First of all: readers who have Henry’s Handbook of Soil and Water can check out pages 124-125 for a detailed discussion of water chemistry, complete with calculations.
Water is considered to be the universal solvent because it is capable of dissolving more substances than any other liquid. Therefore, one of the first things we might want to know about a given water sample is the total dissolved solids (TDS).
TDS can be determined by evaporating a sample to dryness at 105 C and weighing the residue — simple in concept, but time-consuming in practice, so not often used.
TDS can also be determined by measuring all the chemical elements in a sample and adding them up, but that is seldom done. If the chemical elements are previously determined, it is a simple matter to add them up. If just TDS is reported, with no explanation of how it was determined, it may not be reliable.
What chemical elements are dissolved?
Major positively-charged elements are Ca++, Mg++ and Na+, with K+ as an also-ran. Hard water is caused by Ca++ and Mg++, while soft water is dominated by Na+.
Water hardness is expressed as CaCO3 equivalent in parts per million (p.p.m.) and can be easily measured in the field with a Hach testing kit. In our days on the road staying in motels, we could get some idea of water hardness by the way the soap acted in the shower!
- Negatively-charged elements are Cl- and SO4 – -, with NO3- as an also-ran. Most of our waters are SO4 dominated.
Electrical conductivity
Electrical conductivity (EC) is a simple measurement that can be easily made in the field. The units are microSiemens per centimetre (uS/cm), corrected to a standard temperature of 25 C.
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For sulphate-dominated waters, with TDS from 1,000 to 20,000 p.p.m., a useful first approximation is that EC in uS/cm is about equal to TDS in p.p.m.
The level of water hardness in water wells is determined by the geology of that well. If completed in glacial deposits, the water will be hard. Unless completed in sand, most Saskatchewan farm wells have hard water. If completed in pre-glacial (that is, bedrock) deposits, the water will be soft. Most Alberta farm wells are completed in pre-glacial deposits, so the water is soft.
A final note
In this neck of the woods, if we know the EC and hardness of a water sample, we know a lot about that water and what it can be used for.
Many water well drillers have EC and hardness equipment with the drill rig and can measure a water sample as soon as a well is developed and pumping.
