Water Sampling Procedure

The quality of irrigation water plays an important and significant role in the successful production of agricultural crops. The quality of irrigation water can vary substantially between areas and also from one farm well to another. It is essential that growers have a knowledge of the chemistry of their water as well as an understanding of potential problems that may be associated with it use for irrigation.

To accurately evaluate a specific water source, it is important to collect samples in a proper manner. The following guidelines are suggested for proper sample collection:

  1. Collect samples at the appropriate time. Ground water samples should be collected only after the well has pumped for a period of one to two hours. A longer time period is desirable if the well has not been used on a regular basis. Surface water samples should be taken during the period of time when the water is used for irrigation.
  2. Collect an adequate amount of sample. Generally an 8 to 16 ounce sample of water is adequate for most quality assays.
  3. Use a proper sample container. Clean plastic containers may be used in most instances. Samples to be analyzed for organic residues, such as pesticides, should be collected in glass containers. Contact the laboratory for special instructions.
  4. Use preservatives when required. Accurate tests for certain constituents in water require the addition of specific compounds as preservatives during the time the samples is collected. Contact the laboratory for special instructions.
  5. Deliver water samples to the laboratory as soon as possible. Samples should be delivered within 24 hours. Samples may be refrigerated or kept in a portable cooler for short periods. Do not store samples at room temperature or expose them to heat or direct sunlight.

The following explanations of the water analysis can be used to interpret the results of the laboratory analysis. However, caution should be used in the interpretation of laboratory results. Accurate interpretation of laboratory results requires experience and knowledge, therefore, the use of trained and experienced agronomists is strongly recommended.

Explanation of Water Analysis

  1. pH – The pH expresses the acidity or alkalinity of water. A pH value below 7.0 is acidic, 7.0 is neutral, and above 7.0 is alkaline. Most water has a pH between 6.5 and 8.4.

  2. ECw – The total salt content of water is expressed as electrical conductivity (ECw). The ECw provides an indication of both potential salt accumulation and water penetration problems as follows:

Problem Type  No Problems Slight Problems Severe Problems
Salinity <0.75 dS/m 0.75-3.0 dS/m >3.0 dS/m
Penetration >0.50 dS/m 0.5-0.2 dS/m <0.2 dS/m
  1. Ca+Mg – Calcium (Ca) and Magnesium (Mg) are essential plant nutrients that are normally present in water. Together with Sodium (Na), they are used to calculate SAR and adjSAR.

  2. Na – High levels of Sodium are toxic to plants and lead to water penetration problems in the soil. The detrimental effect on soil is primarily related to the SAR and adjSAR.

  3. HCO3+CO3 – Bicarbonates (HCO3) and Carbonates (CO3) are common in natural waters. High levels may lead to sodium dominant soils and to precipitation of calcium and magnesium salts.

  4. Cl – Chloride is an essential nutrient in small amounts. However, excessive levels are toxic. The following guide can be used to predict toxicity problems due to chloride in the water:

Cl levels (meq/L)

Expected plant response

<2 Generally safe
2-4 Sensitive plants show injury (trees/vines)
4-10 Moderately sensitive plants show injury (annuals and short-lived perennials
>10 May cause severe problems (most crops)
  1. SAR and adjSAR – The sodium adsorption ratio indicates the relative activity of sodium, calcium, and magnesium ions in soil or water reactions. The adjusted SAR includes the additional effect of carbonates and bicarbonates. The following guide can be used to relate adjSAR values to potential soil and plant problems.

Problem Type  No Problems Slight Problems Severe Problems
Toxicity <3.0 3.0-9.0 >9.0
Penetration <6.0 6.0-9.0 >9.0
  1. Boron – Small amounts of Boron (B) are essential for plant growth. However, high levels may be toxic. Most trees, vine and bean crops are sensitive to excess boron, while forages, field crops and certain vegetables are relatively tolerant.

  2. Nitrogen – In waters and wastewaters, the forms of nitrogen of greatest interest are nitrate, nitrite, ammonia, and organic nitrogen. All these forms of nitrogen, as well as nitrogen gas, are biochemically interconvertible and thus are components of the nitrogen cycle.

Nitrate generally occurs in trace quantities in surface water, but can reach high levels in groundwater. In excessive amounts, it contributes to the illness known as methemoglobenemia. A limit of 10 mg/L Nitrate-N has been set in order to prevent this disorder.

Ammonia is naturally present in surface and groundwater and in wastewater. It is produced largely by the de-amination of organic nitrogen-containing compounds and by the hydrolysis of urea. It is also produced naturally by the reduction of nitrate under anaerobic conditions.

Organic nitrogen includes such natural materials as proteins and peptides, nucleic acids and urea, and numerous synthetic organic materials.

Traditionally “Kjeldahl nitrogen” is the sum of organic and ammonia nitrogen. JMLord, Inc. uses a LECO Nitrogen analyzer which detects all forms of nitrogen during the analytical process. Thus the “Total Nitrogen” reported is a sum of all the forms of nitrogen discussed here.

  1. TDS – Total Dissolved Solids is another measure of total salt content in water. It may be estimated my multiplying the ECw result by 640 and expressed as mg/L (ppm).


Useful Conversions

1 dS/m = 1 mmho/cm = 1000 micromhos/cm
1 percent = 10,000 ppm
1 ppm = 2.72 lbs/Ac-Ft of water
1 Gallon water = 8.345 lbs
1 Ac-Ft = 325,851 gallons

 
 

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