Conversion factors for groundwater chemistry

Groundwater chemistry data are usually quoted in milligrams per litre (mg/l). In order to plot these data on one of the graphical methods such as a Piper or Schoeller diagram, it is necessary to convert the units to milli-equivalents (meq/l). Use the conversion factors from the table below and divide the concentration of each ion in mg/l by the factor shown.

For example, a Magnesium concentration of 38 mg/lMg is 38.00 12.16 = 3.125 meq/lMg.

Calcium (Ca)20.04
Magnesium (Mg)12.16
Potassium (K)39.10
Sodium (Na)23.00
Chloride (Cl)35.46
Sulphate (SO4)48.04
Nitrate (NO3)62.01
Bicarbonate (HCO3)61.01
Carbonate (CO3)30.01

Determinands may be expressed in different units than the one you want. The following table provides the conversion factors for the most common ones.

To convert fromto multiply by
Nitrate as NNitrate as NO30.2258
Nitrite as NNitrite as NO20.3043
Ammonium as NAmmonium as NH40.7778
Alkalinity as HCO3Alkalinity as CaCO31.22
Hardness as CaHardness as CaCO30.40
Sulphate as SO4Sulphate as SO31.193

When you look through old records chemical analyses may be quoted in other units. Frequently 'parts per million' (ppm) are used and these can be treated as being the same as mg/l. Even older records may express concentrations in 'grains per gallon' and can be converted to mg/l by multiplying by 14.25 for imperial gallons or 17.11 for US gallons

This Handy Hint is based on information in my book Field Hydrogeology.

Copyright © 2004 by Rick Brassington


Eur Geol Eur Ing Professor F.C. Brassington BSc MSc CGeol FGS CEng MICE FCIWEM