Validation of Water Sample Data

This example demonstrates how a water analysis can be (routinely) checked and interpreted by hydrochemistry. We start with a groundwater sample, gw.sol, that belongs to the collection of example files shipped with aqion.

We restrict our demonstration to the following four steps:

  1. input of sample water (raw data of water analysis)
  2. charge-balance error (and parameter adjustment)
  3. aqueous composition
  4. carbonate system & calcite saturation

An example of an incomplete water sample is given here.


aqion_input_panel_with_groundwater

Step 1:  Input of Water Sample Data

Just after program start the input panel opens into which you can enter the parameters of the aqueous solution: pH, temperature, and the element concentrations.

Alternatively, you are able to load an existing aqueous solution (input water) by click on button Open. In this example we select the file gw.sol (groundwater sample). 1

The content of this file feeds the window as shown in the right screenshot. In the top line you find the name of the solution: gw (where the file extension sol is skipped).

There are to possibilities to start a sequence of hydrochemical calculations:

  • Button Start:  Charge balance, chemical equilibrium without/with minerals, carbonate-calcite system, etc.
  • Button Reac:  Addition of chemicals, pH calculations, titration curves

Let us click on button Start.

Step 2:  Charge Balance Error (CBE)

aqion_gw_charge_balance

The very first plausibility check inspects the cation-anion balance. The result is displayed as CBE shown in the right image.

In this example the charge-balance error of 0.33% is really small, which indicates an analysis of excellent quality. Nonetheless, there is always the possibility to establish full charge balance, i.e. an equilibrium solution with CBE ≈ 0.

Charge balance can be established only if the corresponding left checkbox is activated. Then you are able to select one parameter from the a dropdown list. Let us select the pH value (default) and click on the next button to start the pH adjustment.

Charge-Balance Adjustment

aqion_gw_pH_adjustment

In this example, full charge balance is established by adjusting the chosen parameter pH from 6.90 to 6.91 (see right screenshot). This is, in fact, a tiny adjustment – due to the good quality of the water analysis data. (In reality we are often faced with analytical data of less accuracy, i.e. CBE ≥ 3%.)

Technically, the program performs two subsequent calculations before the results are displayed in the right panel:

  • 1st calculation (Output 1):  Parameter adjustment to achieve full charge balance (i.e. chemical equilibrium without mineral phases)
  • 2nd calculation (Output 2):  Chemical equilibrium with mineral phases

If the water is supersaturated with one or more minerals the corresponding mineral(s) will precipitate (as indicated in Output 2). Precipitation is often accompanied with changes of pH, and these changes are displayed too. (In the present example, however, the amount of precipitated Fe(OH)3 is too small to alter the pH.)

By click on button next you obtain the complete aqueous composition of the input and output waters (Step 3).

Step 3:  Output Table

aqion_gw_output_table

The structure of the output table in the right screenshot is explained here. It contains three water compositions labeled as Input, Output 1 and Output 2:

Input. This column represents the input water (raw data).

Output 1. This column represents the aqueous solution after charge balance adjustment. It is the result of an equilibrium calculation.

Output 2. This column displays the aqueous solution after charge balance adjustment and equilibrium with mineral phases. In this example, Fe almost disappears due to the precipitation of amorphous ferrihydride, Fe(OH)3.

In the second calculation, also redox equilibria are established, which influence the Fe(2)/Fe(3) ratio as well as the disproportionation between nitrate and nitrite.2

Other Data. While the screenshot above displays total concentrations of elements additional information is provided in separate tables (in the upper menu bar):

  • Ions – ions and aqueous complexes (complete speciation)3
  • Minerals – precipitated minerals and saturation indices (SI)

By click on the next button one obtains the results shown in step 4.

Remarks & Footnotes

  1. All aqueous solutions (input waters) of aqion are stored in the subfolder aqion\INP in the user’s home directory.

  2. The redox equilibrium between nitrate and ammonium is excluded by default – see here.

  3. Another example for aqueous speciation is here.

[last modified: 2016-05-28]