Levels of concern

Interpreting the Water Analysis

Table 1 provides criteria for assessing the potential of an irrigation water to cause clogging. Other quality considerations include the following.

• Bicarbonate concentrations exceeding about 2 meq/l or 120 mg/l and pH exceeding about 7.5 can cause calcium carbonate precipitation.
• Calcium concentrations exceeding 2 to 3 meq/l can cause precipitates to form during injection of some phosphate fertilizers.
• High concentrations of sulfide ions can cause iron and manganese precipitation. Iron and manganese sulfides are highly insoluble, even in acid solutions.
• Clogging problems by iron and manganese are caused by the conversion of soluble iron and manganese to insoluble forms (see Preventing Iron and Manganese Clogging). This conversion process occurs for iron if the pH exceeds about 5 and for manganese if the pH exceeds about 9.

Table 1. Relative microirrigation system clogging potential of irrigation water

Source: Bucks and Gilbert 1979; Nakayama and Bucks 1991.

Units of Concentration

The concentrations of chemicals in water are expressed on either a weight basis or a volume basis. Concentrations expressed on a weight basis are parts per million (ppm), percent concentration (%C), and milligrams per kilogram (mg/kg). Concentrations expressed on a volume basis are milligrams per liter (mg/l), milliequivalents per liter (meq/l), and millimoles of charge per liter (mmolc/l). Millimoles of charge per liter (mmolc/l) is the designated SI unit (International Standard of Units). Relationships between units are as follows:

1 ppm = 1 mg/l (for all practical purposes in dealing with dilute water solutions)
1 ppm = 1 mg/kg
1% concentration = 10,000 ppm
1% concentration = 1.33 ounce by weight per gallon of water
1 mmolc/l = 1 meq/l

Also keep in mind that 1 ppm means 1 pound of dry chemical material is dissolved in 1 million pounds of water, whereas the concentration percentage is the ratio of the weight of the dry material in the solution to the weight of the solution multiplied by 100. Grains per gallon also may be used as a concentration unit. To convert grains per gallon to mg/l, multiply the grains per gallon by 17.12.

Many laboratories report concentrations of chemical constituents in a water sample as mg/l or meq/l. Sometimes converting mg/l to meq/l or vice versa is desirable. Table 2 provides useful conversion factors.

Table 2. Conversion factors

 Examples:

     120 ppm HCO3 x 0.016 = 1.9 meq/l HCO3

     4 meq/l HCO3 x 61 = 244 ppm HCO3

Guidelines

The following steps will help in evaluating water quality; refer to table 1 for assistance.

1. What is the total dissolved solids concentration? If only the electrical conductivity is given, multiply the EC (mmhos/cm) by 640 to determine the total dissolved solids.
2. What is the calcium concentration? If the calcium concentration exceeds 2 to 3 meq/l, see Preventing Calcium Carbonate Clogging.
3. What is the bicarbonate concentration? If the bicarbonate concentration exceeds about 2 meq/l, see Preventing Calcium Carbonate Clogging.
4. What are the iron and manganese concentrations? If either concentration exceeds about 0.2 ppm, see Preventing Iron and Manganese Clogging.

Examples

Table 3 gives water quality data from the analysis of two irrigation water samples. The water quality criteria from Table 1 are used to evaluate the clogging potential of the two waters.

• Water 1. This is a gypsiferous water because of the large amounts of SO4, Ca, and Mg. The sum of the cations equals 28.9; the sum of the anions equals 28.7, and the ratio of the sum of the cations to EC equals 11.5. Thus, the quality of these data is good. The relatively high total dissolved salts, or TDS, (1,900 ppm) indicates that the water has some clogging potential. This is verified by the relatively high bicarbonate concentration (5.2 meq/l), compared with the standard of 2 meq/l. The calcium and the bicarbonate concentrations together suggest that calcium carbonate could clog the emitters, particularly if the pH were to rise as a result of any chemical injection. The iron and manganese concentrations indicate little potential for clogging from precipitation of those elements.
• Water 2. Two separate samples were collected. Acid was added to one of the samples to prevent the Fe and Mn from oxidizing and precipitating out. Both the sum of the cations and the sum of the anions equals 8.7, indicating that one of the constituents (usually SO4) was not analyzed for but rather determined so that the sums of the anions and cations agreed. The ratio of the sum of the cations to the EC equals 10, suggesting good-quality data. The analysis reveals little potential for clogging from total dissolved salts (560 ppm), but the pH and the HCO3 concentration indicate that clogging might result from calcium carbonate precipitation. The levels of Mn and Fe indicate a severe potential for clogging from manganese and iron precipitation. However, little manganese precipitation should occur unless the pH exceeds 9. Care should be taken when injecting fertilizers or amendments containing calcium into this water.

Table 3. Water quality analysis of two irrigation water samples