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Collecting Samples for Agricultural Irrigation Water Quality Testing

Summary

Irrigation water contains organic and inorganic compounds that influence plant health, soil health and structure, and irrigation system longevity. This article will primarily help agricultural irrigation users (in particular row crop and vegetable farmers, specialty crop growers, and turfgrass managers) understand how to collect a representative water sample based on the source and prepare it for transport to a laboratory for analyses.

Introduction

It is important to regularly test the quality of your irrigation source water. The frequency of testing depends upon use. The analysis should be conducted in the same laboratory over time to create a record of changes in water quality. Keep the reports to create a baseline of stability or seasonal changes in water quality to compare to future reports. Information contained in each report can help you understand how your irrigation water supply may influence plant health, soil health, and irrigation system longevity.

Direct Impacts on Plant Health

Certain chemical compounds in water can directly influence the health and productivity of plants (table 1). For example, water sources can contain macronutrients and micronutrients. If these nutrients are not present in high enough concentrations, then the agricultural manager may need to apply fertilizers as supplemental nutrient sources. Some compounds are toxic to plants if present at threshold concentrations. Ions such as chloride and sodium can cause direct ion toxicity in roots and leaves, while other ions (e.g., boron) become toxic as they accumulate in plant tissue after passive uptake from water.1

Soil Impacts and Subsequent Plant Health Impacts

Water can also impact soil physical, chemical, and biological properties, eventually affecting plant health. For example, source water that contains minimal to no additional elements (termed “pure water”), as well as water that is high in sodium, can cause soils to disperse. Dispersed soil particles settle close to each other, which can result in a loss in soil permeability and porosity; dispersed soils are more susceptible to becoming compacted.1, 2 Clogging and sealing can also reduce gas exchange and water holding capacity, reducing the activity of microorganisms. The roots of some plants also have difficulty penetrating the compacted soil.3

Impacts on Irrigation System Performance

Water quality can also reduce the efficiency of irrigation systems. Lead-based pipes in older irrigation systems may corrode due to iron present in the water. Low pH (acidic) water may accelerate the corrosion of critical irrigation systems components. Limescale may accumulate in an irrigation system if the source contains moderate to high concentrations of bicarbonates. Bicarbonates bind with magnesium and calcium and settle out of the water forming the limescale. Limescale can build up on inner walls of pipes and clog sprayer nozzles.1

Where to Collect Water Samples

The following information relates to how to collect water samples based on the source of irrigation water. Collecting your sample from the correct location is essential, as it influences the accuracy and usefulness of the water quality test results.

Groundwater

Groundwater varies in water quality. Factors include the depth of the well (which aquifer is the well accessing) and proximity to the coast. For well water testing, collect the sample from the pump housing so that results are not influenced by contaminants within the irrigation lines. Collect one sample from each well. If a contaminant is thought to originate within the irrigation lines or sprinkler configuration, collect an additional water sample at a spray head or quick-connect shortly after turning on the pump after it comes to operating pressure and flow. This task will allow the collection of a representative sample of water that has been sitting in the system since the last irrigation cycle. Next, collect a separate sample after the stagnant water has been flushed through the system. The time this takes will vary with the type and size of the irrigation system. If wells are used to fill an irrigation reservoir (pond), sample the wells as outlined above, then follow surface water collection directions outlined below to obtain a separate sample from the reservoir.

Table 1. Typical water quality constituents tested and whether they potentially pose a risk to plants (either by deficiency or toxicity), soils, or irrigation systems.1, 2

Water Quality Constituent Direct Plant Risk Soil Risk Irrigation System Risk
pH X X X
Electrical Conductivity (EC) X X
Total Dissolved Salts (TDS) X X X
Sodium Adsorption Ratio (SAR) X
Adjusted Residual Sodium (adjRNA) X X
Residual Sodium Carbonate (RSC) X X
Bicarbonate X X
Carbonate X X
Phosphorus X X
Potassium X
Calcium X
Magnesium X
Zinc X
Copper X
Manganese X
Iron X X X
Sulfur X
Boron X
Sodium X X
Chloride X
Aluminum X X
Nitrate-nitrogen X

Surface Water

The quality of surface water depends primarily on land uses within the contributing watershed, the frequency, intensity and duration of rainfall, and exchange with other connected surface and groundwater resources. If irrigation water is applied via an overhead sprinkler, a pivot system, or spray stakes, collect the sample directly from a sprinkler head or quick connect. If water is not applied overhead and there is a pump station, follow the directions outlined above for obtaining samples at the pump housing.

