Improve documentation for ion balance and theoretical conductivity calculations

[JamesSample]

I've added calculations for the ion balance and theoretical conductivity based on functions previously written for the Aquamonitor QC app. Where two versions of the calculations are available, I've used Øyvind G's method rather than the one already implemented in RESA.

It would be nice to improve the documentation for these functions so that this code is easier to maintain. @kariaust @catbgun can you help to answer any of the following questions, please?

• What exactly is "A minus"? I assume it's some estimate of organic acids? It's calculated as TOC_mg/l * (1.77 * pH - 3). Can you suggest an improvement for the documentation here?
• Which parameters can be assumed to be zero for ion balance and theoretical conductivity calculations? At present, these functions are only evaluated if all parameters are present. See e.g. the list of parameters here. Are you happy to assume some of these are zero if they are not provided?
• What threshold should we use to flag differences in the ion balance and theoretical conductivity? At present, I'm highlighting differences greater than ±10% for both calculations. Is this OK?
• What exactly are the gamma parameters calculated from the ion strength here?

It would be great if you could find time to test the new functions by e.g. uploading data with known good and bad ion balances and theoretical conductivity.

Thank you! :-)

[catbgun]

Here follows answers to the questions, with inputs from Øyvind Garmo, Rolf D. Vogt and @kariaust

• "A minus" is an estimate of the charge contribution from organic acids. Apparently, there are different ways of calculating this with the different options possibly optimally suitable for different types of waters. The one implemented by Øyvind is an empirical simplification of Hruška, J., et al., 2003. Environmental Science & Technology 37, 1726–1730. https://doi.org/10.1021/es0201552. (Another commonly applied alternative is by Oliver 1983.)
• The following parameters can be set to zero if not provided: NH4-N, LAl, A-
• Threshold of ±10% is ok.
• The gamma parameters are the activity coefficients of the ions, 1 for monovalent and 2 for divalent ions. It is used to compensate for the non-ideal behavior of real waters. Also here, there are different options. Garmo used the Davies approach. (Another commonly used alternative is the extended Debye-Hückel equation.)

Hope this was useful. Let me know when the functions have been updated (setting parameters to zero when data is not provided) and I will upload a test-datafile.

[JamesSample]

Thanks @catbgun!

I've added some details to the documentation and made NH4, LAl and A- optional. Please give it a go and let me know how you get on :-)

Thanks!