refers to the calcite-carbonate system:3

  • saturation pH at given evaluation temperature (which may or may not differ from sampling temperature)
  • Langelier Saturation Index (LSI)
  • state of water: corrosive, scale-forming, or in equilibrium with calcite
  • saturation pH after CO2 exchange
  • saturation pH according to Strohecker/Langelier
  • saturation index (SI) of calcite
  • Calcium Carbonate Precipitation Potential (CCPP)
  • amount of precipitated or dissolved calcite in mg/L

In addition, the CO2 (carbonic acid) is split into “free” CO2 and “bound” CO2. This allows to quantify the

  • CO2 in excess (aggressive CO2) or
  • CO2 deficit

In the given example the water tends to be corrosive (i.e. the water dissolves calcite). The corresponding saturation index as well as the calcium carbonate precipitation potential are both negative. The water has 38.9 mg/L CO2 in excess.

Footnotes

  1. This panel is the left part of a larger window shown here.

  2. In the present example the amount of precipitated Fe(OH)3 is too small to have a noticeable impact on ANC and/or BNC.

  3. This panel is the right part of a larger window shown here.

[last modified: 2014-11-16]