Municipal Water Systems

Municipalities are required by law to test drinking water provided to the public. In many cases, this water is used for irrigation purposes. Some water quality constituents are tested more frequently than others.4 The results of the water quality tests (termed Consumer Confidence Reports) are publicly available by request from the relevant public water utility. Access the SC Department of Health and Environmental Control Testing your Drinking Water website for specific municipality information and regulations.5

Some municipalities also offer clients access to treated wastewater (known as reclaimed water) as a potential irrigation source. Specific guidelines regulate what types of reclaimed water can be used for different irrigation purposes. Consult the EPA’s “Guidelines for Water Reuse” for identifying specific uses.6 In certain instances, reclaimed water can be a better water source than well water for agriculture. For example, a golf course in Bluffton, South Carolina was using well water high in bicarbonates and sodium for irrigation. The course also had access to reclaimed water. Water quality analyses for both sources were compared. The reclaimed water contained less bicarbonates and sodium than the well water and provided additional phosphorus and potassium needed to support the health of turf and ornamental plants.1 The golf course switched to the reclaimed water as an irrigation source and saw a significant improvement in plant health.

Exploring Potential Irrigation Water Sources

Upper picture shows a VanDorn water sample with both ends open - this sample can be lowered into water and the ends opend to collect a water sample at a desired depth. The lower pictures shows a young male collecting a water sample using a long white sample pole.

Figure 1. The upper image is a Van Dorn water sampler, and the lower image shows a sample being collected using a grab sampler. Image credit: Sarah White, Clemson University.

Always test the water quality of potential irrigation water sources to determine water treatment needs and impacts on pesticides and fertilizer additions. Water can be sampled when test wells are bored. For surface water bodies, samples should be collected at the same location and depth the pump intake will be placed. Van Dorn horizontal water samplers and grab samplers (figure 1) are the easiest way to collect samples at a specific depth within the water column, especially if the water is deep. If the water level fluctuates in a potential surface water source, the intake pipe should be placed on a pulley system or float so it can be easily maintained at the desired depth, ideally the middle of the water column.

When to Collect Water Samples

The frequency of water sample collection depends upon the water source. Surface water should be tested seasonally (spring, summer, and fall) since weather, organisms living in the water, and the surrounding plant communities can affect water quality. For shallow groundwater wells, such as those often used along the coast, collect water samples seasonally to identify how rainfall patterns, tidally influenced surface water, and aquifer may influence water quality. Deeper wells tend not to have the fluctuations in water quality typical of shallow wells and surface water bodies; thus, deeper wells can be sampled once every two to three years. Collect separate samples at low and high tide if the source of irrigation water is close to a shallow water table (e.g., marshes) or tidally influenced.

Collecting a Water Sample

Start by rinsing a clean 5-gallon bucket (preferably a newly purchased bucket, or a bucket solely used for water sample collection) three times with the water to be collected, and then fill the bucket approximately halfway with the water. This bucket is what you will use to fill the sampling container. Collecting samples in high-density polyethylene (HDPE) bottles (figure 2) is best because the plastic will not readily react with the water chemistry. Bottles should be opaque to limit light penetration. These bottles can be purchased online in bulk. If buying HDPE bottles is not economically feasible, consider using other containers such as single-use water or soda bottles. However, it is important to make sure the bottles are cleaned and rinsed thoroughly before using to hold and transport a sample. Do not use glass bottles, as the glass can react with the water chemistry. Glass sample bottles can also break in transit to the laboratory.

Picture holds two HDPE bottles one ~500 mL in size and one that is ~50 mL in size.

Figure 2. Opaque, HDPE bottles ideal for holding water samples. Image credit: Dara Park, Clemson University.

