Assessing Water for Its Chemical Precipitate Clogging Potential
The irrigation water used in a microirrigation system should be carefully evaluated to assess any potential clogging problems.
Analyzing Irrigation Water
Irrigation water should be analyzed for the following.
- electrical conductivity (EC), which is a measure of the total dissolved salts (TDS). This equation shows the approximate relation between TDS (ppm) and EC (dS/m or mmhos/cm): TDS = 640 × EC
- pH
- calcium (Ca)
- magnesium (Mg)
- sodium (Na)
- chloride (Cl)
- sulfate (SO4)
- carbonate (CO3) and bicarbonate (HCO3)
- iron (Fe)
- manganese (Mn)
Collecting a Water Sample
Contact the laboratory that will be doing the water analysis to get guidance on collecting the sample. Often the laboratory will also provide sample bottles and any necessary additions, such as acid, for the sample.
As general guidance, when collecting a sample from a well, first allow the pump to run for at least 15 minutes (even longer is preferable). A liter or quart sample volume is usually ample for chemical constituent analyses. Be sure the sample bottle is clean and rinsed. Flush the bottle with the water to be sampled a few times prior to gathering the sample. Collect the sample as close to the well as possible but do not collect samples too close to a chemical injection point since there may be insufficient mixing.
Surface water is more difficult to sample, and it often changes significantly over the season. Try to take as representative a sample as possible. If suspended solids or organic matter, or both, are of concern, a liter-size sample may not be adequate. Contact the laboratory doing the analysis for more guidance.
Deliver the sample to the lab as soon as you can. Storing the sample in a cooler or a refrigerator until it can be delivered is a good idea; keeping it in a hot pickup truck for days before taking it to the lab should be avoided.
If the water is to be analyzed for iron, the sample must be acidified to a pH of 4 or below; otherwise, the soluble (ferrous) iron in solution will precipitate as insoluble (ferric) iron prior to reaching the lab. If this happens, the water analysis will show little iron in solution. Thus, two samples must be collected: one acidified sample for iron and one plain sample for the remaining constituents in solution. The laboratory can provide a sample bottle with the correct amount of acid in it for gathering the iron sample. Do not rinse the bottle prior to collecting the sample.