Water Sample with Incomplete Data

Problem

What is the equilibrium composition and alkalinity of a water sample with incomplete analytical data? The only information we have is that it is a potable water with:

pH 7.5
T 5 °C
DIC 2.6 mM

Answer

Irrespective of the incompletness of the analytical data it is possible to deduce valuable hydrochemical information.

First of all, the above dataset does not contain any cations. Therefore, we assume the existence of Ca as a major ion (which is surely present in any potable water). We set, for example, 2 mM Ca as an initial value,1 and let the exact value determine by the program.

So, we start with pure water (button New) and activate the checkbox Mol. Then, we enter the following parameters:

pH 7.5
T 5 °C
DIC 2.6 mmol/L
Ca 2 mmol/L

Click on Start. Immediately the programm outputs a CBE of 25.66%. To adjust charge balance we select the parameter Ca and click on next which yields:

Ca  =  2.0   →   1.188 mM

Our first-choice value of 2 mM (= 80.16 mg/L) was decreased by an amount of 0.812 mM down to 47.61 mg/L. The obtained solution is now perfectly charge-balanced.

Go on with next to the main output table, then click on the upper button Ions. The ions table offers the complete carbonat speciation of the input solution (raw data) and of two equilibrium solutions (one with and one without mineral precipitation). In this example, the last two columns are equal because there is no precipitation of minerals. These columns contain the results:

CO2 = 0.229 mM
HCO3- = 2.349 mM
CO3-2 = 0.002 mM

The same information will be presented in the next window of aqion which displays the speciation together with alkalinity and buffer capacities:

ANC to pH 4.3 = 2.41 mM
ANC to pH 8.2 < 0
BNC to pH 4.3 < 0
BNC to pH 8.2 = 0.20 mM
M alkalinity = 2.38 meq/L

In fact, the values of HCO3-, of alkalinity, and of ANC are nearly the same. This is correct.

Remarks

  1. The result is independent of the initial choice for Ca; you can take Ca = 0.001 mM or 1000 mM or any other value. (The only condition is that you should enter one value ≥ 0.001 mM, otherwise Ca will not be included in the parameter list for charge-balance adjustment.)

[last modified: 2014-11-18]