Generally, a 0.5 to 1 liter (seventeen to thirty-four ounces) sample bottle should be adequate. Commercial laboratories usually provide guidelines on the sample volume required for different analyses, so check with the laboratory you will use before obtaining sample bottles to ensure you buy the correct size. With a permanent marker and a piece of label tape, indicate the location, sample ID, date, name of the person who collected the sample, and any other relevant information on the bottle. Next, follow the steps below to obtain the water sample7:

  1. Wash hands thoroughly or use gloves. Try not to touch the bottle opening or interior of the cap.
  2. Pour water from the 5-gallon bucket into the sample bottle, fill the bottle one-quarter to one-half full.
  3. Screw on the cap.
  4. Shake vigorously.
  5. Empty the water, ensuring not to contaminate the water in the 5-gallon bucket.
  6. Repeat steps 1 through 5 two more times to complete a “triple rinse.”
  7. Pour water from the 5-gallon bucket into the sample bottle, filling the bottle to the very top and not leaving room for air space. Screw on the cap tightly.
  8. Place the sample into a cooler with ice or cold pack.
  9. Follow the timing and transport instructions specified by the laboratory.

Rinsing the bottle and cap three times with the water being testing is important to ensure (1) there are no contaminants in the bottle, and (2) that constituents in the water that might bind to the plastic will bind prior to the final sample collection. Keeping the sample out of direct sunlight, making sure there is no air space, and keeping the sample cool are important because sunlight and warm temperatures can influence microorganism activity in the water and change the sample water composition.7

To make sure test results are as accurate as possible, always follow protocols specified by the laboratory. For example, the Clemson Agricultural Service Laboratory has guidelines for sampling. Contact the laboratory to find out how quickly they will be able to process the sample(s) and to let them know when to expect your sample(s) and how many there are. Some water quality constituents change over time. Discuss the water quality analyses you want with the laboratory before obtaining the sample to ensure collection, transport, and analyses needed are coordinated to get the best quality results. It is good practice to collect water samples early in the week so that the lab will receive the sample and have time to analyze it before the weekend or a holiday. For the same reason, if samples are to be shipped to the laboratory, plan to sample early morning to ensure shipment the same day. With time-sensitive analyses, plan to ship the samples using one-day or next-day shipping. Samples should be shipped using cold packs to maintain low sample temperature to minimize shifts in water quality composition.

References Cited

  1. Carrow R, Duncan R, Huck M. Turfgrass and landscape irrigation water quality. Boca Raton (FL): CRC Press; 2009. doi:10.1201/9781420081947.211.
  2. Carrow RN, Waddington DV, Rieke PE. Turfgrass soil fertility & chemical problems: assessment and management. Hoboken (NJ): John Wiley & Sons; 2002.
  3. Weil RR, Brady NC. 2017. Nature and Properties of Soils. 15th ed. United Kingdom: Pearson Ed; 2016.
  4. Safe Drinking Water Act (Title XIV of the Public Health Service Act). Ch. 373 of the 78th Congress, 58 Stat. 682; 1944, enacted 2019 [accessed 2020 05 01]. https://www.govinfo.gov/content/pkg/COMPS-892/pdf/COMPS-892.pdf.
  5. SC Department of Health and Environmental Control. Testing your drinking water. Columbia (SC): SCDHEC; 2019 [Accessed 2020 05 01]. https://scdhec.gov/environment/your-home/drinking-water-concerns/testing-your-drinking-water.
  6. US Environmental Protection Agency. Guidelines for water reuse. AR-1530. EPA/600/R-12/618. Washington (DC): USEPA; 2012 [Accessed 2020 05 01]. https://www.epa.gov/sites/production/files/2019-08/documents/2012-guidelines-water-reuse.pdf.
  7. Austin BJ, Espinoza L, Henry C, Daniels M, Haggard BE. How to collect your water sample and interpret the results for the irrigation analytical packages. Fayetteville (AR): Arkansas Water Resources Center; 2017 [accessed 2020 05 01]. FS-2017-03. https://arkansas-water-center.uark.edu/publications/factsheets/FS-2017-03-Irrigation-Analytical-Package-How-to-Collect-Sample-and-Interpret-Results-2.pdf.

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