Anions and Cations

Balances of anions and cations are used throughout bcgwcat in order to assess various water chemistry metrics.

Generally speaking, charge balances are used to assess whether the water chemistry data is valid. For water types, piper plots and stiff plots, samples are only retained if the charge balance is <=10%.

ions	charge balance	water type	piper plots	stiff plots
anions
Cl	X	X	X	X
SO4	X	X	X	X
HCO3		X	X	X
CO3			X
F	X
NO3	X
NO2	X
Meas_Alk	X	*	*	*
cations
Ca	X	X	X	X
Mg	X	X	X	X
Na	X	X	X	X
K	X	X	X
Al_diss	X
Cu_diss	X
Fe_diss	X
Mn_diss	X
Zn_diss	X
NH4	X
pH_lab	X
* Used if HCO3 is missing

Charge balances

Charge balances are calculated based on a formula from ALS formula.

Although EMS data contains charge balances, as well as the anion and cation sums used to calculate these values, potential changes in workflows over the years have made it difficult to ascertain exactly how charge balances were calculated in older samples. This resulted in discrepancies between EMS and locally calculated charge balances. Therefore for consistency, we calculate charge balances for all samples using the ALS formula below.

Note: This means that the original EMS charge balances, anion sums, and cation sums, have been omitted.

One difference between our calculation and that of ALS, is that we use more significant digits when calculating MEQ.

Charge balances use following measures in MEQ

Anions Cl, SO4, F, NO3, NO2, Meas_Alk
Cations Ca, Mg, Na, K, Al_diss, Cu_diss, Fe_diss, Mn_diss, Zn_diss, NH4, and pH_lab

And are calculated as follows

Anion sum = Cl + SO₄ + F + NO₃ + NO₂ + Meas_Alk

Cation sum = Ca + Mg + Na + K + Al_diss + Cu_diss + Fe_diss + Mn_diss + Zn_diss + NH₄ + (10^−pH_lab) * 1000

Charge balance = |100 × (Cation Sum − Anion sum)/(Cation Sum + Anion Sum)|

Missing values are ignored (i.e. generally treated as 0). However, if all values for cations or anions are missing the charge balance is NA.

This means that Charge Balances will be invalid if there are too many missing measures.

Water Types

Water types identify the major cations and anions in a water sample. In bcgwcat, we calculate water types using the following ions:

Anions Cl, SO4, HCO3
Cations Ca, Mg, and Na

Water type is only calculated for samples with valid charge balances. Ions are ranked by proportion MEQ, all greater than 10% are listed in descending order, cations first. If HCO3 is missing, we use Meas Alk as a replacement (indicated by an ’*’ on HCO3 in the water type). Otherwise, missing ions are ignored (i.e. treated as 0). The column missing_ion indicates whether there is a problem with the water type, such that it is missing a cation or anion (i.e. is all cations or all anions).

For example with this data

Sample_Date	SampleID	StationID	Cl_meq	SO4_meq	HCO3_meq	Meas_Alk_meq	Ca_meq	Mg_meq	Na_meq	K_meq	charge_balance
2022-01-01	999990-01	0	0.0226	0.0208	1.54	1.7	0.187	0.49	0.465	0.0665	0.5

We would get the following water type.

water_type	missing_ion
Mg-Na-HCO3	FALSE